23-3-247 Alteration of Micas

Journal of Geosciences, 2016

The Cr-Ni-rich micas, Ni-Co sulphide phases and associated minerals occur in a small body of listvenite, an extensively altered serpentinite, in Lower Palaeozoic paragneisses near Muránska Zdychava village in Slovenské Rudohorie Mts. (Veporic Superunit, central Slovakia). The main rock-forming minerals of the listvenite are magnesite, dolomite and a serpentine-group mineral, less frequently calcite, quartz and talc. Accessory minerals of the listvenite include Cr-Ni-rich micas, chromite, and Ni-Co-Fe-(Cu-Pb) sulphide minerals (pyrite, pyrrhotite, pentlandite, millerite, polydymite, violarite, siegenite, gersdorffite, cobaltite, chalcopyrite and galena). The micas from the Muránska Zdychava listvenite (Cr-Ni-rich illite to muscovite and Ni-dominant trioctahedral mica) contain the highest Ni concentrations ever reported in the mica-group minerals (up to 22.8 wt. % NiO or 1.46 apfu Ni). The Cr concentrations are also relatively high (up to 11.0 wt. % Cr 2 O 3 or 0.64 apfu). Compositional variations in both Cr-Ni-rich mica minerals are characterized by the negative Cr vs. Ni correlation that indicates a dominant role of

Atlantic Geology, 1988

White mica (WM) in peraluminous granitoid rocks of the South Mountain Batholith (SMB) and East Kemptville leucogranite (EKL) of Nova Scotia have been examined to see if parameters can be used to distinguish between primary and secondary grains. Texturally very little of the WM in the SMB can unequivocally be classified as primary, whereas most of the WM in the EKL is consistent with such an origin. Although discriminant diagrams which utilize major element chemistry do not provide unambiguous divisions between primary and secondary WM, there appear to be some chemical trends which indicate that the bulk composition of the host rock is an important control. This is best exemplified by volatile (i.e., F) and trace element contents, including the rare earth elements. For example, F, Li, Rb and Cs are systematically higher in WM from relatively more evolved units of the SMB. The importance of bulk rock composition is also indicated by the enrichment of lithophile elements in WM from the EKL, itself enriched in these same elements. The octahedral impurities in WM of the SMB are accommodated via biotitic and phengitic substitutions, whereas WM from the EKL is dominantly phengitic. Comparison to experimentally determined stability fields for muscovite indicate that WM from the SMB re-equilibrated to 500-600°C in the more primitive units and 400-550°C in the more-evolved units and greisens. Recent experimental data also suggest crystallization of the WM may have occurred at pressures of ca. 2 kb in melts with 2-3 wt.% f^O. Batholite de South Mountain (BSM) et du leucogranite d'East Kemptville (LEK) en Nouvelle-Ecosse afin de determiner si certains paramdtres peuvent servir A distinguer les grains primaires des grains secondaires. A l'dgard de la texture, une trds faible proportion des MB dans le BSM peuvent, sans 1"ombre d'un doute, etre classes comme primaires alors que la plupart des MB dans le LEK sont compatibles avec une telle origine. Bien que les diagrammes discriminant par chimie des elements majeurs ne procurent aucune limite precise entre les MB primaires et secondaires, on semble y d i s c e m e r des tendances chimiques indiquant une influence prApondArante de la composition totale de 1'encaissant. Les contenus en elements volatiles (i.e., F) et en traces, y comprises les terres rares, en sont la meilleure illustration. Par exemple, la teneur en F, Li, Rb et Cs est systdmatiquement plus dlevde dans les MB provenant des unites plus dvoludes, par comparaison, du BSM. L'importance de la composition totale de la roche est aussi attestde par 1 1enrichissement en elements lithophiles des MB du LEK (lui-mdme enrichi en ces elements), L 'accommodation des impuritds octaddriques dans les MB du BSM se fait par le biais de substitutions biotitiques et phengitiques alors que les MB du LEK sont surtout phengitiques. Une comparaison des MB du BSM aux domaines de stajjilitds de la muscovite determines expdrimentalement rAvAle que leur rd-dquilibrage s'est effectud entre 500 et 600°C dans les unites les plus primitives et entre 400 et 550°C dans les unites plus dvoludes et les greisens. A la lumidre de donndes expdrimentales rdcentes, la cristallisation des MB aurait eu lieu A des pressions d'environ 2 kb dans des bains ayant de 2 A 3% en poids d 'H"0. [Traduit par le journal]

American Mineralogist, 1995

Micas with variable Ba content in the interlayer sites occur in silicate-rich bands within the high-grade, metamorphic banded iron formation enclosing massive sulfide bodies in the Broken Hill deposit, Namaqualand Metamorphic Complex, South Africa. In those layers that contain neither potassium feldspar nor muscovite, a green, barian (up to 17 wt°/o BaO), trioctahedral mica occurs that is similar to kinoshitalite. In contrast to the originally described kinoshitalite, however, this mica is Fe-rich (average XFe = 0.65), suggesting the existence of an Fe analogue of kinoshitalite not described to date. Ion chromatographic analysis indicates that about 8°1o of the total Fe in this mica is present as Fe3+, and a typical formula is (Ba1.2K0.4)(Fel1Mno.2Mg2.oAlo.2Fe6.t Tio.2)(AI3.oSi5.o)02o(OHl.SF2.2). The unit-cell parameters were determined as follows: a = 5.383(2), b = 9.328(8), c = 10.055(8) A, {j = 100.44(5)0, with V = 496.5 A3, and Z = 2. The space group is C21m. The Ba-rich mica formed at the peak of metamorphism (T = 670 :t 20°C, P = 4.5 :t 1.0 kbar) at a pH below the muscovite + potassium feldspar buffer, 102buffered by quartz + fayalite + magnetite and fs2 between 10-5 and 10-7. .

Geochimica Et Cosmochimica Acta, 1980

Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with ng [ = Mg/(Mg + Fe)] 0.45-0.65, TiO, 3-6 wt%, AlaOs 1416wt%, no Fe3' required in tetrahedral sites, low NiO (-0.02 wt%), and relatively high na [Na20/(Na2G + KaO)] 0.02-0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO, (0.7-4.0 wt%) and AlaOs (6.8-14.2 wt%), have substantial Fe3+ in tetrahedrat sites, and have relatively low aa. Roth types may have rims with compositions indicative of mica-'tontine' mixtures resulting from reaction with a highly aqueous fluid. The ~~o~aphi~lly-det~mined 'serpentine' is chemically of two types: Fe-rich sernentine and Fe-rich talc. Associated ohases in the aroundmass vary from one kimberhie to another: c&&e, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, miller&e?, heazlewoodite?, quartz.

Clay Minerals, 2018

The Canadian Mineralogist, 2014

Mount Mica is a poorly zoned, sodic LCT-type pegmatite consisting dominantly of quartz, albite, and muscovite in the outer portion of the pegmatite. Micas are the principal K minerals throughout the pegmatite. Potassium feldspar and rare-element minerals such as lepidolite, elbaite, Cs-rich beryl, fluorapatite, and pollucite are restricted to the core zone of the pegmatite. The dearth of rare element (Li, F, Cs, and Rb) substitution in minerals of the outer portion of the pegmatite and the relatively low K/ Rb ratio of K-feldspar in the core zone indicate that the overall degree of fractionation of the pegmatite is moderate. Aluminummicas belong to the muscovite-polylithionite chemical trend, with a compositional gap between muscovite and trilithionite. Lithium is incorporated into the micas mainly via the Li 3 Al -1¨-2 substitution mechanism. Lithium-Al micas in the wall zone are chemically homogeneous, but abruptly evolve into higher Cs + Rb bearing Li-rich muscovite and lepidolite in the core zone. An abrupt change in grain size, texture, and chemical composition is evident in the micas from the core zone. Pods of fine-to coarsegrained lepidolite occur sporadically throughout the core zone. Muscovite crystals located adjacent to lepidolite pods or adjacent to or within miarolitic cavities may be overgrown by a coarse-grained mosaic of lepidolite crystals. Some of the overgrown muscovite crystals display interlayering on a microscopic μm scale. The interlayering suggests late-stage rapid crystallization along a diffusion-controlled crystal-liquid interface. The abrupt increase of the Cs, Rb, and F in K-feldspar, muscovite, and lepidolite and the occurrence of such highly evolved species as F-rich lepidolite, pollucite pods, elbaite tourmaline, Cs-rich beryl, and spodumene in miarolitic cavities in the core zone suggest that incompatible elements were retained in the residual fluid until their concentration was high enough to initiate crystallization of incompatible-rich mineral phases. The relatively low abundance of incompatible elements in the hanging-wall zone (HWZ) and footwall zone (FWZ) suggest that the fractionation process was very efficient in sweeping the incompatibles into the core zone of the pegmatite, producing proportionally small volumes inside the pegmatite with very high enrichment in incompatible elements.

Detailed petrographic studies and microchemical analyses of titanomagnetite from igneous and metamorphic rocks and ore deposits form the basis of this investigation. Its aim is to compare the data obtained and their interpretations with the experimentally deduced subsolidus oxidation-exsolution model of Buddington and Lindsley (1964). The results are also considered relevant for the interpretation of compositional variations in black sands which are recovered for titanium production. The arrangement of the samples investigated is in accordance with textural stages C1 to C5 caused by subsolidus exsolution with increasing degrees of oxidation (Haggerty, 1991). Stage 1 is represented by two types of optically homogeneous TiO 2-rich magnetite: a. An isotropic type considered to represent solid solutions of magnetite and ulvite containing between 5.2 to 27.5 wt% TiO 2 corresponding to about 14.7 to 77.7 mol% Fe 2 TiO 4 in solid solution with magnetite. The general formula of this type is Fe 2‡ 1‡x Fe 3‡ 2À2x Ti x O 4 (x ˆ 0.0±1.0). b. The second type which has not been reported so far is anisotropic and shows complex internal twinning resembling inversion textures. It is thus attributed to inversion of a high-temperature ilmenite modi®cation (with statistical distribution of the cations) which forms solid solutions with magnetite. TiO 2 varies between 9.3 and 24.5 wt% corresponding to about 17.2 to 43.6 mol% ilmenite in solid solution with magnetite. This type is interpreted as a cation-de®cient spinel with the general formula Fe 2‡ 12=12‡1=4x Fe 3‡ 24=12À3=2x & 0‡1=4x Ti x O 4 (x ˆ 0.0±16=12). Isotropic and anisotropic homogeneous magnetites occur in volcanic rocks only; the homogeneity of the solid solutions was explained by fast cooling which prevented the development of exsolution textures. Stages 2 and 3 are represented by magnetite with or without ulvite. The magnetite host contains ilmenite lamellae forming trellis and sandwich textures. In contrast to the requirement of the oxidation-exsolution model, the ilmenite lamellae are concentrated exclusively in the cores of the host crystals. The reverse host-guest relationship may also occur. Stages 4 and 5 are identical with thermally generated martite (ˆ martite due to heating). The textures are characterized by very broad lamellae of ferrian ilmenite or titanohematite dominantly concentrated along the margins of the host crystals. Thermally generated martite is restricted to subsolidus-oxidation reactions. The ilmenite lamellae of trellis and sandwich textures contain low Fe 2 O 3-concentrations (average 4.8 mol%; to a maximum of 8.3), whereas the Fe 2 O 3-content of thermally generated martite is between 32 to 71 mol%. With respect to the Fe 2 O 3-concentrations in the ilmenite lamellae, no transition between the two types was observed. The results of this paper show that the widely accepted oxy-exsolution model of Buddington and Lindsley (1964) which is based on experimental results can ± with the exception of thermally generated martite ± not explain the tremendous variety of magnetite±ilmenite±ulvite relationships in natural rocks and ore deposits.

The sediment-hosted stratiform Cu-Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu-Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with finegrained minerals, whereas the second is a multistage synorogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu-Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e.g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu-Co stage display δ 34 S values (−10.3‰ to +3.1‰ Vienna Canyon Diablo Troilite (V-CDT)), which partly overlap with the δ 34 S signature of framboidal pyrites (−28.7‰ to 4.2‰ V-CDT) and have Δ 34 S SO4-Sulfides in the range of 14.4‰ to 27.8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu-Co stage display δ 34 S signatures that are either similar (−13.1‰ to +5.2‰ V-CDT) to the δ 34 S values of the sulfides of the first Cu-Co stage or comparable (+18.6‰ to +21.0‰ V-CDT) to the δ 34 S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (−9.9‰ to −1.4‰ Vienna Pee Dee Belemnite (V-PDB)) and oxygen (−14.3‰ to −7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu-Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (−8.6‰ to +0.3‰ V-PDB) and oxygen (−24.0‰ to −10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu-Co stage are mostly similar to the δ 13 C (−7.1‰ to +1.3‰ V-PDB) and δ 18 O (−14.5‰ to −7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu-Co stage. This Editorial handling: H. Frimmel Electronic supplementary material The online version of this article (

American Mineralogist, 2007

This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al 3+ , Ti 4+ , and, only in a few samples, to Fe 3+ . The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [ VI Ti 4+VI (Mg,Fe) 2+ -1 (OH) --2 O 2 2-] and Ti-vacancy [ VI Ti 4+VI ■ ■ VI (Mg,Fe) 2+ -2 ] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and signiÞ cantly affects octahedral and interlayer topology as well. Micas with the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fe tot ) diagram, possibly as a consequence of a variation in intensive parameters (T, P, f H 2 , f O 2 , f H 2 O ) during crystallization in the magmatic chamber. In micas with signiÞ cant Fe 3+ contents, the layer charge balance is accomplished by the following mechanisms: VI Fe 2+ -3 VI Fe 2 3+VI ■ ■, VI Fe 2+ -1 VI Fe 3+ (OH) --1 O 2-, and VI Fe 2+ -1 VI Fe 3+IV Si 4+ -1 IV

American Mineralogist, 2002

Organic-rich Carboniferous shales with associated coal seams, from the Bacia Carbonífera do Douro-Beira (north Portugal), have been studied by TEM as well as by a variety of other methods. NH 4 and K micas and berthierine form small subparallel packets of a few layers separated by lowangle boundaries. One-and two-layer ordered polytypes, with some spot enlargement typical of minor disorder, occur in the NH 4 micas (i.e., metamorphism tobelites). They exhibit all the characteristics commonly described for sub-greenschist-facies, including a lack of textural and chemical equilibrium. The compositions of the 2:1 layers vary considerably for both K-and NH 4-micas, and except for Ti, exhibit similar compositional ranges. The most significant compositional variations are explained by phengitic substitution (Si from 3 to 3.25, Fe + Mg from 0.1 to 0.3), but no evidence of an illitic substitution has been found. NH 4 in tobelites was determined by analysis of NH 3 using Nessler's reagent, basal spacing, and 1-(K + Na). The resulting values indicate NH 4 contents ranging from 30 to 59% of the interlayer site occupancy. At very low temperatures of metamorphism (e.g., North Sea), the intergrowths of NH 4 and K in micas is on the nanometer-scale. With an increase in temperature (e.g., Pennsylvania), NH 4-and K-micas are present as different minerals. The Douro-Beira samples represent a still higher-temperature case and, therefore, NH 4 and K can be present in the same interlayer sheet but with one cation being dominant. NH 4-and K-dominated micas have segregated into well-separated packets with scarce intergrowths and almost no mixed-layers. Hence they show a solvus relationship.