The Formation of Cavansite and Pentagonite in the Wagholi Quarries, Pune, India

Abstract

Formation conditions of dimorphic minerals cavansite and pentagonite were previously based on theoretical assumptions. In doing so, the associations with other minerals, especially zeolites, that actually occur in nature, were disregarded or incorrectly taken into account. As a result, formation conditions were assumed that are not consistent with those for the associated minerals in the Deccan Volcanic Province. This relates in particular to overestimated high pressure and temperature values, as well as chronological processes of alteration. Long-term field studies and evaluations of numerous samples led to the conclusion that cavansite and pentagonite formed under temperature (approx. 120 °C to 200 °C) and pressure (0.01–0.03 GPa) conditions that are relevant for the associated low-temperature zeolites. Integration of geological and petrographic conditions, as well as crystallization sequences enabled the presentation of a multi-stage mineralization model. It is also explained that, contrary to the original assumption, characteristic pentagonite fivelings are not formed from five, but from six individuals.

1. Introduction

Secondary minerals from the Deccan Volcanic Province (DVP) reached worldwide fame because of their size, attractive formation, paragenetic association, and abundance. In addition to the zeolites and their characteristic associated minerals, e.g., apophyllite, special attention is devoted to cavansite and pentagonite. The two dimorphic minerals were first discovered in 1960 in Oregon (USA) [1,2,3]. In 1974, the first discoveries of cavansite happened in a quarry at the village of Wagholi near the city of Pune in Maharashtra, India [4,5,6]. Subsequently, pentagonite was also discovered at Wagholi [7]. The frequent occurrence of both minerals in Wagholi led to further studies dealing with their formation, distribution, properties, and morphology [8,9,10,11,12,13,14,15,16,17,18,19,20].

Intensive quarrying of basalt has taken place since the 1970s with countless finds of cavansite and pentagonite specimens. In addition, several new cavansite occurrences were discovered in the DVP [8,21,22]. Cavansite was also found in New Zealand and Brazil [23,24].

The initial descriptions of both minerals were based on relatively small crystals and their minor abundance from locations of Owyhee dam and Goble, Oregon. For the estimation of the mineralization sequence, only little investigation material was available. The co-occurrence of cavansite and pentagonite was observed only once in Oregon. Although a large number of suitable specimens were available from Wagholi and the geological conditions in the quarries were favorable for study, the considerations on the formation conditions of cavansite and pentagonite were based largely on theoretical assumptions [14,20].

Staples et al. [3] stated that this association of minerals suggests that during their formation they passed through the temperature of transition between cavansite and pentagonite, and that the latter is the low temperature form. Ishida et al. [14] suggested that pentagonite can be formed only if hydrothermal solution is under a supercritical condition (>300 °C), and it is therefore a high temperature form. Pujari et al. [20] assume in their theoretical study about the abundance of both minerals that cavansite never occurs together with pentagonite.

This study focuses on the formation conditions of cavansite and pentagonite, taking into account their occurrence in certain rock zones with respect to the paragenesis and mineralization sequence. The goal is to find out why the chemically identical dimorph minerals cavansite and pentagonite were deposited in closely neighboring positions with apparently different abundance. The present study refers to the findings in Wagholi and the conditions there. The mineral dimorph to cavansite owes its name, pentagonite, to the characteristic twinning that yields crystals with an apparent fivefold symmetry. Closer inspection of its crystals reveals some morphologic details that do not support its trivial crystallographic symmetry. Additionally, it turned out that the fivefold morphology is actually the result of six twinned crystals. The aim of this study is therefore to examine and describe the exact morphology of single and twinned pentagonite crystals and to check the correctness of its crystallographic symmetry.

This study on the formation of cavansite and pentagonite is of particular importance, as it is based on the observation of the Wagholi deposit over a period of about 50 years. The large number of specimens found and on which the study is based led to a broad overview of the mineralization sequences and paragenesis with the associated minerals, especially zeolites. As there is an imminent danger that the quarries will be closed in the near future due to the maximum mining rights being reached, the paper is a last-minute attempt for the exploration of the unique world-famous deposit and its exceptional minerals.

The results show that the mineralization of cavansite and pentagonite must be considered in the context of the geological conditions and events of the Deccan volcanism. In particular, the development of barometric and temperature conditions over geologic time is important. The clarification of the formation conditions of the secondary minerals also contributes to a better knowledge of the geological processes in the DVP in the period between the lava eruption and present. Since cavansite and pentagonite occur exclusively together with low-temperature zeolites both in Wagholi and at other sites worldwide, common formation conditions can generally be assumed.

2. Geological Setting and Mineralogy

The village of Wagholi is located about 15 km northeast of the city center of Pune, in the valley of the river Mula-Mutha on the road to Ahmednagar and Aurangabad. Since the 1990s, the quarries around Wagholi have been enormously productive (Figure 1) and today form a 7 km × 1.5 km complex consisting of around 40 mostly shut-down quarries. Between the individual quarries there are areas that have not been quarried and in particular contain facilities for crushing and classifying the basalt. A dense, largely void-free basalt was quarried, which was used as so-called “black rock”.

The basalt lava exposed belongs chemostratigraphically to the Lonavala Subgroup and was extruded around 65 Ma ago as part of the Deccan flood volcanism [25,26]. Between the Wagholi area (600 m amsl) and the highest point of the nearby region, Katrj Ghat (1142 m amsl), a total of 23 lava flows, which largely belong to the Wai Subgroup, are exposed [27]. However, the original paleosurface in the Wagholi—Katraj Ghat area is likely to have been somewhat higher. However, the corresponding layers in the river valley of the Mula-Mutha were removed by erosion. A maximum overburden of 500 to 1000 m can therefore be assumed for the Wagholi quarries. The lavas of the Wai Subgroup are tholeiitic in composition, e.g., [28].

In the area of the Wagholi quarries, there are only two largely horizontal lava flows, which generally have no significant dips. It is assumed that the lower lava belongs to the Khandala Formation. The upper flow could be the first of the four lavas belonging to the Bushe Formation [27]. The lower lava is a massive sheet flow and contains flow top breccias in which cavansite and pentagonite occur. The upper lava flow is a core-dominated (>10 m at places) rubbly lava flow [29]. As the breccias were not of economic interest in most cases, they were only partly excavated, so their actual extent, thickness, and possible connections with each other are unknown.

The secondary minerals of the DVP have been known for more than 200 years and have been valued and researched because of the formation of large idiomorphic crystals and unique paragenesis. The focus has been on the zeolites from the western area of the Main Deccan Province (MDP), the central part of the DVP. Walker [30] carried out the first larger overall view based on a 3-month trip through the DVP and recorded mineral species occurring at 120 sites. Based on the location of the individual finds, Walker believed he recognized three zones of mineralization. Some later researchers uncritically accepted Walker’s theory [31,32]. Walker’s assessment, however, was refuted by later works [33,34]. Ottens et al. [35] established in their study for the first time a multistage mineralization model for the secondary minerals of the Jalgaon area (central MDP). The results are also based on fluid inclusion studies and age dating. The latter implies that in the first stage during the burial, mostly clay minerals and only very small zeolite crystals were formed. Remarkably large crystals of zeolites and their associated minerals, on the contrary, were precipitated only in later stages, partly several million years after the eruption [35]. The results from the investigations in the Jalgaon area may also be principally applied to other areas of the MDP.

Due to the calcium-rich plagioclase in the common tholeiitic host rock of the MDP the zeolites are the Ca members, especially heulandite-Ca, stilbite-Ca, scolecite, mesolite, thomsonite, chabazite-Ca, and mordenite. The zeolites formed in the tholeiitic basalt of the MDP are considered as classical representatives of the zeolite facies [36]. For these, formation temperatures up to about 200 °C at pressures up to 0.1–0.3 GPa are proposed [37].

The intense green color of apophyllite crystals from the quarries at the Pashan Hills in Pune prompted Rossman [38] to investigate the coloration. The cause was found to be a vanadium (VO²⁺ ion) content of 1600 ppm. Further analyses of green apophyllite from other occurrences in the MDP also found vanadium contents between 500 and 3000 ppm [39]. Vanadium availability was present at a general content of about 300 ppm in the basalt of the MDP, e.g., [40]. Values of up to 600 and 750 ppm were reported for the Pune area [41]. The brecciated character of the rock with its high porosity and high permeability plays an important role in alteration.

This study is the first to investigate all secondary minerals associated with cavansite and pentagonite in the Wagholi quarry and to describe their mineralization sequence.

3. Sampling and Method

3.1. Sampling and Visual Evaluation of Minerals

The recording and visual evaluation of the occurring minerals is based on multiple personal visits (BO) and sampling in the quarries from 1995 onwards. Furthermore, reports from quarry operators and mineralogists who visited the quarries were taken into account. This refers in particular to the positions of the respective finds. The main mineral species in Wagholi can be easily distinguished visually. Numerous specimens that were found in the period between 1978 and 2023 and are now in private collections and public museums were included in the evaluations, especially with regard to the mineralization sequence. In the end, a total of over 1000 photos from museums, collectors, dealers, and the mineral platform Mindat.org [42] were evaluated. The tested specimens are listed in

Table 1. List of tested samples.

Sampl.No.	Minerals (Tested Minerals in Cursive)	XRD	SEM EDX	Raman/ Microsc.	Fl. Incl.	Supplementary Figure
A 1	Pen, 2 generations, Mor fibers.	x	x			Figures S1–S4
A 3	Pen, 2 generations.	x	x			Figures S5–S7
D 1	Pen		x			Figure S8
Mu 01	Cal on Pen and Hul.				x
Mu 02	Hul with Mor.		x			Figure S9
Mu 03	Hul.	x				Figure S10
Mu 04	Cal with Mor, Hul and Stb.	x	x	Ram.		Figures S11–S13
Mu 05	Crs with Pen, Oke	x	x	Ram.		Figures S14–S16
Mu 06	Pen and Mor.	x	x			Figure S17
Mu 11	Mor.		x			Figure S18
Mu 14	Pen on Hul.	x	x			Figures S19 and S20
Mu 15	Cav.	x				Figure S21
Mu 16	Hul with Cav, Mor and Durleite	x	x			Figure S22
Mu 17	Stb.	x	x			Figures S23 and S24
Mu 18	Cha, Pen and Qtz.			Micr.
USA 1	Cav.	x	x			Figures S25 and S26

Cav—cavansite, Cal—calcite, Cha—chalcedony, Crs—cristobalite, Hul—heulandite, Mor—mordenite, Oke—okenite Pen—pentagonite, and Stb—stilbite. The “Figures Suppl.” column lists the analyses shown in the supplements.

.

3.2. Analytical Methods

The studies were carried out on macroscopic samples, which were examined with the binocular microscope and magnifying glass to determine paragenetic sequences and assemblages. The minerals were identified by their crystal morphology and X-ray diffraction (XRD) and Raman patterns and by semi-quantitative chemical analysis carried out on a scanning electron microscope SEM/EDX. It is known that the chemical compositions vary within individual zeolite species in the DVP, but also at individual sites and also within individual crystals, without this resulting in a fundamentally different classification in the thermobarometric conditions. Therefore, XRD and SEM/EDX were sufficient for the determination of the species. The SEM-EDX analyses were performed on the zeolites to determine the dominance of the respective cations (alkali and alkaline earth metals) and to determine the mineral type of the subgroup (e.g., heulandite-Ca).

The XRD Diffractometer Bruker, Germany, D2 Phaser with Co-tube (Co Kα), 30 kV and 10 mA, was used for analysis of cavansite. SEM-EDX analyses were carried out using Zeiss EVO MA10 SEM with a Bruker EDX detector XFlash 410-M with Quantax 200 software and a High-Resolution RHoriba Europe GmbH, Germany, LabRAM Raman Microscope with 785 nm laser wavelength.

Polarizing microscopy in transmitted light on polished thin sections was performed on a ZEISS Axio Imager A1m microscope (Carl Zeiss Microscopy Deutschland GmbH, München, Germany).

The original aim of the work was to determine also the homogenization tempera-tures of cavansite, pentagonite, calcite, and chalcedony/quartz by examining the fluid inclusions (FIs). For cavansite and pentagonite, it was not possible to produce thick sections polished on both sides due to the brittle nature and small size of the crystals. No usable fluid inclusions could be determined for chalcedony/quartz. Only the examination of calcite was successful.

Microthermometric measurements of the FIs were performed using a Linkam Scientific Instruments, UK, THSMG600 heating and freezing stage covering a temperature range from −196 °C to +600 °C at the NAWI-Graz Geocenter (Graz, Austria). During cooling and heating, phase transitions were observed with an Olympus petrographic microscope equipped with an 80× ULWD objective. The Synthetic Fluid Inclusion Reference Set (Bubbles Inc., Blacksburg, VA, USA) was used for stage calibration. Temperature measurements are reproducible to within 0.2 °C at a heating rate of 0.1 °C min 1. Calculations of fluid densities were performed with the program Bulk by using the appropriate equations of state (EoS) for aqueous FIs after Wagner and Pruß [43]. Bulk is part of the software package FLUIDS 1 [44]. All FIs were initially cooled down to −190 °C and subsequently heated to determine the temperatures of phase transitions. Depending on the chemical system for any given FI, the following parameters are documented (L = liquid; V = vapor; and S = solid phase): (Te) eutectic temperature or apparent eutectic temperature (e.g., SIceV → IceLV); Te means the minimum temperature of liquid stability in a specified system and was used to identify the aqueous fluid system after Davis et al. [45] and Goldstein and Reynolds [46]; Tm(Ice) is the final melting temperature of ice (IceLV → LV); and Tm(Ice) was taken to calculate salinities of aqueous fluid inclusions using freezing point depression as well as equations after Bodnar [47]. Homogenization temperatures Th(LV → L or V) for aqueous inclusions were measured to avoid leakage or decrepitation and to obtain minimum conditions for formation of homogeneously trapped FIs.

4. Results

A description of the geological setting with more detailed information on the deposits of cavansite and pentagonite is only possible to a limited extent. While the general geological setting is identical regardless of where the vanadium-bearing minerals were found, the specific conditions at the respective occurrences varied. These covered various positions over the period of quarry operation from around 1970 to 2024. The areas of the old quarrying sites are now so heavily degraded that they can no longer be described by current investigations. However, as they are important for the overall assessment, the previous descriptions in the literature [5,6,22,48], the author’s (RD, BO) personal visits, and the recollections of some of the people involved in the excavation were used.

4.1. Geology

The minerals occurring in the respective surrounding rock zone must be taken into account. Generally, there is developed an approx. 10 m thick layer of a largely vesicular poor massive greyish basalt (Figure 2). This layer is almost not weathered and shows no horizontal zoning for trapped gas bubbles. However, if vesicles are present, they contain celadonite → heulandite → mordenite → calcite → rarely apophyllite. The incomplete mineral filling of the vesicles indicates an origin of the corresponding solutions from the directly surrounding zone in the host rock. The solid greyish basalt does not contain cavansite or pentagonite.

Field studies were carried out in 2024 in three quarries: (a) Dhoot Quarry—18°35′42.21″ N; 73°58′30.07″ E, (b) Para Plateau Quarry—18°36′10.97″ N; 73°58′33.96″ E, and (c) Bhawadi Quarry—18°36′30.03″ N; 73°58′47.79″ E. The sections (Figure 2) show exemplarily the location of the cavansite–pentagonite mineralization zones in the south-western part of the quarry complex. Their positions are marked in the map (Figure 1B). Sections in the central to north-eastern areas of the quarries show similar structures of the two lava flows and breccias. Only the mineralization shows lower amounts of cavansite and stilbite, but higher amounts of pentagonite and mordenite.

Interesting aspects of the flow top breccia are revealed in the quarry sections. In the shutdown Dhoot Quarry (Figure 1 and Figure 2a), a 2 to 5 m thick flow top breccia belonging to the lower lava was exposed at the base of the quarry. This flow top breccia overlies the vesicular crust of the lower lava and consists of glassy basalt fragments that for the most part were welded. Pits excavated in the quarry floor yielded rich cavansite finds. A highly mineralized pillar-like zone was also identified in the Dhoot Quarry. The base of the pillar possibly represents a breccia ’ramp’ or breccia mound that was highly mineralized in terms of cavansite and stilbite. The mineralizing veins and stockwork extended well beyond the breccia ramp and into the jointed basaltic massive core of the upper lava, leading to a misconception of mineralized ‘vent’ or ‘feeder dyke’. In subsequent years, the mineralized pillar was also excavated and destroyed.

In a quarry section some 100 m north-northeast of Dhoot Quarry, the same contact between the two lavas is exposed (Figure 2b). Present day excavations at the base of the upper lava expose the ~4.6 m thick flow top breccia. Here too, the massive core of the upper lava is ~12 m thick and is welded to the flow top breccia. Stilbite–cavansite–calcite mineralization is sporadic and confined to small breccia pockets. Another interesting aspect of variability in cavansite–pentagonite mineralization is seen. In a quarry (Figure 2c) about 500 m from the Dhoot Quarry, stilbite–cavansite–pentagonite mineralization is seen in the flow top breccia on the SE side of the quarry floor. As one moves to the higher ground, stilbite–calcite and cavansite mineralization is seen close to the upper flow contact. Further up in the section, cavansite is deposited with mordenite and calcite.

Cavansite and pentagonite occur generally only in the top breccia on the underlying lower flow, which is characterized by an intense alteration-induced red color. The texture of the breccia varies. The breccia first quarried in the so-called Dhoot Quarry consisted of fragments up to several dm in size, extending over almost 15 m in height and up to approx. 2 m in width. While only heulandite and stilbite were found in the upper area, especially in the vent-like zone of the Dhoot Quarry, more and more cavansite appeared with increasing depth [5,6]. In the years after 1990, the extraction of basalt was intensified in a north-easterly direction, which was accompanied by an increase in pentagonite and mordenite findings in addition to the occurrence of heulandite and cavansite. The mostly lenticular brecciated areas (Figure 3) were positioned on the quarry floor under the dense basalt zone and usually had dimensions of a few meters only. It is likely that the different characteristics of the breccias are due to the transformation between lobate and sheet lava and the positions between the individual lobes. The texture of the individual breccias differs in regard to the shape, number, and size of the interconnected open spaces on the one hand and the petrographic texture of the breccia segments on the other. It was clearly visible that the interconnected open spaces formed zones with high porosity and high permeability. Individual breccia fragments, on the other hand, showed a more or less pronounced porosity without open connecting channels.

The reddish brecciated areas also consist of basalt and not of andesitic tuff as erroneously depicted in some reports [5,48]. These excavated separate areas do not form always a visible continuous layer of breccia, but are locally limited. There is further an upper continuous vesicular crust (Figure 2) under the excavated zone, which generally overlies the lower lava flow. This vesicular crust has not yet been investigated in detail, so no further statement can be made about the mineralization in this subzone.

The specific positioning of the various secondary minerals in the breccia is of major importance. Therefore, two basically different positions have to be distinguished (Figure 4). The first area consists of altered rock fragments with small isolated pores and vesicles (iv). The second area consists of irregularly shaped interconnected open spaces (osp) between the individual brecciated rock fragments.

It is remarkable that no clay minerals, such as celadonite or smectite, were found as initial formations in the isolated vesicles and interconnected open spaces of the breccia. No uniform crystallization sequence of the individual mineral types could be noted in the different places of the quarry complex. In some places, stilbite or mordenite or pentagonite or cavansite are completely absent. One of the striking differences is the fact that cavansite and pentagonite are only occasionally found together. The combined occurrence of pentagonite and stilbite is occasionally also observed.

As a part of the preliminary considerations for this study, it became apparent that not only the determination of the mineral species, but also their occurrence in distinct zones of the breccias was essential. In particular, it was important to determine the extent of differences in mineral deposition between isolated vesicles and interconnected open spaces. Figure 5 shows that only heulandite crystallized in the isolated vesicles, regardless of the zone and depth. Further, the other secondary minerals stilbite, cavansite, pentagonite, mordenite, chalcedony, quartz, and calcite formed on top of the first heulandite layer only in the interconnected open spaces. Their occurrence indicates a significant, predominantly vertical, zonation. The interconnected open spaces, containing cavansite and pentagonite, are shaped differently and cannot be assigned to a uniform horizon in the quarry. For these reasons, a schematic representation (Figure 5) of the zones and the minerals crystallized in them was chosen. The individual zones are defined as follows: Zone I—upper zone, in which only heulandite and stilbite occur. In zone II—cavansite, pentagonite, and mordenite occur with decreasing depth. In zone III—cavansite and stilbite are absent, while pentagonite and mordenite dominate. The subzone is the unmined vesicular crust of the deeper basalt layer that lies under the breccia.

The examination results of the secondary minerals are presented below in the order of the chemical mineralogical groups. The XRD 2 theta plots are not shown and the EDX data are only presented in a few cases. Particular emphasis was on the properties of the minerals cavansite and pentagonite and the paragenetically relevant parameters for silica variants, heulandite, stilbite, and mordenite.

4.2. Minerals

4.2.1. Native Copper Cu and Djurleite Cu₃₁S₁₆

Native copper was observed visually several times as small plaques (up to 2 mm) in the host rock near the cavity wall. Djurleite occurs very rarely. The crystals reach a few mm in size and have a short prismatic habit. Identification was made by XRD (Figure S22).

4.2.2. Silica Polymorphs (Opal C, Cristobalite, Chalcedony, and Quartz) SiO₂

Chalcedony appears to occur in the subzone directly on the host rock of the interconnected open spaces as a wall-forming layer with a thickness of approx. 1–2 mm (Figure 6h). However, a close visual examination of several specimens revealed that the original layer of heulandite crystals was more or less replaced by chalcedony and its remnants are still often recognizable (Figure 7). The chalcedony layer is apparently frequently overgrown by other minerals, but it can be noted that the surface of the chalcedony consists of spherical formations (former opal spheres), which are only partially intergrown (Figure 6g,h). However, no chalcedony was observed in the isolated vesicles of the matrix of the same samples, while they contained highly lustrous and fresh heulandite.

Both cavansite and pentagonite crystals were apparently deposited on chalcedony, which in turn was partially or completely overgrown by further chalcedony (Figure 6g). In reality, they grew up on heulandite, which was later more or less displaced by chalcedony (Figure 7). Cavansite, stilbite, and pentagonite that are overgrown by chalcedony or quartz have the same properties, shapes, and/or twinning as those that are not overgrown.

An approx. 10 cm × 10 cm × 5 cm specimen (sample Mu 05) from zone III—subzone shows an unusual mineralization and sequence. Spheres (S1, diameter approx. 1 mm) and octahedrons (O1, approx. 1 mm) were visible within the transition zone on the chalcedony layer, which was partially exposed (Figure 8). The octahedrons showed rounded edges, but an undoubtedly octahedral habit. XRD analyses confirmed that the spheres (S1) and octahedrons (O1) are consisting of quartz. The chalcedony layer was largely overgrown by a mordenite layer up to 3 cm thick. This layer consisted of densely intergrown mordenite fibers, which in turn partially or completely enclosed pentagonite crystals and octahedrons. On the free mordenite and pentagonite surfaces there were again silica spheres (S3) and octahedrons (O3) (Figure 9). The latter were sharp-edged with highly lustrous surfaces. While the spheres (S3) were also identified as quartz by XRD, the octahedrons (O3) were in this case identified as α cristobalite (Figure 8) by XRD and Raman spectroscopy (Figure S14). Pseudo-octahedral crystals (O3) are actually tetragonal low-temperature α cristobalite. A section through the mordenite layer (Figure 8) shows that its interior also contains octahedrons (O2, not analyzed). The section shows that pentagonite was more or less overgrown by mordenite (Figure 8). The sharp-edged pentagonite crystals have a dull surface, but still high transparency and display no other signs of alteration or transformation.

On the surface of the fibrous mordenite mass, a less than 1 mm thick layer of the alkali-free okenite (Ca₁₀Si₁₈O₄₆ · 18H₂O) was in some places determined by XRD (Figure S11). As this plays no role in the placement of pentagonite in the mineralization sequence, the mineral is not described further below.

Among the unusual finds from the subzone are several specimens in which cavansite or pentagonite (rarely together with stilbite) were overgrown by chalcedony/macrocrystalline quartz (Figure 6i). The matrix was overgrown by an approx. 1–2 mm thick layer of chalcedony/quartz, which replaced the original heulandite. Pentagonite crystals formed in two generations on this chalcedony layer. The longest crystals of the first generation were over 50 mm long. The coarsely crystalline quartz was deposited directly on pentagonite.

Quartz crystals exhibit typical Bambauer lamellae (Figure 10A). In addition, many quartz crystals show heterogeneous internal structures (irregular extinction in polarized light), which are the result of the recrystallization of former chalcedony. This leads to feathery or flamboyant textures (Figure 10B), which are typical of the hydrothermal origin at low temperatures.

4.2.3. Calcite CaCO₃

Calcite formed in the interconnected cavities in four generations. The first generation of calcite deposited before or simultaneously with cavansite. Some cavansite crystals were enclosed in calcite crystals in the final growth phases. Their habit is flat rhombohedral. The second generation of calcite sits on top of cavansite, usually in the form of flat or slightly steep rhombohedral crystals. The third calcite generation is in the form of crystals that developed on top of pentagonite or partially enclosed it. The fourth generation is deposited on mordenite or directly on pentagonite if mordenite is present on the same specimen. The crystals of the fourth generation are generally very transparent and of a light yellow color. Calcite is present in all zones in the quarry.

Fluid inclusions of the third calcite generation of sample Mu 01 were investigated. FIuid inclusions in calcite of this generation, that are aligned along intragranular planes of rehealed bended cracks inside the crystals, show locally small channels between the inclusions due to “necking down” [49] and appear mostly as single-phase liquids (up to 95%) without a gas bubble at room temperature (Figure 11A). However, after several cool/heat runs, a considerable number of easily observable bubbles of a constant degree of fill (liquid/vapor proportion) evolved. Their homogenization to the liquid state happened without exception in the temperature range between 137 °C and 233 °C, with a more closed frequency from 170 °C to 201 °C (Figure 11B). Apparent eutectic temperature T_e(ice) was used to elaborate the chemical system of the trapped fluids after [46]. T_e(ice) below −21.2 °C, the eutectic temperature for the presence of NaCl, and below −10.7 °C, which is defined as the eutectic of KCl in the solution, was not clearly detectable. This is due to the restricted presence of thin ice platelets without any relief compared to additional salt crystals of higher relief, e.g., hydrohalite. The following observation of last ice melting T_m(ice) happened between −0.9 °C and 0.0 °C, which is transferable to all inclusions in calcite. Consequently, the fluid system is reduced to a pure aqueous fluid. Calculated densities correspond to the closed temperature frequency and yield 0.86 g/cm³ to 0.89 g/cm³.

4.2.4. Cavansite Ca(VO)Si₄O₁₀ · 4H₂O

Cavansite occurs predominantly in the interconnected open spaces of zone II and frequently crystallizes directly on top of heulandite (Figure 6a and Figure 12). Cavansite is very often intergrown with stilbite, and is commonly being formed first (Figure 6b,c). It was noted on several specimens that it was clearly deposited on stilbite (Figure 6d). However, on several specimens, the opposite was observed (Figure 6o and Figure 13). Cavansite sometimes rests on calcite, but it may also be partially or completely enclosed in it. In numerous cases, small calcite crystals sit on cavansite that is also associated with mordenite that may partially or completely overgrow it. Cavansite was found together with chalcedony. There, it was apparently deposited on heulandite/chalcedony and partly overgrown by a younger chalcedony crust.

Cavansite forms rosettes, spherical aggregates of acicular crystals, occasionally bladed tabular single crystals, and parallel aggregates up to 2 cm in size (Figure 6a,b,e) [5,6]. Orthorhombic crystals from Wagholi, often doubly terminated, are elongated along their c-axes. Two generations are discernible: smaller crystals up to 1 mm sit on the long-prismatic crystals, which reach 1 cm in length.

Cavansite was confirmed by XRD (Figures S21, S25 and S26). The EDX results are shown together with those of pentagonite (

Table 2. EDX results (shown without O) of pentagonite and cavansite as At %. A—first generation, B—second generation, and * probably contaminated by mordenite.

Sample/ Element	Pen A1 A	Pen A1 B	Pen A3 A-D	Pen A3 B-D	Pen * A3 A-R	Pen * A3 B-R	Pen D1 A	Pen Mu 14 A	Cav USA01
Al					0.25	0.06
Si	69.00	68.29	66.55	69.56	67.31	65.98	70.04	68.45	69.70
Ca	16.06	16.04	15.67	15.62	17.74	18.91	15.54	16.05	16.06
V	14.94	15.34	14.24	14.51	14.50	15.78	14.10	15.25	14.23
Cu		0.33	3.54	0.30	0.19	0.27	0.31	0.24
Total	100.00	100.00	100.00	100.00	99.99	101.00	100.00	100.00	100.00

). It is striking that cavansite occurs in different color shades. The variants range from bright blue to shades of green.

4.2.5. Pentagonite Ca(VO)Si₄O₁₀ · 4H₂O

Pentagonite occurs in zones II, III, and the subzone, but it is most frequently in zone III. The mineral is commonly crystallized on heulandite (Figure 6l,m), and less so on heulandite/chalcedony. Associations with cavansite, which crystallized first, are occasionally found (Figure 6o and Figure 13). When cavansite and pentagonite occur together, the nucleation base of the pentagonite is extremely rare on cavansite, but common on heulandite. Additionally relatively rare is the paragenesis with stilbite, which is formed after pentagonite and crystalized on the latter (Figure 6p,q). The association of cavansite with stilbite and pentagonite was observed on a few specimens (Figure 13). On the contrary, a paragenesis with mordenite, which partially or completely overgrows pentagonite, is common (Figure 6r,t). Occasionally, calcite crystals sit on pentagonite and can partially overgrow it. Pentagonite may have a high luster and good transparency if it is not overgrown by mordenite or chalcedony. Dull surfaces are evident on partly overgrown pentagonite crystals.

Pentagonite prefers to form radial sprays or clusters of elongated crystals which occur in two generations (Figure 6l) [21,50]. It is worth mentioning that pentagonite crystals of the first generation can reach up to 70 mm in length. The second pentagonite generation, on the contrary, comes in crystals commonly not exceeding 5 mm. Crystals of this generation sit on those of the first generation without a uniform orientation. The nucleation points of the second generation can be located on all surfaces and at any position of the first generation crystals. No other mineral species were precipitated between the two generations. Twinning of pentagonite is characteristic. Crystals of both generations appear in differently twinned forms. It is striking that pentagonite occurs also in different color shades from bright blue to clear shades of green. The differences in color are possibly due to a variable Cu content (Table 2).

Crystals of the first and second generation of samples A1, A3, D1, Mu 06, and Mu 14 were examined using XRD and EDX (Figures S1–S8, S17, S19 and S20) and (Table 2). Neither XRD nor EDX showed any evidence of substantial differences between the two generations.

4.2.6. Fluorapophyllite-(K) KCa₄(Si₈O₂₀)(F,OH) · 8H₂O

Fluorapophyllite-(K) is generally rare in Wagholi. A combined occurrence with cavansite has been observed only on a limited number of specimens. Green crystals were found on heulandite on which cavansite was also deposited.

4.2.7. Heulandite-Ca (Ca,Na)₅(Si₂₇Al₉)O₇₂ · 26H₂O

Heulandite occurs in all zones in all isolated vesicles and interconnected open spaces of the brecciated rock. The mineral was deposited directly on all cavity walls (Figure 6a). Clay minerals could not be observed between the altered host rock and the wall-lining heulandite. Size of the heulandite crystals depends on the size of the cavities. While in small vesicles the crystals reach up to 1 mm, in larger interconnected cavities they could reach up to 5 mm.

Depending on which zone of the breccia the sample with heulandite comes from, stilbite, cavansite, pentagonite, mordenite, chalcedony, and calcite may occur as associated minerals.

The samples from Wagholi analyzed by XRD and EDX (Figures S9, S10 and S12) confirm heulandite. The EDX data indicate heulandite-Ca (

Table 3. EDX results zeolites in At %.

Sample/ Element	MU 02-1 Hul-Ca	Mu 02-2 Hul-Ca	Mu 17-1 Stb-Ca	Mu 1-2 Stb-Ca	MU 04-1 Mor	Mu 04-2 Mor
O	76.57	77.08	66.40	65.74	56.73	58.60
Si	16.42	16.10	22.31	22.65	32.93	31.49
Al	4.04	4.10	6.20	6.40	5.98	5.63
Ca	2.01	1.93	4.06	3.89	1.59	1.60
Na	0-91	0.78	1.03	1.32	2.71	2.63
K	0.05	0.02			0.05	0.05
Total	100.00	100.00	100.00	100.00	100.00	100.00

) typical for the DVP with low contents of K, Na, and without significant traces of Sr or Ba.

Heulandite associated with mordenite was examined with SEM to determine the extent to which the former shows signs of dissolution. Such a possibility was estimated in particular with regard to the results of Kitsolpoulos [51]. However, no corresponding dissolutions could be detected in the samples Mu 02 and Mu 04. Visual evaluation of a specimen (Figure 6f) revealed highly lustrous heulandite, although mordenite spheres, measuring up to 4 cm in diameter, were sitting on top of the highly lustrous wall-lining heulandite.

4.2.8. Stilbite-Ca NaCa₄[Al₉Si₂₇O₇₂] · nH₂O

In general, stilbite only occurs in the interconnected open spaces and not in the isolated vesicles. Stilbite is found in the uppermost zone I and more frequently in zone II. The mineral formed in different generations. In the first one, small stilbite crystals up to five mm were deposited on top of the heulandite layer (Figure 6b). In the second generation, stilbite crystals up to approx. 3 cm long formed, which are often intergrown or penetrated by cavansite (Figure 6c). This association of stilbite with cavansite is considered the most common. Occasionally, specimens in which cavansite is clearly deposited on stilbite were observed (Figure 6d).

Considering the fact that a large number of samples and corresponding photos were checked, only a relatively small number of specimens was identified in which stilbite occurs together with pentagonite. On all such specimens, stilbite was crystallized after pentagonite and partially penetrated by it (Figure 6p,q). Such stilbite crystals were highly lustrous and up to 20 mm in size. No specimens were discovered where pentagonite grew on stilbite. Stilbite together with mordenite was observed only exceptionally.

The samples analyzed by XRD and EDX (Figures S13, S23 and S24) confirm stilbite. The EDX data indicate stilbite-Ca based on the cation content (Table 3) and are typical for the DVP with small amounts of Na and without significant contents of K, Sr, or Ba.

4.2.9. Mordenite (Na₂,Ca,K₂)₄(Al₈Si₄₀)O₉₆ · 28H₂O

Mordenite occurs sporadically in the zone II and the subzone, and very frequently in zone III. The hair-shaped crystals form radial sprays and clusters, spherical aggregates, as well as layers, which are generally grown on heulandite, chalcedony, cavansite, and pentagonite (Figure 6e,f,r). Mordenite prefers to be deposited directly on heulandite crystals. The latter have highly lustrous, smooth surfaces and commonly show no signs of dissolution. On numerous specimens, mordenite was deposited exclusively on heulandite, while there is no mordenite on cavansite or pentagonite on the same specimen. Among the rarities are specimens in which mordenite forms layers up to approx. 1 cm or more in thickness and completely encloses cavansite or pentagonite.

Mordenite was identified by XRD and EDX (Figures S11 and S18) and (Table 3).

4.2.10. Mineralization Sequence and Frequency

Most common paragenetic mineralization sequences and frequency (

Table 4. Mineral frequency in the interconnected open spaces.

Mineral/Zone	I		II			III				Sub
	a	b	a	b	c	a	b	c	d
Heulandite	xxxx	xxxx	xxxx	xxxx	xxxx	xxxx	xxxx	xx	xx	x
Cavansite			xxx	xxx	xxx	x
Stilbite		xxx	xxx	xxx	xxx	x
Pentagonite					xx	xx	xxx	xxx	xx	x
Mordenite				x	xx	xxx	xxx	xxxx	xxxx	x
Chalcedony						x	xx	xxx	xxx	xxx
Quartz										xx
Calcite		x	xx	xx	xx	xx	xx	xx	xx	x

The frequency is determined by the number of crosses: x—rare, xx—occasionally, xxx—frequently, and xxxx—common.

) of the secondary minerals in the interconnected open spaces in Wagholi:

heulandite → cavansite I

heulandite → cavansite I → stilbite I

heulandite → cavansite I → stilbite I → stilbite II

heulandite → cavansite I → stilbite II → cavansite II → stilbite III

heulandite → cavansite II → mordenite

heulandite → pentagonite

heulandite → pentagonite → mordenite

heulandite → pentagonite → stilbite III

heulandite → cavansite → pentagonite → stilbite III

heulandite → cavansite → chalcedony → quartz

heulandite → pentagonite → chalcedony → quartz

heulandite → pentagonite → chalcedony → mordenite → chalcedony → quartz

Calcite can occur in all stages and is therefore not shown. Mordenite was observed together with stilbite and pentagonite only once on one sample. Djurleite was probably deposited in an additional younger stage without influence on the formation of cavansite and pentagonite.

5. Discussion

The vulcanological evidence of welded breccia and the fused base of the upper lavas at the Dhoot Quarry and elsewhere suggests that the two lavas were emplaced quickly within a short time span. It is likely that the several-meter-thick layer of top breccia and intense vesicular crust acted as the major aquifer and fluid-bearing zone. Hot fluids led to intense alteration in the highly porous and permeable zone, resulting in solutions with a high vanadium content.

Visual inspection of cavansite and pentagonite-bearing host rock shows that smaller vesicles that were formerly inter-connected were partially or completely filled by secondary minerals (predominantly heulandite) during burial and are therefore isolated vesicles in subsequent stages.

In contrast to this work, the earlier reports on the formation of cavansite and penta-gonite did not contain any investigation results on the co-occurring zeolites [3,14,20]. The investigations of the mineral species occurring in Wagholi with regard to their mineralogical and chemical data did not reveal any special features. Cavansite, pentagonite, and the associated zeolites heulandite-Ca, stilbite-Ca, and mordenite were confirmed. For the thermobarometric conditions of the formation of cavansite and pentagonite, the variations in the cations in the zeolites play no role. Regardless of the variations, larger intervals for the temperatures and pressures apply to the respective species anyway. Silica varieties occurring in the deeper zone, which were not described in earlier reports, provided additional insights. As it was not possible to determine homogenization temperatures using fluid inclusions, except calcite, the aim was to restrict the formation temperatures and pressures of cavansite and pentagonite using the thermobarometric conditions known or calculated for the host rock and associated minerals.

5.1. Thermobarometric Conditions

When volcanic rocks are exposed at the Earth’s surface, they are subject to alteration caused by various factors. Specific reactions that take place between volcanic rocks and meteoric and or magmatic water and hydrothermal fluids include, in particular, the temperature–pressure conditions and the composition of the host rock and of the fluids [52]. The temperatures for the formation of the minerals in Wagholi were essentially determined by the extent of burial and the temperature of younger hydrothermal fluids. It is estimated that the thickness of the maximum paleo overburden in Wagholi is up to 1000 m. This range is based on the difference between the assumed paleosurface and the current height of the Wagholi quarry due to erosion [26,53]. There are no other precise calculations for the paleogeothermal gradient at our disposal for the Wagholi area. Ottens et al. [35] determined possible values for the comparable area in Savda/Jalgaon. Taking into account similar conditions in Wagholi and assuming an average paleogeothermal gradient of 80–100 °C /km would result in a probable burial temperature of approx. 70–120 °C, e.g., [52,54,55,56]. The experiments of Barth-Wirsching and Holler [57] and investigations show that heulandite and stilbite preferentially form in a temperature range of 80–140 °C [54,55,58,59]. Temperatures below 140 °C are assumed for the coexistence of heulandite with stilbite [60]. Heulandite, which occurs in all isolated vesicles and open interconnected spaces, probably formed by burial diagenesis based on the estimated paleo burial temperature and its characteristic temperature range.

For the MDP area, age dating reveals that several secondary minerals formed during the post-volcanic time, until several million years after the end of volcanism [35,61]. It is very likely that the overburden in Wagholi was already significantly lower at such younger times than at the end of volcanism due to progressive erosion. A significantly lower geothermal gradient can also be expected. As a result, the respective rock temperatures millions of years after the end of volcanism were also lower and could no longer reach values probably over 50 °C.

In accordance with observations in this study, zeolite crystals formed during burial can reach a few mm in size [62]. Larger crystals mainly formed on the basis of hydrothermal alteration. Stilbite in crystals up to 3 cm in length is present only in the interconnected open spaces, contemporaneous with or after cavansite, or after pentagonite. Their deposits can be explained by younger post-volcanic hydrothermal fluids with temperatures for stilbite more than approx. 120 °C, which cannot be attributed to the burial in Wagholi several million years after the end of volcanism. As stilbite homogenization temperatures cannot be determined by examining fluid inclusions, the generally accepted paleotemperature indicators are used for the formation of zeolites, e.g., [55,58]. The use of zeolites as paleotemperature indicators is based, among other things, on the assumption that the formation temperature of the minerals is independent of the chemical composition of the hydrothermal solution [55].

Since earlier or contemporaneous crystallization with stilbite can be assumed for cavansite, its relevant formation temperature can therefore be set at a value of less than 140 °C, and probably even lower. Powar and Byrappa [10] assume 110–120 °C. On several examined specimens, up to 3 cm long stilbite crystals rest on pentagonite (Figure 6p,q). Whether generally similar formation conditions for pentagonite can be derived from this paragenetic sequence has not been proven. It is likely that these formations are the result of a narrowly tolerated condition.

Pentagonite generally crystallizes in zones II and III after heulandite, and is more or less overgrown by mordenite on numerous specimens. Mordenite covers pentagonite, cavansite, and heulandite. Mordenite is the zeolite with the highest SiO₂ content, for whose crystallization a temperature range of approx. 150–180 °C is calculated, e.g., [51,57,58,59,63,64].

Of particular interest was the question of whether mordenite formed at the expense of heulandite re-dissolution or exclusively from a new Si-rich fluid. Kitsolpoulos [53] investigated the formation of mordenite by hydrothermal alteration of pyroclastics with ignimbrite character on Polyegos Island, Greece. The alteration minerals identified were mordenite, heulandite/clinoptilolite, opal-CT, quartz, and amorphous silica. Zeolite minerals and not smectite were recorded as the predominant phase on Polyegos, which is similar to the results in Wagholi. It is assumed that clinoptilolite and heulandite formed on Polyegos in a semi-open system from the glass of the rhyolitic lava through hydrothermal alteration (hydrolysis). Elevated temperatures may also cause mordenite to form. Excess silica caused formation of opal CT.

Due to renewed intrusions of rhyolitic lava dome on Polyegos, the temperatures increased with subsequent circulation of corresponding hydrothermal solutions. Due to the increased temperatures, the previously formed minerals (clinoptilolite and heulandite) were more or less dissolved. The released ions promoted the formation of mordenite. The remaining heulandite crystals show strong etching and dissolution traces, whereby mordenite was to a certain extent deposited on the remaining clinoptilolite/heulandite crystals. Mordenite thus formed at the expense of heulandite. It is known that the existence of mordenite is favored over heulandite at higher temperatures and that there is a relationship between temperature and the occurrence of clinoptilolite and mordenite [65]. Bish et al. [66] found that for clinoptilolite-rich tuff from the Yucca Mountains, mordenite formed from clinoptilolite after prolonged hydrothermal leaching at 120 °C and 180 °C. However, Kitsopoulos [51] does not rule out the possibility that a smaller amount of mordenite formed directly from a gel-like material.

Both the SEM examinations and the visual evaluations with the microscope on samples from Wagholi, which contain heulandite and mordenite, showed no signs of dissolution compared to those from Polyegos. It can therefore be assumed that mordenite in Wagholi was formed by the incursion of new SiO₂-rich fluids. The occurrence of thick mordenite layers and mordenite spherulites as well as the formation of silica varieties indicate SiO₂-supersaturated fluids in the corresponding stage.

In the subzone of the Wagholi quarry, pentagonite crystallized apparently on chalcedony and was consequently overgrown by mordenite and/or chalcedony/macrocrystalline quartz. The formation of chalcedony in basalts occurs in the temperature range between 150 and 300 °C [66]. However, when chalcedony forms by transformation from opal, which is likely in Wagholi, the formation temperatures are significantly lower [67]. Ottens et al. [35] determined a homogenization temperature of approx. 150 °C, based on the results of fluid inclusions, for the formation of chalcedony in the basalt of Savda/Jalgaon, which is an area with a comparable burial depth to Wagholi. Quartz that overgrows previously formed pentagonite shows typical Bambauer lamellae, which are a significant indication of epithermal (100–200 °C) formation (pers. comm. Götze, 2024). In addition, those quartz crystals show heterogeneous internal structures (Figure 10) (irregular extinction in polarized light), which are formed by recrystallization of former chalcedony. This results in the so-called feathery or flamboyant textures. They are typical for a low-temperature hydrothermal formation (pers. comm. Götze, 2024).

Fluid inclusions in the secondary minerals occurring could only be detected in calcite of the second generation. The mineralization sequence was heulandite → pentagonite → calcite. Since calcite formed in all mineralization phases, it is not suitable for the narrower temperature determination of the paragenetically occurring minerals.

The visual evaluation of a large number of specimens pointed to pentagonite crystallization after heulandite and cavansite, but before mordenite and before stilbite. The number of reported findings in which pentagonite occurs with stilbite is not very high in comparison to the total number of pentagonite findings. However, they are clear evidence that both minerals can form under largely identical conditions (Figure 6p,q).

A pressure of 0.1 to 0.3 GPa is generally accepted for the formation of zeolites at temperatures below 200 °C, e.g., [68,69]. The pressure during the precipitation of zeolites increases from a minimum of 1 bar (atmospheric pressure) to pressures between 0.015 and 0.03 GPa, which corresponds to the hydrostatic or lithostatic pressure at the maximum burial depth of 1100 m [54]. Frey et al. [70] give a maximum pressure of 0.18 GPa for the formation of heulandite.

A burial depth of 500 to max. 1000 m is assumed for the investigated basalt layer in Wagholi. Such values are in accordance with the generally accepted values for the formation of zeolites, e.g., [54]. The breccia-like porous-rich rock in Wagholi with the interconnected open spaces, in which the zeolites and the two vanadium–calcium minerals occur together, is to be regarded as a connected semi-open system with the same pressure conditions. If hot hydrothermal fluids in a mineralization stage caused an increase in vapor pressure, this would affect all areas and all minerals of the system. However, since the zeolites heulandite, stilbite, and mordenite formed before, contemporaneously, and after the deposition of cavansite and pentagonite, an effective pressure of approx. 0.03 GPa can also be assumed for both vanadium silicates.

In previous studies [14], it was reported that cavansite and pentagonite do not occur together and that pentagonite is grown on stilbite. However, several specimens were found to contain both mineral species. The study of the mineralization sequence showed that on all specimens containing cavansite and stilbite, the latter was grown on pentagonite (Figure 6p,q). It is also assumed on the basis of theoretical calculations [14] that cavansite crystallizes below 300 °C under conditions with sufficiently available H₃O⁺ and OH⁻ ions. For pentagonite, on the other hand, it is assumed that its crystallization is only possible under supercritical conditions of over 300 °C and a pressure of 0,01 GPa. Cavansite is regarded as the low-temperature form and pentagonite as the high-temperature form. Such assumptions regarding the temperature that are relevant for the formation of pentagonite are mostly contrary to the recognized and probably effective conditions that led to the mineralization of pentagonite in Wagholi.

In another study [20] based on theoretical calculations, an attempt was made to determine the reason for the relative abundances of cavansite and pentagonite. However, the real conditions and the abundance of the minerals occurring were not properly taken into account in the underlying considerations. In particular, the simultaneous occurrence of pentagonite with cavansite or stilbite was excluded. The assertion that pentagonite exists only in smaller crystals than cavansite is incorrect, and in fact, the opposite is true. The frequency of deposited crystals of cavansite or pentagonite varies in dependence of their finding place within the quarry. While cavansite predominates in the upper zones of the breccia, the opposite is the case in the lower zone. The apparently more frequent occurrence of cavansite compared to pentago-nite can be explained by the fact that the upper cavansite-bearing zones in Wagholi were intensively mined over many years. Only at the lower zones does pentagonite dominate. It is assumed [20] that 99% cavansite and only 1% pentagonite form at a temperature of approx. 600 K (326 °C). The theoretical assumption of such a high temperature seems unrealistic, as cavansite forms in the same temperature range as stilbite, i.e., below 140 °C. They did not take into account that both minerals crystallized in a common semi-open system at the same pressure, in which only different temperatures and different ionic compositions of the solutions are conceivable.

The formation of the two hydrous vanadium silicates cavansite and pentagonite should not only be seen from the point of view of the common crystallization with zeolites, but also with the related structures. For cavansite, structures of 4-fold and 8-fold rings, and for pentagonite of 6-fold rings, and analogous structures to gismondite are given [2]. Evans [2] and Staples [1] state that cavansite belongs to the layer silicate structure types and that the H₂O molecules are probably zeolitic.

5.2. Mineralization Model

In stage I, heulandite was precipitated during burial in all isolated vesicles and interconnected open spaces (Figure 14).

In stage II, the fluids that led to hydrothermal metamorphism probably formed a long time, up to several Ma, after the end of volcanism [35,50,52]. The same pressure and temperature conditions must have prevailed in the breccia in which the vanadium-containing minerals were deposited. The temperatures depended on the lower overburden height due to erosion and the geothermal gradient of the respective period (decrease in pressure and temperature). Host rock temperatures of probably less than 50 °C and a pressure between 0.01 and 0.03 GPa are estimated.

It is evident that brecciated areas of interconnected open spaces formed, with a basically uniform vertical and stretched horizontal zonal sequence of mineralization. The mineralization observed and its distribution can be explained by different phases/pulses in which the supply of a relatively hot, vanadium-bearing, as well as vertically and horizontally ascending fluid took place. Multiple fluid pulses also explain the crystallization of several generations of the same mineral species. Hot fluids cooled with increasing altitude and immigration distance due to heat exchange with the cold host rock. Pentagonite and mordenite are therefore deposited in deeper and more north-easterly areas, while stilbite and cavansite occur preferentially in higher and more south-westerly areas. It is very likely that factors other than temperature, such as pH or the chemical composition of the liquids, influenced the different phases of cavansite or pentagonite formation.

In stage III the precipitation of mordenite can best be explained by new, ascending fluids at approx. 150–180 °C, e.g., [71], of which some contain vanadium. According to this, pentagonite could occasionally crystallize simultaneously with mordenite and be enclosed by the latter. As the temperature of ascending fluids decreases, mordenite is consequently often deposited on pentagonite and occasionally on cavansite and heulandite, which were precipitated in stage I or II. Formation of mordenite in Wagholi is probably based on an elevated SiO₂ concentration of new hydrothermal fluids in stage III. In natural systems, the crystallization of mordenite is favored by the following: (1) a high SiO₂ content of the original glass, and (2) high a pH of pore water, which influences the Si:Al ratio of the pore water and the precipitated zeolite [64].

The occurrence of cristobalite together with opal-C and mordenite in Wagholi allows a complementary aspect of pentagonite genesis [72]. Cristobalite was detected once before in connection with mordenite in the MDP by Valkenburg and Bule [73]. The samples were found in an area of the Ellora caves in geological conditions basically comparable to those in Wagholi. Twinned, macroscopically identical crystals could be determined both as cristobalite and as pseudomorphs of quartz after cristobalite. The crystals were all perched on fibrous mordenite. According to current knowledge, they were low-temperature cristobalite crystals for which formation temperatures of 200–280 °C are assumed [74]. Previously formed cavansite or pentagonite were more or less overgrown by mordenite and opal/chalcedony. The opal converted over time to quartz.

It is not known which events led to the formation of younger, hot fluids from which the secondary vanadium minerals cavansite and pentagonite crystallized. No dykes were reported in the vicinity of the brecciated zones in Wagholi. There was no renewed volcanic activity or intrusion several million years after the end of the DVP eruptions. There are also no known tectonic events in the MDP, such as folding, that led to fluids with higher temperatures and pressures.

5.3. Comparison with Other Cavansite and Pentagonite Localities

Cavansite and pentagonite were also found in Owyhee Dam, Oregon (USA) and Aranga Quarry (New Zealand). However, a comparison of the mineralization in Wagholi with that in Aranga and Oregon is only possible to a limited extent. Only a small number of pentagonite specimens was found at Owyhee Dam, which is not sufficient for the reliable determination of the mineralization sequence. In Oregon, cavansite and pentagonite, heulandite, stilbite, apophyllite, analcime, and calcite were found as associated minerals, which overall indicates a formation environment suitable for low temperature zeolites.

In the Aranga Quarry, a larger variety of different mineral species were noted [75]. In addition to cavansite and pentagonite, the low temperature zeolites chabazite, cowlesite, lévyne, and thomsonite, as well as the silicates apophyllite and okenite, play a major role. Native copper, cuprite, and chrysocolla were also observed. The Aranga Quarry is sited in a single flow of an augite–olivine basalt which was intruded by less than 2 m wide feeder dykes of basalt [75]. The vanadium minerals and zeolites occur in a brecciated zone. The host rock which is bright red in color is heavily oxidized. No investigation of the mineralization sequence is available. However, visual analysis of photographs indicates the formation of the two vanadium minerals together with chabazite and okenite. Vanadium minerals sit on top of the low temperature chabazite that does not show any signs of dissolution. Remarkable for the Aranga Quarry is the proximity of a huge dyke. This could be the cause of hydrothermal alteration at higher temperatures, resulting in vanadium and copper-bearing fluids.

6. Conclusions

• There are two lava flows outcropping in the Wagholi quarry area. The lower lava is massive and contains flow top breccias, in which the mineralization of cavansite and pentagonite occurs.
• Pentagonite occurs occasionally together with cavansite, the latter precipitated first.
• Pentagonite occasionally occurs together with stilbite, which crystallized later.
• Cavansite, pentagonite, stilbite, and calcite in the Wagholi quarries crystallized in several generations.
• The deposition of cavansite, pentagonite, stilbite, and mordenite was controlled by ascending fluids whose temperature decreased towards the flow and breccia surface and immigration distance.
• A multistage mineralization model applies to the formation of the secondary minerals in the Wagholi quarry:

  ○

  Stage I—Deposition of heulandite by burial diagenesis in isolated vesicles and interconnected open spaces.

  ○

  Stage II—Multiple pulses of ascending vanadium-bearing hydrothermal fluids through interconnected open spaces. Deposition of cavansite, pentagonite, and stilbite induced by decreasing temperature and constant pressure in a semi-open system. Cavansite and pentagonite formed at pressure 0.1 GPa to 0.3 GPa and temperature ˂ 200 °C, which are conditions that also apply to low-temperature zeolites.

  ○

  Stage III—Ascending of Si-rich fluids, with deposition of silica varieties and mordenite.

• Cavansite crystals are always singles and reach up to 2 cm in length. Cavansite crystals that are embedded in mordenite develop characteristic sheaf-like-shaped crystals with heavily curved faces. Pentagonite, on the contrary, are predominantly twinned and reach up to 7 cm.
• Further work is required to determine the different conditions for the formation of the two dimorphic minerals cavansite and pentagonite. The actual mineralization sequences and the joint geological conditions of cavansite, pentagonite, and their associated minerals have to be taken into account.