Mineralogy and Geochemistry of the No. 5−2 High-Sulfur Coal from the Dongpo Mine, Weibei Coalfield, Shaanxi, North China, with Emphasis on Anomalies of Gallium and Lithium
2. Geologic Setting
3. Samples and Analytical Methods
3.1. Sample Collection
3.2. Analytical Methods
4.1. Coal Chemistry and Petrology
4.2. Minerals and Their Modes of Occurrence
- Clay minerals. Kaolinite is the most abundant mineral in the No.5−2 coal (except in sample DP5−2-5), and it is also the dominant component of most of the roof/floor/parting samples throughout the coal seam. In the coal, most of the kaolinite occurs as cell-fillings (Figure 3B,F, Figure 4A,E,F) and massive lumps (Figure 4B), indicating authigenic and terrigenous origins. In some cases, the kaolinite occurs as vermicular forms (Figure 4C), indicating a volcanic and subsequent in-situ precipitation origin . In a few cases, the kaolinite occurs as fracture-fillings (Figure 6C), indicating an authigenic origin through epigenetic precipitation . By contrast, in the roof/floor/parting samples, kaolinite generally occurs as a matrix (Figure 4D, Figure 7E,F, Figure 11C), and to a lesser extent, it replaces plant tissue (Figure 4D). This type of authigenic kaolinite has also been reported in several tonstein beds [55,56]. Additionally, all studied samples contain illite, ranging from 0.51% to 9.93% in the coals, 5.25% to 13.64% in the partings, and 11.56% to 14.67% in the host rocks. Chlorite was detected in sample DP5−2-2, and illite/smectite (I/S) was detected in roof samples by XRD.
- Silicates. Quartz occurs as fine-grained particles (Figure 5A,B) and as phenocrysts, which are mostly coated by clay minerals, indicating a syngenetic detrital origin. It is worth noting that some of the quartz in sample DP5−2-1-r has sharp edges rather than rounded edges (Figure 5A). This may be indicative of high temperatures and the influence of volcanic activity [21,24,54,57]. Wang et al.  also found high temperature quartz derived from a volcanic source in Wangshiwa Mine, which is close to Dongpo Mine. Additionally, lumpy and dispersed albite (Figure 5C,D and Figure 7C) was observed in DP5−2 coal using SEM-EDS. Some of the albite grains were altered and corroded, and some were filled by other minerals, such as goyazite (Figure 5C).
- Carbonate minerals. The carbonate minerals detected in the samples from Dongpo Mine include calcite, ankerite, and dolomite. Calcite is more abundant in samples DP5−2-4 (41.8%) and DP5−2-5 (50.5%) than in the other samples. The calcite generally occurs as fracture-fillings (Figure 3A, Figure 6A–C), and in a few cases, as cell-fillings (Figure 6D). Figure 6A,B shows calcite fracture fillings cutting across the bedding layers of kaolinite. Additionally, the ankerite and dolomite occur as cell- (Figure 4F and Figure 6D) or fracture-fillings (Figure 6E,F), indicating an epigenetic origin.
- Sulfides. The main sulfide mineral present in the study area is pyrite. Pyrite in the Dongpo coals mainly occurs as pore-fillings (Figure 3B,E) and as discrete crystals (Figure 7A), suggesting an authigenic origin. Most of the euhedral pyrite particles in study area are less than 3 μm (Figure 7A). In some cases, disseminated particles of pyrite are widely distributed within the veined calcite (Figure 7B). Small amounts of chalcopyrite were observed using SEM, but were not detected by XRD analysis (Figure 7C,D). Pyrite in the partings generally occurs as massive (Figure 7E) and fracture-fillings (Figure 7F), indicating a syngenetic and epigenetic origin.
- Phosphates. Phosphate minerals, such as apatite (Ca5F(PO4)3), svanbergite (Sr Al3(PO4)(SO4)(OH)6), and goyazite (SrAl3(PO4)2(OH)5·H2O) were detected in several of the coal samples from Dongpo Mine using XRD and SEM-EDS. Svanbergite and goyazite are intimately associated with other minerals, such as kaolinite (Figure 4F,H) and albite (Figure 5C,D), and they contain minor amounts of F, i.e., 1.5% and 2.37%, respectively. Apatite mainly occurs in samples DP5−2-5 and DP5−2-8 with concentrations of 1.4% and 4.4%, respectively, detected by XRD (Figure 8 and Figure 9). SEM analysis reveals that the apatite occurs as dispersed particles (Figure 10A,C) and banded composite layers of apatite and aluminosilicate minerals in the coal samples (Figure 10B,D). Additionally, the apatite crystals varies, including light green, yellowish brown, and transparent under reflected light. Two forms of apatite crystals were identified in this study: (1) Single rounded apatite crystals (Figure 10E–H,J,K) indicate transportation and a detrital material input from the sediment-source region. (2) Single enhedral and hexagonal apatite crystals (Figure 10E,I–M), indicate a volcanic origin . In some cases, the delicate elongated apatite grains may have fractured as a result of compaction  or have been altered . Volcanic debris, shells and/or fecal matter may have acted as the phosphorus source. The apatite probably formed in the pores of the organic matter and in the collodetrinite if Al was not available to react with the precipitated phosphatic material [60,61].
- Other minerals. In addition to the mineral phases discussed above, trace amounts of zircon and anatase (rutile) were also detected in the partings by SEM-EDS. They were not detected by the XRD probably because they were under the detection limit of XRD. Zircon occurs as incomplete quadrilateral bipyramid (Figure 11A,B), while anatase/rutile occurs as individual particles (Figure 11C,D).
4.3. Geochemistry of the Coal and Host Rocks
4.3.1. Major Element Oxides
4.3.2. Trace Elements
4.3.3. Rare Earth Elements and Yttrium
5.1. Sediment Source Region
- The Al2O3/TiO2 value has been widely used as a useful provenance indicator of sedimentary rocks [75,76] and the sediment-source region of coal deposits [57,73,77], and to infer the original magma composition of volcanic ashes in coal and coal-bearing strata [78,79]. Typical Al2O3/TiO2 ratios of sediments derived from mafic, intermediate, and felsic igneous rocks are 3–8, 8–21, and 21–70, respectively . The coal benches in Dongpo Mine have Al2O3/TiO2 values ranging from 13.85 to 51.52, with an average of 31.93 (Figure 17), indicating intermediate-felsic compositions for the sediment-source region. In addition, similar to those of the coal benches, the Al2O3/TiO2 ratios of the host rocks and partings also suggest terrigenous source rocks with intermediate–felsic compositions (Figure 17). In addition, the ratios of Zr/Sc and Th/Sc can reveal variabilities in the mineral composition, the degree of sorting, and the heavy mineral content  and is suitable for studying the sedimentary source rocks of coal deposits . All of the Dongpo Mine samples fall within the same area, indicating that their source rocks are felsic (Figure 18). An intermediate-felsic source rock for the Dongpo Mine samples is in agreement with the chemical compositions of the N and W Middle Proterozoic moyite in the Yinshan Oldland . During the Late Paleozoic, the Xingmeng Trough on the northern side of the basin subducted under the North China Platform, resulting in the uplift and orogeny of the Yinshan Oldland on the northern margin . During deposition of the No.5−2 coal, the basin’s terrain was high in the north and low in the south , so the intermediate-felsic lavas that erupted from the Yinshan Oldland was an important sediment source during coal formation.
- The provenance of the No.5−2 coal can be further deduced by the rare earth element (REY, or REE if yttrium is not included) and trace-element assemblages [1,57]. The Dongpo coals (including partings, roofs, and floor) and the Junger coals, e.g., the Heidaigou and Haerwusu coals (Figure 19) [17,19], have similar geochemical characteristic, i.e., negative Eu anomalies and enrichment in Li, Ga, Zr, Pb, and Th. Therefore, they may share a similar provenance—the Middle Proterozoic moyite in Yinshan Oldland and the Benxi Formation bauxite. Thus, as with the Junger coals, the Dongpo coal bench samples would also be expected to have high REY contents, with the Benxi Formation bauxite providing most of the REY. However, in addition to variations throughout the seams in the Dongpo Mine and the Junger Coalfield, i.e., high REY contents in the overlying partings and low REY contents in the coal benches of Dongpo Mine, while high REY contents in the coal benches but low REY contents in the overlying partings of the Haerwusu, Heidaigou and Guanbanwusu coals [17,19,20], the REY contents of the coal benches (73.412 μg/g on average, respectively) from Dongpo Mine, are two to three times lower than those of the coals in the Junger Coalfield (e.g., 154 μg/g on average for the Guanbanwusu coals; 228 μg/g on average for the Haerwusu coals; 259 μg/g on average for the Heidaigou coals) [17,19,20]. In addition, according to Dai et al. , besides coals being enriched in REY from leaching of the partings, high REY contents in the coal benches in Junger coals are also attributed to the bauxite of the weathered surface. Thus, no or weak influence of the weathering crust bauxite leads to a lack of REY enrichment in the No. 5−2 coals from Dongpo Mine.
- The Al2O3 content of the Dongpo coals, which range from 3.87% to 5.09% (4.47% on average, on a whole-coal basis), is lower than that of the Heidaigou (10.56% on average), Haerwusu (8.89% on average) and Guanbanwusu (9.34% on average) coals [17,19,20]. In addition, a number of minerals, including diaspore and boehmite, which are highly enriched in the No. 6 coal in the Junger Coalfield [2,17,19,20], were not observed in the coal samples from Dongpo Mine. Studies by Dai et al.  showed that these minerals are thought to have formed from colloidal aluminous gels or solutions, generated in conjunction with the development of bauxitic soils from the Benxi Formation bauxite in the northern and eastern parts of the Junger Coalfield. Then, this material was transported to the peat swamp and reacted to form gibbsite, with the gibbsite altering to boehmite or diaspore through compaction and dehydration. Thus, the evidence further suggests that the anomalies of Ga and Li in the No. 5−2 coal from Dongpo Mine were not caused by the Benxi Formation bauxite.
- As described above, the modes of occurrence of Li in the No. 5−2 coals are similar to those in the Haerwusuand Guanbanwusu coals, which are believed to be associated with aluminosilicate minerals, indicating the same source region—the Middle Proterozoic moyite from the Yinshan Oldland region [19,20]. However, the modes of occurrence of Ga in the No. 5−2 coal, which occur in aluminosilicate minerals and pyrite, differ from those of the Junger Coalfield, indicating that unlike the source of Ga in the Junger Coalfield, the enrichment of Ga in the Dongpo coals is unlikely influenced by the bauxite of Benxi Formation. In addition, the rare metal content of the raw rock determines the content in the rock’s weathering products . Since the Li content of the moyite is 26 μg/g , and the Ga in intermediate-acid rocks ranges from 19 μg/g to 21 μg/g , the contents of Ga and Li in the Middle Proterozoic moyite from Yinshan Oldland are not high enough to provide the high proportion of Ga (from 26 μg/g to 34 μg/g, with an average of 31.55 μg/g) and Li (from 75.7 μg/g to 91.7 μg/g, with an average of 84.98 μg/g) in the No. 5−2 coal from Dongpo Mine. Therefore, the Middle Proterozoic moyite in the Yinshan Oldland region provided the basic material source of the trace elements in the No. 5−2 coal. If the input of this source rock resulted in the enrichment of Ga and Li, it may only lead to slightly higher Ga and Li contents than those of world hard coals, but it could not cause the much higher Ga and Li contents mentioned above. The synchronization volcanic ash, hydrothermal fluids and marine environment, which will be discussed below would cause the secondary enrichment of Ga and Li in the No. 5−2 coals.
5.2. Influence of Synchronization Volcanic Ash
- Eu anomalies in coal are generally not thought to have originated from weathering processes in the sediment source region or during metal transportation from the sediment source region to the peat swamp, but instead inherited from rocks within the sediment source region . The distribution patterns of rare earth elements in the No.5−2 coal (including partings, host rocks) are similar to those of intermediate-felsic volcanic rocks that have negative Eu anomalies compared to the upper continental crust. In addition, the enrichment of Li, Ga, Be, and U in the No.5−2 coal samples from Dongpo Mine, which are typically enriched in felsic volcanic materials , also indicate a felsic volcanic input.
- Although it was below detection limit of XRD, a small proportion of zircon has been identified in the samples by SEM-EDS analysis (Figure 11A,B). Studies by Zhou et al.  showed that detrital zircons in normal sediments of terrigenous origin are quite different in crystal habit and morphology from those of pyroclastic origin. The former are characterized by tetragonal bipyramids with relatively short prisms, making the ratio of length to width (c/a values) around 2 . However, the zircon population from pyroclastics, as a whole, is represented by long, well-developed, tetragonal prisms doubly terminated by pyramids, with c/a values above 2.5 . The c/a values of the zirons in our samples are about 2.8, indicating a pyroclastic origin.
- The avanbergite and goyazite detected in the Dongpo coals are intimately associated with the kaolinite (Figure 4F,H) and albite (Figure 5C,D), indicating an authigenic origin. Previous works by Ward et al.  concluded that the phosphorus that formed the goyazite-group phosphate minerals may have originally been part of the volcanic ash that was input as a dilutant into the original peat deposit and/or was released from the organic matter in the peat, and then picked up by the Al-rich solutions to form the essentially insoluble aluminophosphates in the pores of the organic matter. Other authors, such as Triplehorn and Bohor , Spears et al. , and Hill , have reported that the goyazite-group minerals in tonsteins associated with coal seams were derived from volcanigenic material altered in a peat swamp. Dai et al. [17,19,20] found that goyazite occurs in the boehmite-rich coal benches of the Heidaigou, Haerwusu and Guanbanwusu Mines and was probably derived from the surface-weathered Benxi Formation bauxite. As discussed above, the No. 5−2 coal benches in Dongpo Mine were unaffected by the Benxi Formation bauxite. It is likely that the avanbergite and goyazite are related to the volcanigenic material.
- The high temperature quartz with euhedral hexagonal crystals in the Wangshiwa mine, neighboring the Dongpo Mine, was observed by Wang et al. , suggesting the mineral was derived from a volcanic source. In this study, the quartz grains with sharp edges were observed in the roof sample DP5−2-1-r (Figure 5A). These sharp edges indicate that they may not be a detrital material of terrigenous origin [21,24,54,73]. Yang et al.  also found that in sandstone, quartz from a volcanic origin is irregular in shape and the edge angles are clear, with no trace of abrasion. Furthermore, some of the single apatite crystals occur as enhedral and hexagonal grains (Figure 10E,I–M), further indicating the volcanic origin of this mineral . The vermicular kaolinite also indicates a volcanic and subsequent in-situ precipitation origin [22,54,59].
- In the Pennsylvanian Taiyuan Formation, the tectonic activity gradually became more intense in the southern part of the Ordos Basin with the development of the Qinling orogeny in the Indo-Chinese epoch . Yang et al. , used 158 exploratory wells (almost entirely covering each block of the southern Ordos Basin) to determine that the sandstone of each of the coal deposits contained volcanic material. The wide distribution of this volcanic material suggests the frequent and intense volcanic activity in the southern Ordos Basin in the Late Palaeozoic. The geochemical characteristics mentioned above and the mineral association of zircon, avanbergite, goyazite, high temperature quartz, apatite, and book-like kaolinite further confirm that the synchronization volcanic ash input during formation of the No. 5−2 coal in Dongpo Mine. The diameters of the pyroclastic minerals in the vertical coal profile vary widely, for example, the diameter (the clinodiagonal diameter was used for irregular shapes) of the high temperature quartz in the roof samples is 100 μm to 150 μm (Figure 5A), the diameter of the zircon in the parting sample is about 24 μm (Figure 11A) and the diameter of the single apatite crystals with a volcanic origin in the coal benches is 25 μm to 100 μm (Figure 10E,I–M). The types and sizes of the minerals from a volcanic origin in the coal seams, partings, and host rocks are quite different, indicating that the volcanic activity included multiple eruptions, in a relatively short time interval, that the volcanic material was intermediate-felsic in composition (discussed above), and that the quantity of volcanic ash was too low to form a tonstein layer within the coal seam.
- According to Dai et al.  and Ward , volcanic ash affects the trace element concentrations of coal in two ways: (1) Intra-seam tonsteins, widely distributed in the Late Permian coals of southwestern China, may be incorporated with mined coal products, and if not removed in the preparation plant, become part of the feed coal; and (2) Similar volcanic ash that forms tonstein bands intimately dispersed in the peat, and during diagenesis the originally pyroclastic matter becomes part of the inherent mineral matter in the coal seam. Although the mechanism by which alkaline volcanic ash causes enrichments in Nb, Ta, Zr, Hf, REE, and Ga has been adequately explained by Dai et al. , the mechanism by which intermediate-felsic volcanic ash causes elemental enrichments is still unclear. Since the intermediate-felsic volcanic ash that falls from the sky are enriched in Ga and Li , later leaching by ground water or hydrothermal solutions could transport elements, such as Ga and Li, from the volcanic ash into the coal seam, resulting in the enrichment of Ga and Li in the No. 5−2 coal from Dongpo Mine.
5.3. Influence of Hydrothermal Fluids during the Syngenetic or Early Diagenetic Stage
- In coal seams, the disseminated pyrite particles are distributed throughout the veined calcite (Figure 7B). The paragenetic association of the pyrite and calcite indicates a pyrite-rich calcite hydrothermal origin. Additionally, in the partings, pyrite generally occurs as fracture-fillings (Figure 7F). The precipitation of sulfide minerals in fractures may be derived from sulfate-rich hydrothermal fluids .
- The relationship between the fracture-filling kaolinite and calcite, and the early formation of kaolinite relative to calcite (Figure 6C) also indicate multiple injections of hydrothermal fluids. Additionally, ankerite and dolomite occur as cell- (Figure 4F and Figure 6D) or fracture-fillings (Figure 6E,F), indicating an epigenetic hydrothermal origin.
- Ti and Zr, are normally resistant to leaching, but they are relatively soluble in highly acidic conditions . The study by Dolcater et al.  demonstrated that in Ti-containing kaolinite, 86% of the total Ti was in the TiO2 form, primarily as anatase or as anatase with smaller amounts of rutile. It is probable that acidic hydrothermal solutions are favorable for the subsequent substitution of Ti for Al in developing kaolinite crystals .
- Because the sediment-source region is mainly felsic to intermediate in composition, the REY distribution patterns of the Dongpo coals should exhibit L-REY enrichment type . However, the REY patterns of the middle and upper parts of the coal seams are dominated by M-H-types and H-types respectively, and only the lower part of the seam is L-type REY (Figure 17). The M-REY enrichment of coal is usually caused by the following factors: naturally acidic water , including acidic hydrothermal solutions [103,104]; sediment-source regions composed of high-Ti and low-Ti alkali basalts; and the higher absorption of MREY by humic matter compared to LREE and HREE [105,106]. The H-REY enrichment of coals is generally caused by a wide spread of natural waters enriched in HREY, which may circulate in coal basins . In this study, the abundant calcite found in samples DP5−2-4 and DP5−2-5 (middle part of the No.5−2 coal seam), and the fracture-filling calcite in the collodetrinite in sample DP5−2-5 (Figure 3A) indicate that acidic calcium - rich solution is most likely responsible for the M-and H-type of REY enrichment of the middle part of the No.5−2 coal seam in addition to the higher sorption of MREY and HREY by humic matter than LREY.
- Compared with the Li contents, the contents of Ga in No. 5−2 coals from Dongpo Mine exhibit a significant positive correlation with the amount of volatile matter (R2 = 0.7256, Figure 20A,B). Based on the geologic setting that deep fractures are well developed in the study area, which cut the Mesozoic strata , and according to Wang et al, , the Ga may migrate up to the surface from the deeper strata with the geogas. Additionally, as a metal with a low melting point (29.78 °C) , Ga is significantly affected by temperature and will pervade along fractures with the fugitive constituents in hydrothermal fluids . Although Li has significantly higher melting point in comparison with Ga , it could be mobilized by chlorine associated to hot fluids.
5.4. Marine Environments
- Many studies have shown that negative Ce anomalies are considered to be an indication of a marine depositional environment, although the REY concentrations of seawater are low and the transference of REY from the seawater into the peat is still unknown . Generally, δCe < 0.5 is indicative of coals formed in oxic marine water, 0.5 < δCe < 0.9 is indicative of suboxic marine water, and 0.9 < δCe < 1 is indicative of anoxic marine water (δCe = EuN/EuN*) . The δCe value of the No.5−2 coal varies from 0.85 to 0.98 with an average of 0.93, indicating that the No.5−2 coal was influenced by suboxic to anoxic marine water.
- The high content of total sulfur (3.55% on average) and the high proportion of vitrinite-group macerals (79.6 % on average, mineral-free) further suggest a marine influence during deposition of the No. 5−2 coal deposition and indicate a reduction environment. Although the elevated concentrations of sulfur in coal are not necessarily the supportive evidence for marine environments [112,113,114] but reflect epithermal solutions, the high sulfur content of the No. 5−2 coal from Dongpo Mine is within the range possible for normal sulfate reduction  and the No. 5−2 coal formed in a tidal flat environment, which was influenced by seawater during peat accumulation . These evidences combined with the syngenetic pyrite in the analyzed samples, which is often considered to be an indicator of marine influence , indicate that the high sulfur contents of the Dongpo coals is attributed to marine water.
- The enrichment of gallium and lithium by seawater is mainly seen in the following aspects: On the one hand, marine plankton can enrich some trace elements and provide abundant material sources. On the other hand, seawater changes the pH value, the Eh value, and the H2S content of peat marshes and produces specific geochemical barriers, which are conducive to the enrichment of trace elements . Qin et al.  concluded that the Li contents of coals that were more influenced by seawater are significantly higher than those of coals that were not influenced by seawater, e.g., the coals from the Fugu mining district, Shanbei Permo-Carboniferous Coalfield.
Conflicts of Interest
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|Chinese coal a||8.47||0.33||5.98||4.85||0.22||1.23||0.015||0.16||0.19||0.092||1.42|
|World coal a||14||2||3.7||28||17||6||17||16||28||6||18||100||8.4||2.1||0.2||0.04||1||1.1||150||11||23|
|World clay b||54||3||15||120||110||19||49||36||89||16||133||240||31||1.6||0.91||0.063||1.3||13||460||48||75|
|World coal a||3.4||12||2.2||0.43||2.7||0.31||2.1||0.57||1||0.3||1||0.2||0.58||9||1.1||3.2||1.9||0.3||36||1.2|
|World clay b||10||36||8||1.2||5.8||0.83||4.4||0.9||1.9||0.5||2.5||0.39||1.3||14||0.38||14||4.3||1.4||190||5|
|Correlation with Ash Yield|
|Group a: rash = 0.8-1.0 Ga(0.89) Li(0.87) Zr(0.82)|
|Group b: rash = 0.5-0.8 U(0.67) Th(0.77) Ta(0.6)|
|Group c: rash = 0.3–0.5 Pb(0.42) Bi(0.38)|
|Group d: rash = −0.3-0.3 Er(−0.05) Be(0.09) Co(−0.16) Ce(−0.27)|
|Correlation coefficients with selected element combinations|
|rAl-Si > 0.8 Ga, Th, Zr, U, TiO2|
|rAl-Si = 0.5–0.8 Li, Ta, Bi, MgO, CaO|
|rAl-Si = 0.3–0.5 K2O |
rAl-Si = −0.3–0.3 Er, Be, Co, Ce, Pb, Fe2O3, NaO
|rs > 0.8 Be, Ga|
|rs = 0.5-0.8 Co, Er, Pb, U|
|rs = 0.3–0.5 Bi, Ta, Zr, SiO2, Al2O3|
rs = −0.3-0.3 Th, Li
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Qin, G.; Cao, D.; Wei, Y.; Wang, A.; Liu, J. Mineralogy and Geochemistry of the No. 5−2 High-Sulfur Coal from the Dongpo Mine, Weibei Coalfield, Shaanxi, North China, with Emphasis on Anomalies of Gallium and Lithium. Minerals 2019, 9, 402. https://doi.org/10.3390/min9070402
Qin G, Cao D, Wei Y, Wang A, Liu J. Mineralogy and Geochemistry of the No. 5−2 High-Sulfur Coal from the Dongpo Mine, Weibei Coalfield, Shaanxi, North China, with Emphasis on Anomalies of Gallium and Lithium. Minerals. 2019; 9(7):402. https://doi.org/10.3390/min9070402Chicago/Turabian Style
Qin, Guohong, Daiyong Cao, Yingchun Wei, Anmin Wang, and Jincheng Liu. 2019. "Mineralogy and Geochemistry of the No. 5−2 High-Sulfur Coal from the Dongpo Mine, Weibei Coalfield, Shaanxi, North China, with Emphasis on Anomalies of Gallium and Lithium" Minerals 9, no. 7: 402. https://doi.org/10.3390/min9070402