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At least 19 recordsLinked to original sources
USGS2018

Compositional variation in minerals of the chevkinite group

The composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formula A4BC2D2Si4O22, where A = (La,Ce,Ca,Sr,Th), B = Fe2+, C = (Fe2+,Fe3+,Ti,Al,Zr,Nb) and D = Ti. The A site is dominated by a strong negative correlation between (Ca+Sr) and the REE. Chondrite-normalized REE patterns are very variable, e.g. in LREE/HREE and Eu/Eu*. The C site is dominated by Ti, Al and Fe2+, in very variable proportions. Most chevkinites and perrierites are close to stoichiometric, with cation sums between 12.9 and 13.5, compared to the theoretical 13. There is no single, generally applicable charge balancing substitution scheme in the group; however, the general relationship (Ca+Sr)A + TiC + REEA + M3C+2+ defines a linear array with r2 = 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO* Al2O3 and Ce2O3 abundances. Chevkinite forms mainly in chemically evolved parageneses, such as syenites, rhyolites and fenites associated with carbonatite complexes. Perrierite is more commonly recorded from igneous rocks of mafic to intermediate composition. The compositional characteristics and possible structural formulae of other members of the chevkinite group are reviewed briefly.

Mineralogical Magazine↗
USGS2018

Origin of placer laurite from Borneo: Se and As contents, and S isotopic compositions

We examined grains of the platinum-group mineral, laurite (RuS2), from the type locality, Pontyn River, Tanah Laut, Borneo, and from the Tambanio River, southeast Borneo. The grains show a variety of morphologies, including euhedral grains with conchoidal fractures and pits, and spherical grains with no crystal faces, probably because of abrasion. Inclusions are rare, but one grain contains Ca-Al amphilbole inclusions, and another contains an inclusion of chalcopyrite+bornite+pentlandite+heazlewoodite (Ni3S2) that is considered to have formed by a two-stage process of exsolution and crystallization from a once homogeneous Fe-Cu-Ni sulphide melt. All grains examined are solid solutions of Ru and Os with Ir (2.71-11.76 wt.%) and Pd (0.31-0.66 wt%). Their compositions are similar to laurite from ophiolitic rocks. The compositions show broad negative correlations between Os and Ir, between As and Ir, and between As (0.4-0.74 wt.%) and Se (140 to 240 ppm). Laurite with higher Os contains more Se and less Ir and As. The negative correlations between Se and As may be attributed to their occupancy of the S site, but the compositional variations of Os. Ir and As probably reflect the compositional variation of rocks where the crystals grew. Ratios of S/Se in laurite show a narrow spread from 1380 to 2300, which are similar to ratios for sulphides from the refractory sub-are mantle. Sulphur isotopic compositions of laurite are independent of chemical compositions and morphologies and are similar to the chondritic value of 0???. The data suggest that S in laurite has not undergone redox changes and originated from the refractory mantle. The data support the formation of laurite in the residual mantle or in a magnia generated from such a refractory mantle, followed by erosion after the obduction of the host ultramafic rocks. ?? 2004 The Mineralogical Society.

Mineralogical Magazine↗
USGS2018

A reexamination of the turquoise group: The mineral aheylite, planerite (redefined), turquoise and coeruleolactite

The turquoise group has the general formula: A (sub 0-1) B 6 (PO 4 ) (sub 4-x) (PO 3 OH) x (OH) 8 .4H 2 O, where x = 0-2, and consists of six members: planerite, turquoise, faustite, aheylite, chalcosiderite and an unnamed Fe (super 2+) -Fe (super 3+) analogue. The existence of "coeruleolactite" is doubtful. Planerite is revalidated as a species and is characterized by a dominant A-site vacancy. Aheylite is established as a new member of the group, and is characterized by having Fe (super 2+) dominant in the A-site. Chemical analyses of 15 pure samples of microcrystalline planerite, turquoise, and aheylite show that a maximum of two of the (PO 4 ) groups are protonated (PO 3 OH) in planerite. Complete solid solution exists between planerite and turquoise. Other members of the group show variable A-site vacancy as well. Most samples of "turquoise" are cation-deficient or are planerite. Direct determination of water indicates that there are 4 molecules of water. Planerite, ideally []Al 6 (PO 4 ) 2 (PO 3 OH) 2 (OH) 8 .4H 2 O, is white, pale blue or pale green, and occurs as mamillary, botryoidal crusts as much as several mm thick; may also be massive; microcrystalline, crystals typically 2-4 micrometres, luster chalky to earthy, H. 5, somewhat brittle, no cleavage observed, splintery fracture, D m 2.68(2), D c 2.71, not magnetic, not fluorescent, mean RI about 1.60. a 7.505(2), b 9.723(3), c 7.814(2) Aa, alpha 111.43 degrees , beta 115.56 degrees , Gamma 68.69 degrees , V 464.2(1) Aa 3 , Z = 1. Aheylite, ideally Fe (super 2+) Al 6 (PO 4 ) 4 (OH) 8 .4H 2 O, is pale blue or green, and occurs as isolated and aggregate clumps of hemispherical or spherical, radiating to interlocked masses of crystals that average 3 micrometres in maximum dimension; porcelaneous-subvitreous luster, moderate to brittle tenacity, no cleavage observed, hackly to splintery fracture, not magnetic, not fluorescent, biax. (+), mean RI is about 1.63, D m 2.84(2), D c 2.90. a 7.400(1), b 9.896(1), c 7.627(1) Aa, alpha 110.87 degrees , beta 115.00 degrees , gamma 69.96 degrees , V 460.62(9) Aa 3 , Z = 1.

Mineralogical Magazine↗
USGS2018

Recommended nomenclature for zeolite minerals: Report of the subcommittee on zeolites of the International Mineralogical Association, Commission of New Minerals and Mineral Names

This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission of New Minerals and Mineral Names. In a working definition of a zeolite mineral used for review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundance in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome, pollucite and wairakite in the phillipsite and analcime series. Differences in space-group symmetry and in order-disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si:Al ratio except for heulandite (Si:Al < 4.0) and clinoptilolite ( Si : Al ⩾ 4.0 ). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term 'ideal formula' should be avoided in referring to a simplified or averaged formula of a zeolite. Newly recognized species in compositional series are as follows: brewsterite-Sr.-Ba: chabazite-Ca.-Na.-K; clinoptilolite-K, -Na, -Ca: dachiardite-Ca, -Na; erionite-K, -Ca: faujasite-Na, -Ca, -Na: paulingite-K. -Ca; phillipsite-Na, -Ca, -Ka; stilbite-Ca, -Na. Key references, type locality, origin of name, chemical data. IZA structure-type symbols, space-group symmetry; unit-cell dimensions, and comments on structure are listed for 13 compositional series, 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Mineralogical Magazine↗
USGS2022

Evaluation for internal consistency in the thermodynamic network involving fluorite, cryolite and villiaumite solubilities and aqueous species at 25°C and 1 bar

Thermodynamic data are constrained by the interrelated thermodynamic equations in addition to the observational measurements and their uncertainties. The consequence is a network of thermodynamic properties that can be evaluated for their internal consistency. In this study, three fluoride minerals that can cause high fluoride concentrations in groundwaters are evaluated for their solubilities and their internal thermodynamic consistency with calorimetric, isopiestic and electrochemical measurements: fluorite, CaF 2 , cryolite, Na 3 AlF 6 , and villiaumite, NaF. This evaluation involves the three solids and 13 aqueous species, the free ions of Ca 2+ , Na + , Al 3+ and F – , and the hydroxido and fluorido complexes of Al 3+ , and the CaF + ion pair. For the fluorite–cryolite–villiaumite–aqueous species network, the number of components is minimal, and the solubility studies are mostly of high quality. Re-evaluations of original data using PHREEQC helps to broaden the quantitative evaluation of thermodynamic properties and to resolve apparent discrepancies. A check on this thermodynamic network shows that through a careful appraisal of the literature, a highly consistent set of values can be derived. The resultant infinite-dilution solubility-product constants at 25°C and 1 bar are: for fluorite solubility, log K sp = –10.57 ± 0.08; for cryolite solubility, log K sp = –33.9 ± 0.2; and for villiaumite solubility, log K sp = –0.4981 ± 0.003.

Mineralogical Magazine↗
USGS2018

Dwornikite, (Ni,Fe)SO 4 .H 2 O, a member of the kieserite group from Minasragra, Peru

A new nickel sulphate monohydrate is described from V-sulphide ore from Minasragra; it occurs associated with patronite (VS 2 ) with various sulphates, sulphur and bitumen. Dwornikite forms fine-grained white aggregates mixed with other oxidation products. Indexed X-ray powder data are tabulated; strongest lines 3.342(100) , 4.732(70), 3.024(70), 4.754(50), 3.293(35), 2.491(35) A; a 6.839, b 7.582, c 7.474 A, beta 117.85o; space group C2/c. XRF gave NiO 39.0, FeO 9.3, SO 3 42.4, = 90.7; D calc. 3.34 g/cm 3 ; mean refr. ind. approx 1.63. Unit-cell data for the synthetic end-member compounds NiSO 4 .H 2 O and FeSO 4 .H 2 O and new X-ray powder data for retgersite (NiSO 4 .6H 2 O) are given. The name is for E. J. Dwornik, mineralogist with the U.S. Geological Survey.-R.A.H.

Mineralogical Magazine↗
USGS2018

Zinc- and Y-group-bearing senaite from St Peters Dome, and new data on senaite from Dattas, Minas Gerais, Brazil

A Zn- and Y-group-bearing senaite, formulated as AM 21 O 38 (Pb 0.63 Na 0.46 Ba 0.11 ) Σ1.20 (Ti 14.64 Zn 1.74 Y‒group REE 0.70 Mn 0.38 Nb 0.20 Sn 0.03 Zr 0.03 Th 0.02 ) Σ20.9 O 38 , a member of the crichtonite group, is a newly characterized phase associated with murataite at the St Peters Dome area, Colorado. The Zn- and Y-group-bearing senaite is uranium-free and nonmetamict, but otherwise is comparable to known senaites and davidites in X-ray diffraction pattern, symmetry, and structure. The REE distribution shows a strong dominance of Y and the Y-group REE which are present in M (1). Megascopically, the mineral is black, submetallic, and opaque; in polished section it appears to be white and moderately bright compared with murataite. Cleavage is absent but twinning on {52 0} is present. Senaite and hitherto unreported zinc-bearing senaite from Dattas, Diamantina, Minas Gerais, Brazil, have the following respective structural formulas as determined by electron microprobe: (Pb 0.76 Sr 0.20 La 0.08 Ce 0.07 Ba 0.04 ) Σ1.15 (Ti 12.89 Mn 0.88 Y 0.59 Zn 0.12 Mg 0.07 Nb 0.03 Cr 0.02 Th 0.02 ) Σ20.61 O 38 and (Pb 0.89 Ba 0.15 Ce 0.02 Sr 0.02 ) Σ1.08 (Ti 14.57 Zn 1.76 Na 0.63 Mn 0.50 Y 0.28 Nb 0.14 Mg 0.04 Cr 0.02 ) Σ20.83 O 38

Dattas, Minas Gerais↗
USGS2018

Uranoan thorite in lithophysal rhyolite - Topaz Mountain, Utah, U.S.A.

Uranoan thorite crystals have been found occurring as a sparse constituent in lithophysae in 6.1 to 6.8 Ma alkali rhyolite flows at Thomas Mountain, Utah, USA. The crystals are associated with sandidine, quartz, topaz, hematite, magnetite, and calcite; they are leek to dark grass green, transparent, well-formed, euhedral prisms, showing development of forms {100}, {101}, and {111}. The mineral is both optically isotropic, with a refractive index of 1.86(1), and X-ray amorphous, indicating its metamict state. Electron microprobe analysis yields: SiO 2 17.3, ThO 2 56.8, UO 2 25.4, total 99.5%, and a structural formula of (Th 0.72 U 0.31 ) Σ1.03 Si 0.97 O 4 . This appears to be the first reported occurrence of thorite in volcanic rocks of rhyolitic composition, and may account for part or all of the Th reported in alkali rhyolites and so-called ‘topaz-rhyolites’ of the western USA.

Utah↗
USGS2018

The 1875 eruption of Askja volcano, Iceland: Combined fractional crystallization and selective contamination in the generation of rhyolitic magma

Major and trace element and Sr, Nd and O isotopic data are presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja. Trace element modelling and Sr and Nd data largely confirm previous major element calculations that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb (and Th and U?), depletion in Cs, and low values of δ 18 O/ 16 O, in the rhyolites are not explained by this mechanism alone. The silicic magmas were selectively contaminated by diffusion from partially molten granitic wall rocks, now found as xenoliths in the eruptive products, the process being particularly marked by lower δ 18 O and Cs/Rb ratios in the rhyolites than in the associated basalts. This is the first record of a combined fractional crystallization-selective contamination process in an Icelandic silicic complex.

Askja volcano↗
USGS2018

Delindeite titanosilicates and lourenswalsite, two new from the Magnet Cove region, Arkansas

Delindeite and lourenswalsite are two new barium titanosilicate minerals found as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite is found as aggregates of flake-like crystallites in compact spherules, light pinkish grey in colour, with a resinous, pearly lustre. The flakes are biaxial positive with average n ∼ 1.813; the measured density is 3.3 g/cm 3 . Electron diffraction revealed a monoclinic unit cell in space group C 2 l m or subgroup, with a = 21.617(13), b = 6.816(5), c = 5.383(3) Å, β = 94.03(5)° (refined from X-ray powder data). The strongest X-ray lines are ( hkl , d obs , I rel ): (200, 10.80, 100); (311, 3.54, 24); (6̄01, 3.083, 28); (601, 2.888, 31); (2̄21, 2.806, 20); (910, 2.262,18). The crystals are submicroscopically twinned on (100) and also produce additional continuous diffraction streaks parallel to a *, which double the b and c axes. The formula derived from electron and ion probe analyses (H 2 O by difference), as constrained by density and molar volume data, is approximately (Na,K) 2.7 (Ba,Ca) 4 (Ti,Fe,Al) 6 Si 8 O 26 (OH) 14 , with Na > K, Ba ≫ Ca, Ti ≫ Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in rosettes, silver grey to light brownish grey in colour. The crystals are biaxial negative with very low 2 V angle. Indices of refraction are n α = 1.815, n β ≈ n γ = 1.840; the measured density is 3.17 g/cm = 1.840; the measured density is 3.17 g/cm 3 . X-ray and electron diffraction show a sharp pseudohexagonal lattice with a = 5.244 Å, but extremely diffuse diffraction streaks normal to the hk 0 plane. In these streaks a period of 20.5 Å can be discerned. A hexagonal unit cell with a = 5.244(2) Å, c = 20.49(3) Å can be refined from the powder diffraction data but does not account for some lines, probably because of extreme layer disorder as shown by precession single-crystal patterns. The strong X-ray powder lines are (002, 10.22, 20); (-, 3.93, 20); (111, 2.608, 100); (300, 1.5145, 80); (220, 1.3111, 25). The formula given by microprobe analyses, constrained by density and molar volume data, is approximately (K,Ba) 2 (Ti,Mg,Ca,Fe) 4 (Si,Al,Fe) 6 O 14 (OH) 12 with K > Ba, Ti ≫ (Mg,Ca,Fe), Si > Al > Fe; Z = 1. These minerals are formed under oxidizing weathering conditions, and iron is assumed to be in the Fe 3+ state.

Arkansas↗
USGS2018

Diagenetic palaeotemperatures from aqueous fluid inclusions: re-equilibration of inclusions in carbonate cements by burial heating

Diagenetic palaeotemperatures determined from aqueous fluid inclusions can be affected by re-equilibration during burial heating. Calculations based on the observed behaviour of inclusions in fluorite under external confining pressure allows prediction of the temperatures and depths of burial necessary to initiate re-equilibration of aqueous inclusions in the common size range 40 to 4 µm. Heating of 20° to 60°C over the initial trapping temperature may cause errors of 10° to 20°C in the homogenization temperature. This suggests re-equilibration may cause aqueous inclusions in carbonates to yield a poor record of their low-temperature history, but a useful record of the maximum temperature experienced by the host rock. Previous work suggests inclusions containing petroleum fluids will be less susceptible to re-equilibration.

Mineralogical Magazine↗
USGS2018

Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera

Mesothermal, gold-bearing quartz veins are widespread within allochthonous terranes of Alaska that are composed dominantly of greenschist-facies metasedimentary rocks. The most productive lode deposits are concentrated in south-central and southeastern Alaska; small and generally nonproductive gold-bearing veins occur upstream from major placer deposits in interior and northern Alaska. Oreforming fluids in all areas are consistent with derivation from metamorphic devolatilisation reactions, and a close temporal relationship exists between high-T tectonic deformation, igneous activity, and gold mineralization. Ore fluids were of consistently low salinity, CO 2 -rich, and had δ 18 O values of 7‰- 12‰ and δD values between −15‰ and −35‰. Upper-crustal temperatures within the metamorphosed terranes reached at least 450-500°C before onset of significant gold-forming hydrothermal activity. Within interior and northern Alaska, latest Paleozoic through Early Cretaceous contractional deformation was characterised by obduction of oceanic crust, low-T/high-P metamorphism, and a lack of gold vein formation. Mid-Cretaceous veining occurred some 50-100 m.y. later, during a subsequent high- T metamorphic/magmatic event, possibly related to extension and uplift. In southern Alaska, gold deposits formed during latter stages of Tertiary, subduction-related, collisional orogenesis and were often temporally coeval with calc-alkaline magmatism.

Alaska↗
USGS2018

Sodium metasomatism along the Melones Fault Zone, Sierra Nevada Foothills, California, USA

Albitite, locally aegirine- and riebeckite-bearing, formed as a result of sodium metasomatism of felsic dykes and argillites along the Melones Fault Zone near Jamestown, California. Pyrite, magnetite, hematite and titanite are common in small amounts in altered dykes. The dykes were originally plagioclase-hornblende porphyritic, and had major and trace element abundances typical of calc-alkaline rocks, whereas they now have Na 2 O contents as high as 11.40%. Associated fracture-filling veins are dominated by albite, but locally include aegirine, analcime, paragonite, calcite and sodic scapolite. Quartz is present in most albitic rocks, but is absent in riebeckite- and aegirine-bearing samples. Albitization predated CO 2 metasomatism and formation of sericite-pyrite assemblages that are typical of gold deposits of the Mother Lode Belt. Alkaline fluids responsible for Na-metasomatism had elevated Na + /K + and Na + /H + relatively high f O2 , and low a H 4 SiO 4 . The presence of titanite indicates fluid. The presence of titanite indicates fluid X C O 2 was low, in contrast to fluids that formed later carbonate-bearing assemblages. Sodic scapolite suggests that, at least locally, the fluids attained very high salinities. Mass balance calculations indicate that alteration involved addition of large amounts of sodium, and the removal of SiO 2 and K 2 O. Textural preservation, combined with volume factors calculated from specific gravity and whole rock analytical data, indicate that Na-metasomatism was essentially isovolumetric. Sodium-rich zones along the Melones Fault Zone are closely associated with fault-bounded bodies of ultramafic rock, typically altered to talc-carbonate or quartz-magnesite-Cr muscovite assemblages. Carbonatization and talc-forming reactions in the ultramafic rocks may lead to SiO 2 -undersaturated fluids. Expansion of the muscovite stability field in terms of Na + /K + -Na + /H + , as a result of incorporation of Cr (up to 7.7% Cr 2 O 3 ) in muscovite, would result in H + - and K + -depletion as the fluid interacts with ultramafic rocks. This could lead to fluids with elevated Na + /K + and high pH, as documented in this occurrence.

California↗
USGS2018

Meurigite, a new fibrous iron phosphate resembling kidwellite

Meurigite is a new hydrated potassium iron phosphate related to kidwellite and with structural similarities to other late-stage fibrous ferric phosphate species. It has been found at four localities so far — the Santa Rita mine, New Mexico, U.S.A.; the Hagendorf-Sud pegmatite in Bavaria, Germany; granite pegmatite veins at Wycheproof, Victoria, Australia; and at the Gold Quarry Mine, Nevada, U.S.A. The Santa Rita mine is the designated type locality. Meurigite occurs as tabular, elongated crystals forming spherical and hemispherical clusters and drusy coatings. The colour ranges from creamy white to pale yellow and yellowish brown. At the type locality, the hemispheres may reach 2 mm across, but the maximum diameter reached in the other occurrences is usually less than 0.5 mm. A wide variety of secondary phosphate minerals accompanies meurigite at each locality, with dufrenite, cyrilovite, beraunite, rockbridgeite and leucophosphite amongst the most common. Vanadates and uranates occur with meurigite at the Gold Quarry mine. Electron microprobe analysis and separate determination of H 2 O and CO 2 on meurigite from the type locality gave a composition for which several empirical formulae could be calculated. The preferred formula, obtained on the basis of 35 oxygen atoms, is which simplies Qualitative analyses only were obtained for meurigite from the other localities, due to the softness and openness of the aggregates. Because of the fibrous nature of meurigite, it was not possible to determine the crystal structure, hence the exact stoichiometry remains uncertain. The lustre of meurigite varies from vitreous to waxy for the Santa Rita mine mineral, to silky for the more open sprays and internal surfaces elsewhere. The streak is very pale yellow to cream and the estimated Mohs hardness is about 3. Cleavage is perfect on {001} and fragments from the type material have a mean specific gravity of 2.96. The strongest lines in the X-ray powder pattern for the type material are ( d obs , I obs , hkl ) 3.216(100)404; 4.84(90)111; 3.116(80)205; 4.32(70)112; 9.41(60)201; 3.470(60)800. The X-ray data were indexed on the basis of a monoclinic unit cell determined from electron diffraction patterns. The cell parameters, refined by least squares methods, are a = 29.52(4), b = 5.249(6), c = 18.26(1) Å, β = 109.27(7)°, V = 2672(3) Å 3 , and Z = 4. The calculated density is 2.89 gcm −3 . The space group is either C 2, Cm or C 2 / m . X-ray powder data for meurigite are closely similar to those for kidwellite and phosphofibrite, but meurigite appears to be characterised by a strong 14 Å reflection. The relationship between these three minerals remains uncertain in the absence of structural data. On the available evidence, meurigite and kidwellite are not the respective K and Na-endmembers of a solid solution series. The meurigite cell parameters suggest it belongs to a structural family of fibrous ferric phosphates, such as rockbridgeite, dufrenite and beraunite, which have a discrete 5 Å fibre axis. Meurigite occurs in widely varying environments, its formation probably favoured by late-stage solutions rich in K rather than Na.

Mineralogical Magazine↗
USGS2018

Alkali-deficient tourmaline from the Sullivan Pb-Zn-Ag deposit, British Columbia

Alkali-deficient tourmalines are found in albitized rocks from the hanging-wall of the Sullivan Pb-Zn-Ag deposit (British Columbia, Canada). They approximate the Mg-equivalent of foitite with an idealized formula □(Mg 2 Al)Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 4 . Major chemical substitutions in the tourmalines are the alkali-defect type [Na* (x) + Mg* (Y) = □ (x) + Al (Y) ] and the uvite type [Na* (x) + Al (Y) = Ca (x) + Mg* (Y) ], where Na* = Na + K, Mg* = Mg + Fe + Mn. The occurrence of these alkali-deficient tourmalines reflects a unique geochemical environment that is either alkali-depleted overall or one in which the alkalis preferentially partitioned into coexisting minerals (e.g. albite). Some of the alkali-deficient tourmalines have unusually high Mn contents (up to 1.5 wt.% MnO) compared to other Sullivan tourmalines. Manganese has a strong preference for incorporation into coexisting garnet and carbonate at Sullivan, thus many tourmalines in Mn-rich rocks are poor in Mn (<0.2 wt.% MnO). It appears that the dominant controls over the occurrence of Mn-rich tourmalines at Sullivan are the local availability of Mn and the lack of other coexisting minerals that may preferentially incorporate Mn into their structures.

British Columbia↗
USGS2018

Nomenclature of the micas

End-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials, and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

Mineralogical Magazine↗