Trapiche-like Quartz from Dongwuqi Area, Inner Mongolia, China

Abstract

Trapiche-like quartz, which is scarcely reported, has a unique texture with a core, six arms, and quartz growth sectors. This study is based on six trapiche-like quartz from the Dongwuqi area, Inner Mongolia, China. Many experiments have been carried out to investigate the basic gemological characteristics, crystal structures, and chemical compositions. Magnification observation has demonstrated that the texture is due to the distribution of white-colored chalcedony in growth sectors. Spherical and black inclusions have been identified as iron oxides by Raman spectra and EPMA. EPMA and LA-ICP-MS data have shown that the samples were grown in an acidic environment with an average medium-high temperature of 325 °C. Combined with the crystal growth theories and infrared experiments, we propose that the trapiche-like texture is the result of dislocation glide, which is caused by the existence of water and changes in temperature.

1. Introduction

The special “trapiche” structure of various minerals has attracted considerable attention since its discovery. The most famous trapiche structure was first discovered in emerald [1,2,3,4]. Subsequently, similar phenomena were found in other gem materials, such as ruby [5,6,7,8,9,10,11], sapphire [12,13,14], tourmaline [15,16], aquamarine [17,18], quartz [19,20,21,22,23], spinel [24], rhodochrosite [20], etc.

The trapiche-like phenomenon in Myanmar quartz was first mentioned by Win [11]. So far, it has been verified that trapiche-like quartz is widely distributed in many areas. Krzemincki and Laurs [19] observed a batch of trapiche-like quartz from Colombia and found many “fibrous” inclusions in its growth sectors. They believed that the trapiche-like pattern was formed by the incomplete dissolution of the elongated fibrous inclusions, which are contemporaneous with the crystal body. Kawasaki et al. reported a light yellow trapiche-like quartz produced in Japan, formed in stages by dendrite growth and layer growth [25]. Chen [26] proposed that the trapiche-like pattern in the smoke quartz of the Huanggangliang area was formed by the regular distribution of chromogenic elements along a certain crystallographic direction, while the trapiche structure of the transparent quartz of Dongwuqi was caused by the parallel growth of crystals. Gabriela et al. [22] studied the trapiche-like quartz formed by inclusions of primary clay (ferrosaponite or nontronite) from Inner Mongolia in depth. In addition, there have been many reports on trapiche-like quartz, most of which just describe trapiche-like phenomena. The growth mechanism has not been discussed in detail. Therefore, the formation mechanism of trapiche-like quartz is still an open question.

To enrich the studies of trapiche-like minerals, six samples of trapiche-like quartz from Dongwuqi area were selected. Their structures, compositions, and spectral characteristics were obtained by microscopy, cathodoluminescence spectra (CL), electron probe microanalysis (EPMA), Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), Raman spectroscopy, and infrared spectroscopy. Combined with the crystal growth theories and previous research results on trapiche-like phenomena, the possible formation environments and mechanisms of trapiche-like quartz were deduced.

2. Materials and Methods

Six different quartz crystals, obtained from the Dongwuqi area of Xilin Gol League, Inner Mongolia, China, were used as the original samples. All samples were cut along the direction perpendicular to the c-axis, with water being used as a lubricant during cutting. A chromium trioxide and alumina polishing solution was adopted to polish all sections, and an ultrasonic cleaning machine with alcohol was used to clean for microscopic examination and other experiments. The six slices were labeled as q1, q2, q3, q4, q5, and q6, respectively (Figure 1). These samples’ diameters varied from approximately 3.8 cm to 5.0 cm, and their thicknesses from 0.3 cm to 0.7 cm. The q3 and q6 were selected for CL. The q3, q4, and q6 were later carbon-coated for EPMA, and q3 for LA-ICP-MS analysis.

Conventional gemological properties and structural characteristics were studied in the Gem Laboratory of the School of Gemmology, China University of Geosciences, Beijing (CUGB).

EPMA experiments were carried out at the State Key Laboratory of Tibetan Plateau Earth System, Environment and Resources at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS) (JXA-8230 electron probe, JEOL Ltd., Tokyo, Japan), using a 5 μm beam spot diameter, a 20 nA beam current, and a 15 kV accelerating voltage. The measurement time of all element characteristic peaks was 10 s, and the measurement time of the upper and lower background was half of the peak measurement time. All test data were processed by ZAF correction. The microprobe analytical standards were albite for Na, corundum for Al, olivine for Mg, diopside for Si and Ca, orthoclase for K, rutile for Ti, rhodonite for Mn, hematite for Fe, chromium oxide for Cr, and nickel oxide for Ni. The detection limit was ~100 ppm. The diffraction crystals were TAPL and PET crystals. Before the experiment, a carbon film 20 nm thick was uniformly plated on the samples.

Trace elements were analyzed at the Beijing Zhongkekuangyan Testing Technology Co., Ltd., Beijing, China, using the Agilent 7500 ICP-MS instrument (Agilent Technologies, Co., Ltd., Palo Alto, CA, USA), which was equipped with an ESI NWR 193 laser ablation system. The size of the laser spot was 30 μm, and the laser frequency was 6 Hz. Glasses of BHVO-2G, BCR-2G, and BIR-1G were used as external standards for trace element calibration. Trace element analytical data were processed using the software ICPMSDataCal [27]. Each time-resolved analysis included approximately 20–30 s of blank signal and 50 s of sample signal.

Raman spectra was collected with a Horiba HR (Horiba, Ltd., Tokyo, Japan) Evolution-type micro laser Raman spectrometer at the Gem Laboratory at the School of Gemmology, CUGB, with a 532 nm laser, 10–30 mW laser power, and 4 cm⁻¹ resolution. The spot-size of the Raman spectra was approximately 1 μm. The transverse and vertical spatial resolution were better than 1 μm and 2 μm, respectively. All data were recorded from 400 to 4000 cm⁻¹. The scan time was 20 s. A single crystal silicon wafer, with a 520.7 cm⁻¹ single Raman displacement, was used to correct the Raman spectrum before conducting the experiments.

The infrared spectra were obtained using a Tensor 27 Fourier transform infrared spectrometer (Bruker Co., Ltd., Karlsruhe, Germany) of the Gem Laboratory at the School of Gemmology, CUGB, with a scanning range of 4000–400 cm⁻¹ and a resolution of 4 cm⁻¹ by 16 scans.

3. Gemological Properties and Internal Texture

3.1. Gemological Properties

All quartz samples, which were surrounded by country rocks, had transparent cores and arms, as well as opaque growth sectors (Figure 1). The refractive indices of the samples were measured by a refractometer as Nω = 1.544, Nε = 1.553, and DR = 0.009. The samples were shown to be uniaxial (+) under a polarizer. There was no dichroism. The microhardness of the samples was measured using a microhardness tester and converted to a Mohs hardness value of 6–7 by the formula Hv = 3.25 Hm³. Higher amounts of impurities in the crystals may have been the cause of samples with hardness values below 7. The specific gravity was not measured because of the country rock. The samples did not show twinning. White-colored fibrous inclusions in the quartz slices, i.e., component parts of the trapiche-like pattern, could be clearly seen by the naked eye.

3.2. Internal Texture of Trapiche-like Quartz

3.2.1. Massive Texture of Trapiche-like Quartz

The cross-section of the trapiche-like quartz was hexagonal, which is in good agreement with its symmetrical characteristics; it had six transparent arms running from the core to six corners of the hexagonal growth structure, as well as clear boundaries of six crystallographic sectors (Figure 1).

According to the crystal shape and symmetry of the six transparent arms, their symmetrical grades can be divided into three types: better, good, and worse. The symmetry of the q1 sample was good, with six arms and a more transparent hexagram in the center (Figure 2a). The crystal symmetries of the q3, q4, and q6 samples were better. Six thin arms were averagely distributed and intersected at a point in the core. The intersection angle of the adjacent arms was about 60° (Figure 2c,d,f). The symmetry of q2 and q5 was relatively worse, because the growth of crystal was uneven, resulting in irregular thickness and length and an uneven intersection angle among the arms (Figure 2b,e). The fibrous inclusions, the primary cause of the trapiche-like pattern, were concentrated along the diagonal axis of the crystal and oriented in the growth sectors, showing gradual dense to sparse changes from the core to the edge (Figure 2g).

It can be seen that the colors of the six samples were not uneven (Figure 1). The q1, q2, and q3 samples were white, while the q4 sample was yellow with many dark inclusions appearing in the growth sectors. The q5 sample was colorless in the middle and yellow on the edges. There were brown-colored inclusions in the middle and a light red color near the edges of the q6 sample. All samples showed obvious wavy extinction under cross-polarized light. By comparing their images under plane-polarized light, it was found that the obvious wavy extinction only appeared in the area where fibrous inclusions were densely and directionally arranged, and the interface in this area was relatively rough (Figure 2h,i). The wavy extinction in the growth sections indicated that there was dislocation glide during the crystal growth process [28]. The color of the country rock under plane-polarized light was darker than that of the quartz (Figure 2h), and it appeared bright white under cross-polarized light (Figure 2i), which is consistent with the characteristics of carbonate.

Cathodoluminescence Features

The CL images of the samples were blue-purple in color, with white cracks or orange carbonate present as well (Figure 3).

Compared to under polarization (Figure 3a), growth bands parallel to the hexagonal cylinder (dash lines in Figure 3c) were faintly visible when observed under CL. The formation of the growth band reflects a relatively stable growth environment for deep magmatic reservoirs [29]. Impurity elements in the quartz are generally contained in solid inclusions or fluid inclusions, and the fluid inclusions are mostly distributed along the cracks [30]. Therefore, the cracks in samples had brighter CL images (Figure 3b,d). Orange impurities could also be seen in the cracks under CL (Figure 3g); this was caused by the carbonate surrounding the rock. Concave corners caused by parallel grouping also appeared more clearly under CL (Figure 3f,h). The CL images demonstrate that the growth environment of the samples changes periodically, and the color under CL is affected by impurity elements during the growth process.

Raman Spectra

Raman spectra of six samples after subtracting the background are illustrated in Figure 4a. The characteristic peaks of the samples are located at 127 cm⁻¹, 205 cm⁻¹, 354 cm⁻¹, 392 cm⁻¹, 464 cm⁻¹, 692 cm⁻¹, 813 cm⁻¹, and 1162 cm⁻¹, respectively. According to relevant research, the characteristic peaks of Si-O are mainly divided into four ranges, namely, the low-frequency peak band of 300–100 cm⁻¹, the symmetric stretching vibration of Si-O at 600–300 cm⁻¹, the symmetric stretching vibration of Si-O-Si at 800–600 cm⁻¹, and the asymmetric stretching vibration of Si-O-Si at 1200–1000 cm⁻¹ [31]. Therefore, Raman shifts at 692 cm⁻¹, 813 cm⁻¹, and 1162 cm⁻¹ are, respectively, caused by the symmetric stretching vibration and asymmetric stretching vibration of Si-O-Si. The peaks at 354 cm⁻¹, 392 cm⁻¹, and 464 cm⁻¹ are due to the bending vibration of Si-O.

It has been verified that the FWHM of the characteristic peak is related to the minerals’ crystallization degrees, which also indicate the growth process of the minerals [32,33,34]. The smaller the FWHM of the 464 cm⁻¹ peak is, the higher its crystallinity [35,36].

Gaussian fitting was used to calculate the FWHM of the 464 cm⁻¹ shift in six samples (q1 to q6 in Figure 4b), resulting in values of 10.87 cm⁻¹, 10.95 cm⁻¹, 10.23 cm⁻¹, 10.49 cm⁻¹, 10.92 cm⁻¹, and 10.35 cm⁻¹, respectively. These values are close to each other, indicating that the crystallinity of each sample was similar. However, it still can be found that the lowest FWHM value among the six samples was 10.26 cm⁻¹ (Figure 4b), which is in good agreement with the most symmetrical shape of the q3 sample (Figure 1 and Figure 2c).

Infrared Spectra

Infrared spectra of the q1–q6 samples are illustrated in Figure 5. The characteristic peaks of the samples are located at 472 cm⁻¹, 549 cm⁻¹, 692 cm⁻¹, 798 cm⁻¹, 1102 cm⁻¹, and 1180 cm⁻¹, respectively, which is consistent with the absorption peaks of natural quartz. The samples also show absorption peaks between 1500 cm⁻¹ and 1700 cm⁻¹. According to previous research [37], 1200 cm⁻¹–900 cm⁻¹ are related to the telescopic vibrations of Si-O, 798 cm⁻¹ is due to symmetric telescopic vibrations of Si-O, 472 cm⁻¹ and 549 cm⁻¹ are the Si-O bending vibrations, and the peaks located in the range of 1500 cm⁻¹ to 1700 cm⁻¹ represent the bending vibration of H₂O. This indicates that the crystal contains H₂O.

3.2.2. Inclusions in Trapiche-like Quartz

Inclusions in the sample consisted of fibrous inclusions, spherical inclusions, and other inclusions.

Fibrous Inclusions

The fibrous inclusions, which were densely arranged in the parallel direction to the diagonal axis, were white-colored and formed the trapiche-like pattern. When observed under the polarizing microscope, their color was grayish-black (Figure 6a). When the magnification was strong enough, we found that these “fibrous” inclusions were composed of many intermittent elongated inclusions (Figure 6b–d). The Raman results showed this to be consistent with quartz (Figure 6e). This fibrous (with grain size of 1–50 μm) quartz is called chalcedony [38].

Spherical Inclusions

Spherical inclusions could be found in the interior of the crystal (Figure 7a), the country rock (Figure 7b), and the boundary between them (Figure 7c). These red irregular spherical inclusions were shown to have rough surfaces, red edges, and black insides under the polarizing microscope (Figure 7). According to their appearance characteristics, it is speculated that these spherical inclusions may be iron oxides.

The speculation was verified by Raman spectra of spherical inclusions in q4 (Figure 8). The Raman shifts of spherical inclusions were located around 216 cm⁻¹, 281 cm⁻¹, 400 cm⁻¹, 486 cm⁻¹, 596 cm⁻¹, 1323 cm⁻¹, and 1583 cm⁻¹, almost identical to the vibration modes of iron oxides.

Other Inclusions

In addition to fibrous and spherical inclusions, some other inclusions could also be found in the samples (Figure 9). The impurity inclusions were composed of two kinds; yellowish-brown inclusions were distributed in the quartz, and were often accompanied by directional fluid inclusions (Figure 9a). The black-brown impurities were mainly distributed in the cracks (Figure 9b).

The interiors and the edges of the black mineral inclusions had different colors. It is clear that the main bodies were black, and the edges were brown-red (Figure 9c), which may have been caused by their different thicknesses. The Raman shifts of the black mineral inclusions were almost identical to the vibration modes of hematite (Figure 10). Some small quartz crystals could also be observed in the sample (Figure 9d).

Figure 11 illustrates the detailed spot positions (the red spots) where EPMA data were taken. The chemical compositions of the black mineral inclusions are shown in

Table 1. Chemical compositions of black inclusions, obtained by EPMA (wt%).

No.	K2O	CaO	TiO2	Na2O	MgO	Al2O3	SiO2	Cr2O3	MnO	FeO	NiO	Total
1	0.023	0.007	0.018	0.021	0.067	0.204	0.756	-	0.395	86.307	-	87.798
2	0.037	0.068	0.004	0.103	0.066	2.985	0.752	-	0.346	82.360	0.060	86.781
3	0.027	0.031	0.033	0.058	0.030	0.099	0.472	0.036	0.421	87.357	-	88.553

“-” represents a value below the detection limit.

.

The total chemical composition amounts of the black inclusions, obtained by EPMA tests, were about 87%, which may be due to the alteration. According to the test results, the content of FeO accounted for about 97%, which is consistent with the hematite value reported in the reference (FeOT: 92.58%–99.47%) [39]. Therefore, it was deduced that these inclusion consisted of hematite, which is in line with the previous Raman results.

4. Trace Elements of Trapiche-like Quartz

4.1. Electron Probe Microanalysis

In order to verify whether there were differences among their chemical compositions, the q3, q4, and q6 samples were used to perform line scanning of EMPA. Figure 12 illustrates the detailed line positions (the red lines). The detailed chemical compositions of q3, q4, and q6 are shown in

Table 2. Chemical compositions of q3, q4, and q6 from core to edge, obtained by EPMA (wt%).

Sample	No.	K2O	CaO	TiO2	Na2O	MgO	Al2O3	SiO2	Cr2O3	MnO	FeO	NiO	Total
q3	1	-	-	-	-	0.012	0.230	99.056	-	-	-	-	99.333
	2	0.035	0.025	-	-	0.014	0.226	98.722	-	-	-	-	99.032
	3	-	-	-	0.013	-	0.208	98.745	-	-	-	0.021	98.991
	4	-	-	-	-	-	0.190	99.153	0.042	-	-	-	99.389
	5	-	-	0.002	0.012	-	0.172	99.087	-	0.024	-	-	99.314
	6	-	-	-	-	-	0.144	99.035	-	-	0.027	0.019	99.233
	7	0.015	-	0.019	-	-	0.188	98.948	-	-	-	-	99.190
	8	-	0.022	-	0.022	-	0.201	98.577	-	-	-	-	98.831
	9	0.018	0.014	-	0.035	-	0.233	98.758	0.017	0.016	-	-	99.094
	10	-	0.026	0.013	-	-	0.065	99.086	-	0.027	0.018	-	99.252
	11	-	-	-	-	-	0.031	98.499	0.017	-	-	0.030	98.587
	12	-	-	-	-	-	0.019	98.997	-	-	0.033	0.034	99.101
	13	0.024	-	-	0.024	-	0.014	99.348	-	0.037	-	-	99.455
	14	-	-	0.008	-	-	-	99.209	-	-	-	-	99.220
	15	0.022	-	-	-	0.024	0.057	98.628	-	-	0.079	-	98.819
q4	1	0.023	0.015	-	0.036	-	0.196	98.522	-	-	0.018	-	98.823
	2	-	-	0.007	-	-	-	99.530	-	-	-	0.046	99.605
	3	-	-	-	-	-	0.081	99.163	-	-	-	0.029	99.294
	4	-	-	-	-	0.023	0.163	98.891	0.019	-	0.020	-	99.135
	5	0.011	-	0.021	-	-	0.064	99.707	0.024	-	-	-	99.849
	6	-	-	-	-	-	0.090	99.555	-	-	-	-	99.676
	7	-	0.018	-	-	-	0.192	99.074	-	-	0.020	-	99.322
	8	-	-	0.062	0.015	0.010	0.139	99.607	-	-	-	-	99.833
	9	-	-	-	-	-	0.186	99.943	-	-	-	-	100.144
	10	-	0.017	0.006	-	-	0.131	98.883	-	-	-	0.043	99.102
	11	0.020	0.034	0.008	0.034	-	0.208	99.520	-	0.029	-	-	99.869
	12	-	0.030	-	-	-	0.158	99.139	-	-	-	-	99.355
	13	-	-	0.040	-	0.013	-	99.316	-	0.017	-	-	99.390
	14	-	-	-	-	-	0.258	99.035	-	-	-	-	99.342
	15	-	-	0.020	-	-	0.200	99.011	0.026	-	-	-	99.292
q6	1	-	-	0.026	0.036	0.015	0.032	99.228	-	-	-	-	99.362
	2	-	0.013	-	0.025	-	0.251	98.579	-	-	-	-	98.883
	3	-	0.014	0.006	0.015	-	0.242	98.769	-	-	-	-	99.065
	4	0.032	-	-	0.037	-	0.312	97.860	-	-	-	-	98.251
	5	0.017	-	-	0.014	-	0.266	98.229	-	-	-	-	98.549
	6	-	-	-	0.063	-	0.240	97.558	-	-	-	-	97.873
	7	-	0.014	0.012	-	0.027	0.252	98.716	-	0.039	0.161	-	99.239
	8	0.015	0.020	0.010	0.016	0.012	0.136	99.139	-	-	-	-	99.362
	9	-	-	0.016	-	-	0.015	99.550	-	-	-	-	99.595
	10	-	0.018	0.004	0.014	-	0.180	99.433	-	-	0.033	-	99.682
	11	0.025	-	-	-	-	0.168	99.124	-	-	-	-	99.360
	12	-	-	-	0.034	-	0.081	99.187	-	-	-	-	99.317
	13	-	-	-	-	-	0.062	99.433	-	-	0.042	-	99.555
	14	0.019	0.014	-	-	-	0.020	99.453	-	-	-	-	99.522
	15	0.015	-	0.003	-	-	0.059	99.981	-	-	0.030	-	100.088

“-” represents a value below the detection limit.

.

The main composition of the three samples was SiO₂. There were some trace elements present, such as K, Ca, Na, Mg, Al, Fe, Ti, Mn, Cr, and Ni. The content of Al₂O₃ was the highest among all of the impurity elements. The changes of Al₂O₃, TiO₂, and FeO in the quartz are illustrated in Figure 13 and analyzed in detail. The contents of Al₂O₃, TiO₂, and FeO were unevenly distributed on the cross-section of the whole slice. The content of Al₂O₃ experienced multiple increases and decreases from the core to the edge. According to previous studies, Al and Ti are activators of CL [40,41,42,43,44]. The trapiche-like quartz studied in this paper was α-quartz, which is a pure silica crystal that rarely contains structural defects or impurity ions. It is stable under natural conditions. Therefore, it has few luminescent centers, and its CL is very weak. The blue-purple CL (Figure 3) is attributed to the impurity element Al, which produces an A1³⁺ + O hole color center after irradiation. When compensation ions enter the hole color center, they cause the crystal to show blue violet light under CL.

The total Al content of q4 was lower than that of q3 and q6 (Table 2). Some studies have shown that the change in pH can affect the distribution of Al in quartz lattice. For example, at 200 °C, the lower the pH value, the higher the solubility of Al in hydrothermal fluid [45]. According to the substitution mechanism Si⁴⁺ = Al³⁺ + H⁺, the solubility of Al increases correspondingly with the decrease in pH. Compared with q3 and q6, the impurity in q4 were widely and unevenly distributed with lower Al content, which may have been caused by the increase in pH, making the impurity in the crystallization solution more stable and decreasing the solubility of Al. Based on the interaction between pH and Al concentrations, it is speculated that the uneven distribution of Al in quartz crystal can reflect the changes in formation fluid chemistry.

There was no Fe content in points 1–5 of q6 (Table 2), while the Fe content from point 6 to point 15 was higher than that in the same areas of q3 and q4. Previous studies have shown that pink in the quartz is caused by the high content of Fe and Ti in the quartz lattice [46,47], which explains the change in Fe content in q6.

4.2. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

The detailed positions (the red circles) where LA-ICP-MS data were taken are illustrated in Figure 13. Trace elements of the core (point 1), arm (point 6), and growth sector (points 2–5) in q3 are shown in

Table 3. Trace element concentrations of the investigated samples, analyzed by LA-ICP-MS.

		1	2	3	4	5	6
Na2O	wt%	0.062	0.000	0.000	0.007	0.001	0.000
MgO	wt%	0.049	0.003	0.000	0.000	0.006	0.003
Al2O3	wt%	0.033	0.078	0.127	0.115	0.230	0.122
SiO2	wt%	99.510	99.748	99.707	99.490	99.467	99.723
P2O5	wt%	0.004	0.006	0.005	0.004	0.004	0.006
K2O	wt%	0.007	0.001	0.000	0.002	0.001	0.000
CaO	wt%	0.083	0.003	0.006	0.011	0.000	0.003
TiO2	wt%	0.001	0.000	0.001	0.000	0.000	0.000
MnO	wt%	0.001	0.003	0.000	0.000	0.007	0.001
FeO	wt%	0.000	0.008	0.001	0.000	0.024	0.002
Total	wt%	99.750	99.850	99.846	99.629	99.739	99.859
Li	ppm	11.214	73.334	125.007	138.775	235.210	114.830
Be	ppm	0.000	0.000	0.712	0.349	0.000	0.000
B	ppm	3.098	3.852	3.001	1.635	2.597	2.490
S	ppm	516.220	439.509	501.207	510.244	700.310	318.969
Cl	ppm	716.168	385.817	189.653	253.920	477.981	451.262
Sc	ppm	0.991	1.075	1.245	0.761	1.113	1.501
V	ppm	0.069	0.009	0.000	0.086	0.010	0.000
Cr	ppm	439.624	3.216	1.428	1286.984	0.000	0.901
Co	ppm	0.099	0.017	0.004	0.073	0.129	0.000
Ni	ppm	4.283	6.575	2.554	0.000	2.909	3.312
Cu	ppm	29.118	0.170	0.023	85.279	0.416	0.000
Zn	ppm	12.260	0.179	1.065	34.728	1.339	0.101
Ga	ppm	0.068	0.049	0.104	0.000	0.043	0.179
Ge	ppm	0.283	0.438	0.940	2.419	0.921	0.569
As	ppm	0.437	2.098	4.935	5.237	11.719	4.838
Se	ppm	0.566	0.000	0.027	0.000	1.329	0.000
Rb	ppm	0.185	0.000	0.000	0.074	0.131	0.000
Sr	ppm	0.511	0.058	0.151	0.038	0.134	0.113
Y	ppm	0.000	0.000	0.000	0.043	0.050	0.000
Zr	ppm	0.791	0.000	0.009	0.000	0.000	0.000
Nb	ppm	0.019	0.000	0.000	0.000	0.013	0.024
Mo	ppm	0.000	0.000	0.012	0.000	0.000	0.026
Rh	ppm	0.009	0.007	0.000	0.009	0.007	0.003
Pd	ppm	0.000	0.000	0.000	0.064	0.081	0.000
Ag	ppm	0.040	0.042	0.085	0.006	0.055	0.066
Cd	ppm	0.407	0.227	0.061	0.050	0.000	0.000
In	ppm	0.000	0.000	0.009	0.000	0.018	0.000
Sn	ppm	0.659	0.723	0.000	0.783	0.880	0.405
Sb	ppm	1.671	32.542	35.814	58.389	129.545	37.531
Cs	ppm	0.010	0.034	0.021	0.342	0.014	0.072
Ba	ppm	2.345	0.000	0.220	0.062	0.000	0.000
La	ppm	0.141	0.267	0.000	0.034	0.006	0.543
Eu	ppm	0.017	0.012	0.026	0.000	0.007	0.036
Gd	ppm	0.267	0.295	0.101	0.479	0.091	0.101
Tb	ppm	0.000	0.000	0.000	0.000	0.006	0.000
Dy	ppm	0.000	0.000	0.000	0.050	0.013	0.058
Ho	ppm	0.009	0.000	0.007	0.000	0.000	0.022
Er	ppm	0.068	0.038	0.060	0.010	0.000	0.000
Tm	ppm	0.012	0.009	0.016	0.003	0.000	0.000
Yb	ppm	0.000	0.011	0.000	0.000	0.012	0.000
Lu	ppm	0.000	0.000	0.004	0.000	0.000	0.000
Hf	ppm	0.000	1.362	0.033	0.011	0.000	0.000
Ta	ppm	0.000	0.000	0.000	0.002	0.010	0.000
W	ppm	0.000	0.000	0.000	0.000	0.011	0.000
Re	ppm	0.004	0.000	0.000	0.004	0.009	0.000
Pt	ppm	0.036	0.000	0.019	0.000	0.000	0.022
Au	ppm	0.000	0.000	0.028	0.000	0.029	0.012
Tl	ppm	0.003	0.000	0.007	0.003	0.000	0.008
Pb	ppm	1.362	0.071	0.086	4.100	0.000	0.000
Bi	ppm	0.000	0.016	0.000	0.000	0.029	0.017

.

It can be seen from the LA-ICP-MS results that the geochemical composition of the q3 sample was uneven. The concentration of Al varied greatly from 330 to 2300 ppm, and the concentration of other elements also changed in a large range, such as Mg (30–490 ppm), Na (10–620 ppm), Ca (30–830 ppm), Li (11–235 ppm), Cr (1–1287 ppm), Sb (2–130 ppm), S (319–700 ppm), and Cl (190–716 ppm).

Previous studies have shown that the sharp changes in the concentrations of trace elements in quartz may be caused by changes in fluid composition [48,49,50]. The concentration of Cr in the core area was high, reaching an anomalous peak of 1287 ppm at point 4. Flem et al. [51] believed that Cr existed in the form of fluid inclusions or extremely small solid inclusions in quartz. According to the internal structural characteristics of the sample (Figure 2h), there were fewer solid inclusions in the q3 sample. Therefore, the highest Cr content is speculated to have been caused by the presence of a large number of fluid inclusions. The change in Al concentration may reflect the change in its solubility in the ore-forming fluid, which is obviously controlled by the pH of the fluid [48,49]. According to the substitution mechanism, Si⁴⁺ = Al³⁺ + H⁺ shows that Al concentration is negatively correlated with pH [52]. Combined with the coupling substitution relationship of Al³⁺ + Li⁺ → Si⁴⁺, it was found that Li and Al showed a good linear correlation (Figure 14). Quartz contains Sb with higher concentrations of S, which may be due to the fact that the fluid contains more S [53]. Some studies have stated that fluids rich in Li, Al, and Sb have lower pH values and stronger acidity [54]. A large amount of Ca and a small amount of Bi were also detected in the sample. The abundance value of Bi in intermediate acid magma is 0.01 ppm [55]. Because Bi³⁺ has a large ion radius (1.16 Å), which is close to Ca²⁺ (1.01 Å), it tends to be enriched in minerals with high Ca content, and the abundance of Bi in acid magma is slightly higher. This effectively explains that the Bi content is between 0.01–0.03 ppm. In the Dongwuqi area, Inner Mongolia, except for the alkaline granite type Nb-Y deposit, other deposits can be regarded as belonging to the Yanshanian deep source intermediate acid hypabyssal-ultrahypabyssal porphyry copper polymetallic metallogenic series [56]. Based on the above analysis, it is speculated that our sample grew in an acidic, magmatic environment.

5. Discussion

5.1. Formation Temperature of Trapiche-like Quartz

Ti⁴⁺ easily replaces Si⁴⁺ in the form of isomorphism in a quartz structure. Based on this, our predecessors established a Ti geothermometer through a series of experiments to calculate the formation temperature of quartz. At present, Ti geothermometers have been widely used to calculate the formation temperature of quartz. Wark and Watson established a geological thermometer which is widely used to calculate the formation temperature of magmatic quartz [57,58,59]. The expression of the geological thermometer (Titani Q) [60] is:

$$\mathrm{T}\left(\text{°}\mathrm{C}\right)=\frac{-3765}{\mathrm{l}\mathrm{o}\mathrm{g}({\mathrm{C}}_{\mathrm{T}\mathrm{i}}/{\mathsf{\alpha }}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2})-5.69}-273$$

(1)

${\mathrm{C}}_{\mathrm{T}\mathrm{i}}$ in the formula represents the content of Ti in quartz (ppm). ${\mathsf{\alpha }}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ represents the activity coefficient of TiO₂, which is expressed as 1 for calculation. Because the value of ${\mathsf{\alpha }}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ cannot be determined, the calculation results are only for reference and need to be further verified by other test methods.

Kawasaki and Osanai derived another Ti geothermometer (Formula (2)) [61] from natural rocks of granulite origin in Antarctica. Where ${\mathrm{X}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}^{\mathrm{Q}\mathrm{t}\mathrm{z}}$ is the mole fraction of TiO₂:

$$\mathrm{T}\left(\text{°}\mathrm{C}\right)=\frac{-5895}{\mathrm{l}\mathrm{n}{\mathrm{X}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}^{\mathrm{Q}\mathrm{t}\mathrm{z}}+1.729}-273$$

(2)

Through EPMA line scanning, we obtained the Ti content of q3, q4, and q6 from the center to the edge. It was found that the concentration of Ti in the whole sample was high and unstable, indicating that the formation temperature was relatively high. The calculated temperature is shown in

Table 4. Titanium concentration at each point, measured by EPMA, and temperature calculated using a Ti geothermometer.

Sample	No.	Ti/ppm	Ti (p.f.u. O = 2)	Temperature/°C
				Wark and Watson (2006) [60]	Kawasaki and Osanai (2008) [61]
q3	5	20	0.00005	585	448
	7	190	0.00043	831	706
	10	130	0.00030	780	651
	14	80	0.00018	721	582
q4	2	70	0.00016	706	568
	5	210	0.00047	845	720
	8	620	0.00140	1026	944
	10	60	0.00014	689	552
	11	80	0.00018	721	582
	13	400	0.00091	946	845
	15	200	0.00045	838	713
q6	1	260	0.00059	877	760
	3	60	0.00014	689	552
	7	120	0.00027	770	636
	8	100	0.00023	747	614
	9	160	0.00036	807	678
	10	40	0.00009	648	504
	15	30	0.00007	621	479

, and was obtained by taking different TiO₂ contents into account, such as 1, 3, 7, 8, and 9 EPMA points in q3; 2, 5, 8, 10, 11, 13, and 15 EPMA points in q4; and 1, 3, 7, 8, 9, 10, and 15 EPMA points in q6.

The calculated results show that the temperature periodically changed from the core to the edge. The temperature difference calculated by Formulas (1) and (2) was about 130 °C. Since the value of ${\mathsf{\alpha }}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$ in Formula (1) could not be determined, the temperature calculated by Formula (2) is more believable. The content ranges of Ti in the growth zones of a single crystal were 20–190 ppm, 60–620 ppm, and 30–260 ppm, respectively. The calculated temperatures corresponded to 448–706 °C, 552–944 °C, and 479–760 °C, respectively. In the process of crystal formation, it is impossible to have such a large temperature fluctuation, because the thermal quality of the surrounding rock will inhibit the temperature change and the quartz will dissolve when the temperature rises [48].

The LA-ICP-MS result shows that the main chemical components of the core, arms, and growth sectors in q3 were SiO₂. The contents of Al₂O₃, TiO₂ and FeO were unevenly distributed on the cross-section; this is in line with the EPMA test results. We took the TiO₂ contents (keeping four decimal places of raw data, ppm, the second row in

Table 5. Titanium concentration at each point, measured by LA-ICP-MS, and temperature, calculated using a Ti geothermometer.

No.	Ti/ppm	Ti (p.f.u. O = 2)	Temperature/°C
			Wark and Watson (2006) [60]	Kawasaki and Osanai (2008) [61]
1	10	0.000025	529	392
3	7	0.000017	502	364
4	1	0.000003	398	263
5	3	0.000008	451	316
6	2	0.000005	423	290
average	-	-	461	325

) of points 1, 3, 4, 5, and 6 into Formulas (1) and (2) to calculate the quartz formation temperature, as shown in Table 5.

It can be seen from Table 5 that the growth environment of the sample had a medium-high temperature, and the average crystallization temperatures of the sample were 461 °C and 325 °C, respectively. Rusk et al. [48] believed that Al concentrations formed at temperatures <350 °C consistently in the range of several thousand ppm in all quartz, and rarely in quartz that formed at >400 °C. The Al concentration of the sample was between 330–2300 ppm. Therefore, the second calculated temperature (325 °C from ICP-MS results and Formula (2)) is more reliable.

5.2. Growth Mechanism of Trapiche-like Quartz

Based on the microstructure characteristics, it can be proposed that the white-colored fibrous inclusions, which form the trapiche-like patterns in the samples, consist of many elongated grains. The core was smoother under orthogonal polarization, which indicates that the quartz was crystallized during the nucleation of ordinary quartz at the beginning. The growth sectors showed obvious wavy extinction, indicating that the crystal growth environment changed greatly in a certain period, resulting in a dislocation slip of the crystal. The temperature, differential stress, and activity of chemical components (such as water) led to the initiation of the slip system [62]. According to the EPMA and LA-ICP-MS data, the crystallization temperature was medium-high and changed periodically. It is speculated that this is an important factor leading to dislocation slip. The infrared spectrum data show that the samples contained H₂O, and previous studies have shown that the presence of water in the environment can cause hydrolysis weakening of Si-O bonds, thereby reducing the Peierls stress (the stress required for dislocation intergranular slip) in the slip system and providing a driving force for dislocation slip and particle boundary migration [63,64]. When the subgrains generated by the dislocation slip rotate and recrystallize [65], the newly formed grains are pushed outward, layer by layer, in the direction of the diagonal axis. They then grow into intermittent elongated inclusions, forming fibrous inclusions which are densely arranged in the direction of the diagonal axis and can be observed by the naked eye.

6. Conclusions

Trapiche-like quartz from the Dongwuqi area has six transparent arms that run from the core to the six corners of the hexagonal growth structure. Trapiche-like patterns in quartz samples are caused by the white fibrous chalcedony distributed in the growth sectors. According to the magnification results, white fibrous chalcedony was formed by a large number of elongated inclusions arranged in a dense orientation. Besides chalcedony, other inclusions were also observed in the quartz crystals, such as spherical inclusions, impurity inclusions, and secondary crystals. The spherical inclusions were iron oxides, according to Raman spectra. There was also some water in the sample crystals, according to the infrared results.

The results of EPMA and LA-ICP-MS demonstrated that there were no differences in chemical composition among the core, arms, and growth sectors, indicating that the texture of trapiche-like quartz is irrelevant to its chemical composition. The high concentration of the Cr element indicated that there were a large number of fluid inclusions. Combined with the content of Bi and the metallogenic characteristics of the Dongwuqi deposit, it is speculated that the samples grew in an acidic environment. Coupled with the CL results, the blue-purple color was attributed to the Al₂O₃ content. According to the TiO₂ content, it was calculated that the samples’ crystallization occurred in a medium-high temperature environment, at an average temperature of 325 °C.

The formation of a trapiche-like texture is related to dislocation glide. Wavy extinction and a rough interface were observed in the growth sectors under cross-polarized light, indicating that dislocation glide had occurred in the crystal. The periodic changes in environmental temperature and the decrease in Peierls stress inside the crystal led to the activation of the slip system, resulting in the occurrence of dislocation slip. The subgrains generated by dislocation glide were recrystallized, forming new grains that continuously grew along the diagonal axis direction. Thus, a large number of elongated inclusions were formed in the diagonal axis direction of the crystal, forming the trapiche-like pattern in the quartz.