Volume 111, Issue 2

Tonglushan is the largest porphyry-skarn Cu polymetallic deposit in the Edong ore district, Eastern China, with proven metal reserves of 86.3 Mt @ 1.78% Cu, 39.4% Fe, 0.94 g/t Au, and 0.13% Mo. The ore bodies exhibit distinct sub-lateral metal zonation, from proximal intrusion to distal carbonate, i.e., porphyry-type Cu-Mo, to skarn-type Cu-Fe-Au, carbonate replacement-type Cu-Zn-Pb-(Au), and distal vein-type Cu-Pb-Zn mineralization. However, the genetic factors governing both the temporal and spatial metal zoning remain uncertain. Here, we present trace element and S isotope data for sulfide stage (early-stage pyrite and late-stage sphalerite) across various ore types in a well-constrained paragenetic framework. The early stage contains two distinct types of pyrite, including colloform Py0 and euhedral Py1. Py0, observed in both skarn- and carbonate replacement-type ores, is selectively enriched in Cu, Ag, Au, Pb, and Bi relative to later Py1, reflecting a steep temperature gradient process. By contrast, Py1 exhibits increasing concentrations of Co, Ni, As, and Se with increasing distance from the intrusion, but shows depletion in the most distal vein-type ore. Redox exerts the dominant control on this spatial pattern, whereas temperature plays a subordinate role. Values of δ 34 S py generally increase from −0.4‰ within the intrusion to 1.5‰ in skarn-type ore, following a decrease to −0.6‰ in carbonate replacement-type ore and −2.15‰ for distal vein-type ore, which is dominantly controlled by temperature. Late-stage sphalerite (Sp2) exhibits systematic spatial trends, with high Fe, Co, and Mn contents in skarn-type ores evolving to elevated Cd, Hg, and Sn concentrations in carbonate replacement- and distal vein-type ores. These trends are controlled by redox and temperature variations, as recorded by Mn/Sn and Fe/Cd ratios. These findings offer valuable insights into the spatial evolution of ore-forming fluids and metallogenic zonation from proximal porphyry to distal vein-type mineralization, providing a potential guide for future mineral exploration.

Bauxite overlying the paleokarstic surface is rich in rare earth elements (REE), but its occurrence is debatable, which has severely hampered its extraction and utilization. More than 5 billion tons of karstic bauxite were deposited in the North China Craton (NCC) in the Late Carboniferous. This study thoroughly analyzed REE concentrations and occurrences in the major minerals of karstic bauxite in the northern NCC, with the aim of elucidating their migration and enrichment. Bauxite occurs in the Carboniferous Benxi Formation and consists of three layers from bottom to top: Fe-bearing claystone, bauxite, and claystone. The lower Fe-bearing claystone contains primarily illite, kaolinite, diaspore, hematite, goethite, and anatase; the middle bauxite is dominated by diaspore, kaolinite, hematite, goethite, illite, and anatase; and the upper claystone consists primarily of kaolinite, illite, goethite, hematite, diaspore, and anatase. Total REE (ΣREE) increases from top to bottom along the profile, mainly due to the decrease in pH and leaching. REE are released in the acidic conditions of surface weathering, migrate downward, and accumulate in the alkaline environment of the bottom Fe-bearing claystone (up to 961 ppm). In situ elemental analysis revealed anomalously high REE diaspores (up to 11 666 ppm), suggesting that the diaspore is the main host mineral for REE. The similar geochemical behavior of Al, Ti, and REE in single and different diaspores, as well as the significant negative correlation between ΣREE and Al (r = –0.36), coupled with the slightly larger diaspore cells of this study compared to the standard diaspore cells, confirms that numerous larger REE 3+ enter the diaspore lattice and replace Al 3+ during supergene precipitation. The differences in the concentration and occurrence of REE in minerals formed at different stages (e.g., diaspore, anatase, kaolinite, hematite, and goethite) indicate that the occurrence of REE is closely related to the crystal structure and formation process of these minerals. The diaspore and anatase formed during the metallogenic stage are controlled by the rapid nucleation and crystallization process of minerals, resulting in the incorporation of REE into the mineral lattice mainly by isomorphic substitution. Oolitic hematite is also formed by rapid crystallization, but, due to its dense crystal structure and large surface area, REE are adsorbed on the mineral surface primarily via inner-sphere complexation. In minerals formed during the early metallogenic stage (e.g., kaolinite, hematite, and goethite), prolonged weathering processes lead to the predominant incorporation of REE into the mineral lattice via isomorphic substitution. Clarification of the distribution and occurrence of REE in the NCC karstic bauxite provides insight into the enrichment and occurrence of REE in global karstic bauxite deposits.

Nickel-rich sulfide mineralization from the Onça Preta orebody, Jaguar orefield, Carajás, Brazil, comprises pentlandite, violarite/bravoite, pyrite, sphalerite, and minor chalcopyrite. The orebody, placed at the contacts between units of the Neoarchean Serra do Puma layered complex and granite, is interpreted as a hybrid of Fe-Zn-Ni skarn and iron oxide-copper-gold (IOCG) mineralization. We use micro- and nanoanalytical methods to characterize the sulfides and their potential for a platinum-group elements (PGE) signature. Sulfide mineralization occurs as disseminations and along shears in magnetite-rich ores. Microtextures preserved in pentlandite are concordant with cycles of replacement via coupled dissolution-reprecipitation reaction (CDRR). Grain-scale element mapping reveals evidence for remobilization and reprecipitation of metals as inclusions and/or at grain boundaries. Rare accessory nickel-bismuth minerals, including arsenohauchecornite [(Ni 16.5 Fe 1.28 Co 0.39 Zn 0.17 Cu 0.01 ) 18.35 (Bi 2.9 As 0.77 Sb 0.08 Te 0.08 ) 3.8 (S 15.85 Se 0.01 ) 15.9 ], ognitite [(Ni 1.02 Fe 0.01 Pd 0.004 ) 1.04 (Bi 0.58 Pb 0.002 As 0.001 ) 0.585 (Te 1.36 S 0.02 Se 0.001 ) 1.38 ], and parkerite [(Ni 2.85 Fe 0.045 Ag 0.007 Pd 0.001 ) 2.9 (Bi 2.024 Pb 0.005 ) 2.03 (S 2.02 Te 0.038 Se 0.008 ) 2.07 ] are relatively abundant. The extensive solid solution inferred between ognitite (NiBiTe; Ogn) and melonite (NiTe 2 ; Mlt) is confirmed by imaging and modeling using mixed Bi-Te sites for the intermediate member Ogn 60 Mlt 40 found at Onça Preta. The accessory mineral assemblage also includes Bi-tellurides of the tetradymite group and Pb-Bi sulfosalts from the lillianite homologous series. Nanoscale analysis, combined with crystal structure models and simulations of arsenohauchecornite, confirms the I -centered tetragonal superstructure previously described. Aggregates of parkerite found at boundaries between ognitite and pentlandite are associated with abundant defects and/or misorientation fabrics crossing into pentlandite. Such nanostructures delineate fluid pathways, as they host nanoparticles of various tellurides (moncheite, volynskite, hessite, altaite, tellurobismuthite, tsumoite), galena, and native bismuth. Self-patterning during crystal growth is considered based on rhythmic chemical banding (Bi) and antiphase boundaries in ognitite. Formation of nickel-bismuth minerals is attributed to CDRR replacement of pentlandite during percolation of Bi-Te-bearing fluids. Likewise, the formation of bismuth tellurides and sulfosalts resulted during CDRR replacement of pentlandite by pyrite or violarite/bravoite. Identification of moncheite and Pd in ognitite (up to 0.68 wt% Pd), pentlandite, and violarite/bravoite (at ppm levels) at Onça Preta further extends the potential for PGE in hydrothermal Ni sulfide ores in the southern Carajás Domain. This finding has implications for exploration in the region and analogous terranes elsewhere.

We report a unique quartz-petalite intergrowth that occurs in the core zone of a zoned pegmatite sheet from the Yichun Li-Cs-Ta-Nb mine, China. Within this deposit, a rare-metal-mineralized granite underlies and partially intrudes into the earlier, zoned pegmatite, both of which have previously been interpreted to have originated from the same parental magma. The pegmatite core zone mostly comprises an early quartz generation that is fractured and free of mineral inclusions. A less abundant later quartz partially replaced the early quartz and is characterized by abundant globular to vermicular inclusions of petalite (LiAlSi 4 O 10 ) and lesser K-feldspar (orthoclase/sanidine). These two types of sequentially crystallized quartz have δ 18 O values (12.7–13.0‰) similar to magmatic graphic quartz in the pegmatite wall zone. Hydrothermal quartz in later veins has higher δ 18 O (13.7–14.2‰). Titanium-in-quartz thermobarometry indicates similar crystallization temperatures (ca. 500 °C) for both quartz generations. Textural features and mass-balance calculations suggest that most petalite inclusions formed by exsolution from a magmatic quartz-petalite solid solution containing 1729–3135 ppm Li and >6649–12 118 ppm Al, which resembles stuffed quartz observed in experiments involving the crystallization of Li-rich granitic melts. In addition, some petalite inclusions define growth patterns indicating co-precipitation with the host quartz and represent direct crystallization from the last aliquot of pegmatite melt containing ≤5000 ppm Li. Orthoclase/sanidine inclusions formed in early quartz crystals as co-precipitates with quartz (forming micrographic textures), and later via exsolution together with petalite. To attain ∼2200 ppm Li in the core zone, 90% fractionation of an initial granitic melt containing ca. 300 ppm Li is sufficient. Although the rare-metal-mineralized granite at Yichun was suggested to have originated from a melt of similar compositions to the initial granitic melt that formed the pegmatite, the up to 1.2 wt% Li in the mineralized granite cannot be achieved through similar degrees of fractional crystallization. This highlights the indispensable role of metasomatism in the formation of economic-grade, granite-hosted Li mineralization at Yichun and, possibly, in analogous systems.

The formation processes underlying super-large gold deposits present a captivating area of study, with a particularly enigmatic aspect being the mechanisms by which gold forms and accumulates within under-saturated hydrothermal fluids, notably in the presence of thick, high-grade ore bodies. To address this issue, we conducted comprehensive mineral textural, geochemical, isotopic, and machine learning analyses on gold ore-related sulfides from the North Sanshandao gold deposit (∼562 t @ 4.35 g/t), which is the second-largest deposit in the world-class Jiaodong gold province, and is recognized as China’s inaugural super-large offshore gold resource. The deposit is hosted in the Jurassic Linglong granite and has four stages of alteration and mineralization: (I) quartz-pyrite-K-feldspar; (II) quartz-pyrite-chalcopyrite-native Au; (III) quartz-pyrite-galena-sphalerite-native Au; (IV) ore-barren siderite-ankerite-calcite. Four types of auriferous pyrite were distinguished, including (stage II) porous Py2a and its Py2b overgrowth, and (stage III) Py3a and its Py3b overgrowth. The gold in Py2a occurs mainly as nano-particles, whereas Py2b, Py3a, and Py3b contain high contents of lattice gold and coarse-grained native gold, with gold and arsenic contents strongly positively correlated. The characteristics of a porous core and a smooth edge suggest the occurrence of coupled dissolution-reprecipitation (CDR) processes during the early mineralization stage. Low-melting-point chalcophile elements (LMCE, e.g., Bi, Te, Sb) are frequently associated with gold, indicating that the LMCE melt may play a role in the remobilization of gold, in addition to fluids. Combined with the trace element composition, we show that the CDR process led to Au-LMCE liberation and Au remobilization in the ore fluid and LMCE melt. This process may have significantly upgraded the ore, forming large visible gold grains and local high-grade ore zones. Mineral geochemistry of stage I arsenopyrite suggests that the mineralization temperature was 300–400 °C (concentrated from 342–379 °C), log ⁡ f S 2 $\log f_{\mathrm{S}_2}$ was −11.5 to −6.5 (concentrated from −9.5 to −7.5), and log ⁡ f O 2 $\log f_{\mathrm{O}_2}$ was −34.9 to −24.7 (concentrated from −30.2 to −26.7). The sphalerite composition suggests that the temperature dropped by 320 °C during stage III. Both the North Sanshandao gold deposit (δ 34 S V-CDT = 10.75–13.31‰, n = 61) and the Jiaodong gold province have high δ 34 S V-CDT values (δ 34 S V-CDT = 7.1–10.8‰, n = 1646 1st and 3rd quartile), implying that sulfate reduction was a key ore-material source. Our study provides a new model for the widespread development of over 80% of the disseminated-type mineralization in the Jiaodong Peninsula, emphasizing Au remobilization as a key factor in the formation of high-grade, high-tonnage ore zones, and sheds new light on multistage ore-material enrichment in large-scale gold mineralization.

Thermoelastic properties of upper continental crust or terrigenous sediments (similar composition to the upper continental crust) are the basis for identifying them and investigating their fate in the Earth’s mantle. Potassium feldspar, one of the most abundant minerals in the upper continental crust, transforms to liebermannite under high-pressure conditions, which subsequently transitions to its high-pressure phase, K-hollandite II. However, the thermoelastic properties of liebermannite and K-hollandite II remain unknown. This study employs first-principles calculations to determine their equations of state and elastic properties under high temperatures and pressures. Elastic moduli of both liebermannite and K-hollandite II exhibit strong nonlinear pressure and temperature dependencies. Liebermannite experiences shear instability and unusually large V S anisotropy (∼200%) before its transition to K-hollandite II. The shear instability shifts to higher pressures at higher temperatures with a slope of ∼3.8 MPa/K and generates a ∼50% V S jump from liebermannite to K-hollandite II. We find that the upper continental crust exhibits higher velocities than the surrounding mantle to ∼550 km, which explains the high-velocity anomalies observed under continent-continent collision zones. At the top of the lower mantle, the softening of liebermannite’s shear modulus turns the upper continental crust into a low-velocity body with potential seismic anisotropy, and the liebermannite to K-hollandite II phase transition leads to a large velocity contrast, causing the upper continental crust to appear again as high-velocity anomalies. Therefore, the presence of upper continental crust could be a reasonable explanation for strong anisotropy around low-velocity scatterers at the top of the lower mantle. At depths greater than ∼800 km, both oceanic and upper continental crust have higher velocities than the surrounding mantle, but because their velocities are similar, distinguishing them is challenging.

Scandio-fluoro-eckermannite (IMA 2024-002), a new Sc-dominant amphibole-supergroup mineral, has been discovered in the Bayan Obo REE-Nb-Fe polymetallic deposit, China. The new mineral was collected from banded Fe-REE ores that have formed due to the fenitization caused by carbonatite intrusion, in the Main and East open pits at Bayan Obo. Associated minerals include monazite, bastnäsite, magnetite, biotite, fluorite, bazzite, thortveitite, and magnesio-fluoro-arfvedsonite. The new mineral occurs as euhedral to subhedral crystals and aggregates, appearing both as inner zones of a crystallization sequence from scandio-fluoro-eckermannite to magnesio-fluoro-arfvedsonite as well as homogeneous fine-grained particles, reaching up to 350 μm in size and ∼7 wt% in Sc 2 O 3 content. Scandio-fluoro-eckermannite displays a light yellow to light blue color under plane-polarized transmitted light, with perfect cleavage on {110}, non-magnetic, and no fluorescence. The hardness is 5–6 by analogy to eckermannite, and the calculated density is 3.097 g/cm 3 . Electron microprobe analyses determined the main components (average value in wt%): Sc 2 O 3 6.39; SiO 2 54.30; MgO 13.42; Na 2 O 8.38; Al 2 O 3 1.29; MnO 1.47; CaO 1.21; K 2 O 0.47; FeO calc 6.43; Fe 2 O 3calc 3.80; F 3.01; H 2 O + calc 0.67; F≡O –1.27; total 99.74. The composition normalized on the basis of 24 anions (O, OH, F, Cl), with the assumption of (OH, F, Cl) = 2 apfu, corresponds to the empirical formula A (Na 0.52 K 0.09 □ 0.39 ) Σ1.00 B N a 1.81 C a 0.19 Σ 2.00 C M g 2.87 F e 0.77 2 + M n 0.18 3 + S c 0.80 F e 0.41 3 + Σ 5.03 T S i 7.78 A l 0.22 Σ 8.00 O 22 W   F 1.36 ( O H ) 0.64 Σ 2.00 . ${ }^{\mathrm{B}}\left(\mathrm{Na}_{1.81} \mathrm{Ca}_{0.19}\right)_{\Sigma 2.00}{ }^{\mathrm{C}}\left(\mathrm{Mg}_{2.87} \mathrm{Fe}_{0.77}^{2+} \mathrm{Mn}_{0.18}^{3+} \mathrm{Sc}_{0.80} \mathrm{Fe}_{0.41}^{3+}\right)_{\Sigma 5.03}{ }^{\mathrm{T}}\left(\mathrm{Si}_{7.78} \mathrm{Al}_{0.22}\right)_{\Sigma 8.00} \mathrm{O}_{22}{ }^{\mathrm{W}}\left[\mathrm{~F}_{1.36}(\mathrm{OH})_{0.64}\right]_{\Sigma 2.00} .$ It leads to the simplified formula (Na, □ )(Na,Ca) 2 [(Mg,Fe 2+ ) 4 (Sc,Fe 3+ ,Mn 3+ )][(Si,Al) 8 O 22 )](F,OH) 2 and the ideal formula NaNa 2 (Mg 4 Sc)Si 8 O 22 F 2 . The crystal structure was refined in the monoclinic system, space group C 2 /m (#12). Its unit-cell parameters are: a = 9.8212(3) Å, b = 18.0866(5) Å, c = 5.3091(2) Å, β = 103.767(4)°, and Z = 2, with the a:b:c ratio of 0.543:1:0.294. The crystal-structure refinement indicates that Na is the dominant cation at the A ( m ) and M (4) sites, Mgis the dominant cation at the M (1) and M (3) sites, Sc is the dominant trivalent cation at the M (2) site, and F is the dominant cation at the O(3) site. Therefore, this is the Sc-dominant variety of fluoro-eckermannite. This discovery highlights the importance of amphibole in controlling Sc in this type of ore-forming system. Scandio-fluoro-eckermannite might also be used as a potential recorder to investigate the enrichment process of Sc in the Bayan Obo deposit.

The new minerals auropolybasite, ideally Ag 15 AuSb 2 S 11 , and auropearceite, ideally Ag 15 AuAs 2 S 11 , were found at the Šibeničný vrch near Nová Baňa in Slovakia. They were found as anhedral grains up to 0.2 mm in size, usually rimmed by acanthite. Both new minerals are associated with acanthite, argentopolybasite, argentopearceite, pyrargyrite, proustite, stephanite, selenostephanite, rozhdestvenskayaite-(Zn), zvěstovite-(Zn), zvěstovite-(Fe), naumannite, Au-Ag alloys, uytenbogaardtite, iodargyrite, and bromargyrite. They are dark grey to black, opaque, with a black streak and metallic luster. The Mohs hardness is ∼3. They are brittle with no observable cleavage and with a conchoidal fracture. The calculated densities are 6.622 g/cm 3 for auropolybasite and 6.606 g/cm 3 for auropearceite. In reflected light, auropolybasite and auropearceite are gray with a bluish tint, no observable bireflectance, and very weak pleochroism. They show moderate anisotropy in crossed polars with weak greenish and green-blue tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA are (R min /R max , %): 33.5/31.6 (470 nm), 32.9/31.1 (546 nm), 32.1/30.6 (589 nm), and 30.2/29.3 (650 nm) for auropolybasite and 30.6/29.8 (470 nm), 30.3/29.2 (546 nm), 29.7/28.7 (589 nm), and 28.1/27.4 (650 nm) for auropearceite. The empirical formula (based on 29 apfu) for auropolybasite is Ag 15.27 Au 0.84 (Sb 1.51 As 0.46 ) Σ1.97 (S 10.55 Se 0.30 Cl 0.06 ) Σ10.91 and that for auropearceite is Ag 15.84 Au 0.88 (As 1.33 Sb 0.44 ) Σ1.76 (S 10.46 Se 0.05 Te 0.01 ) Σ10.52 . The ideal end-member formulas are Ag 15 AuSb 2 S 11 for auropolybasite and Ag 15 AuAs 2 S 11 for auropearceite. Their symmetry is trigonal, space group P 321. Latice parameters for auropolybasite are: a = 15.1091(5) Å, c = 12.1518(5) Å, V = 2402.42(15) Å 3 , Z = 4, and those for auropearceite are: a = 14.995(3) Å, c = 12.115(2) Å, V = 2359.3(8) Å 3 , Z = 4. Crystal-structure analysis of auropolybasite confirmed that its atomic arrangement is isotypic to that of the other members of the polybasite group and it is isostructural with auropearceite. These minerals formed in the waning stages of epithermal systems, where reduced, S-rich, low-salinity fluids transported Au, Ag, Sb, and As as thiocomplexes. These fluids were unable to transport Cu, Pb, and Zn, thus explaining the chemical composition of these minerals and the entire mineral assemblage.

The phase transition of garnet-spinel is one of the major parameters that affect lithosphere dynamics. The transition processes of garnet to spinel, however, remain unclear. Spinel-pyroxene symplectites around garnet from one olivine-websterite xenolith hosted by the Cenozoic Xilinhot basalts in northeast China were investigated in this study to decipher the transition processes of garnet to spinel and their implications for the Cenozoic episodic lithospheric exhumation of northeast China. Symplectites occur as two concentric coronae surrounding relict garnet or as pseudomorphs of garnet grains. The outer symplectite is made of relatively coarse-grained assemblages of orthopyroxene + spinel + clinopyroxene and exhibits a large range of mineral and bulk chemical compositions. It contains more Mg and less Al relative to relict garnet. The inner symplectite is a radial arrangement of intergrown fine-grained orthopyroxene+spinel ± glass ± clinopyroxene. Mineral and glass compositions show wide variations within each inner symplectite, but the bulk compositions are relatively homogeneous and show close affinities to the garnet relics. These observations demonstrate that the outer symplectite was formed by a slow reaction between garnet and olivine, whereas the inner symplectite is inferred as a rapid isochemical breakdown product of garnet. The Na- and K-poor glass in the inner symplectite could be the in situ melt from the decomposition of garnet, providing the first physical evidence of incongruent melting during the garnet-spinel transition. Combined with geothermobarometer and P - T pseudosection calculations, a slow-to-fast phase transition from garnet to spinel was discerned in the lithospheric mantle beneath northeast China during the Cenozoic, resulting from heating and decompression. These processes could be induced by asthenosphere upwelling and lithospheric extension associated with the Pacific subduction during the Cenozoic. We speculate that outer symplectite formed before the Miocene, whereas the inner symplectite has only begun to form since the Miocene due to enhanced lithospheric extension. Therefore, the two-stage phase transition in the lithospheric mantle could reflect the two-stage lithospheric exhumation of northeast China during the Cenozoic.

For eight decades, the number of interference figures in optical mineralogy has remained constant: three figures for uniaxial minerals and seven for biaxial ones. Each figure has its own specificities and formation conditions. However, the grossular garnets of the Quixeramobim region in Brazil exhibit a possible new type of interference figure: the “mosaic.” This rare characteristic is present in seven samples of grossular garnet from the region. In this paper, we explain how this new interference figure, named “mosaic,” is formed using optical mineralogy principles. When viewed with the quartz-gypsum accessory plate, these samples show an agglutination of colors, with blue, yellow, and purple appearing together in a mosaic configuration. This figure is formed by the different directions of vibration of light as it passes through these anisotropic garnets. Thus, in this configuration, the optic axes and acute and obtuse bisectrix occur in such a way that they are at the same time inclined (//) and perpendicular (⊥) manners. So far, the interference “mosaic” has been observed only in garnets of the grossular group; it occurs only in minerals that require a high degree of optical disorder.

The absorption of light by Fe/Ti and Fe/Fe intervalence charge transfer (IVCT) bands has previously been found in aluminum oxide and Al 2 SiO 5 aluminosilicate minerals to decrease markedly at elevated temperatures. Given the abundance of iron at depth in the Earth, assessing the generality with which and extent to which IVCT mineral phases become more optically transparent at temperatures than they are under ambient conditions has potentially significant implications for the modeling of mantle geophysical processes such as radiative conductivity. A broad experimental survey of the optical absorption spectra at elevated temperatures of various mixed-valence iron minerals has been conducted. The minerals considered here are cordierite, chloritoid, lazulite, dumortierite, jeremejevite, beryl, osumilite, biotite (mica), pargasite (amphibole), and aegirine (pyroxene). All samples transiently lose significant Fe/Fe IVCT feature intensity at elevated temperatures. In beryl, osumilite, biotite, pargasite, and aegirine, spin-allowed Fe 2+ d-d features also decrease in integral intensity at higher temperatures; in all but beryl, the intensity loss is significant. This trend is consistent with d-d band enhancement via Fe 2+ /Fe 3+ exchange coupling, which has not previously been identified in the majority of these minerals. It is contrasted against the behavior of ordinary spin-allowed Fe 2+ d-d bands in non-IVCT minerals forsterite (olivine) and elbaite (tourmaline). The depletion of Fe/Fe IVCT and enhanced Fe 2+ d-d band intensity at elevated temperatures may both be important mechanisms by which iron-bearing mineral phases become more optically transparent under conditions at depth.

The enigma of ammonium mineral speciation in the solar system has no proven solution due to the lack of data on the real minerals serving as space ammonium carriers. We herein report on the discovery of the first ammonium mineral in meteoritic substance and show its relevance to compositional and spectral characteristics ascribed to hypothetical ammonium phases in cometary and asteroidal bodies. Chemically distant from previously inferred volatile organics or ammoniated phyllosilicates, the mineral is an aqueous metal-ammonium sulfate related to the picromerite group—a family of so-called Tutton’s salts. Nickeloan boussingaultite, (NH 4 ) 2 (Mg,Ni)(SO 4 ) 2 ·6H 2 O, was discovered in Orgueil, a primitive carbonaceous chondrite closely related to (162173) Ryugu and (101955) Bennu, the C-type asteroids. The available spectroscopic, chemical, and mineralogical data signify that natural sulfates related to boussingaultite-nickelboussingaultite series perfectly fit into the role of bound ammonia carriers under conditions of cometary nuclei and carbonaceous asteroids. The potential technogenic contamination of astromaterial samples and the difficulties in electron microprobe determination of ammonium are discussed in the context of recently published reports on the discovery of lunar and asteroidal ammonium-containing minerals.