1M Polytype Research Articles

Synthesis, Structure, and Electric Conductivity of Ferrous Tainiolite and Its Oxidative Conversion into Coarse-Grained Swellable Smectite

Ferrous tainiolite, Cs[Fe2Li]o[Si4]tO10F2, has been synthesized from the melt in gastight molybdenum crucibles. The electric conductivity has been measured by impedance spectroscopy. As compared to natural micas, which are insulators, good conductivity is observed. The layered silicate can be further modified by ion exchange, which makes it a promising host structure for sensor materials. The estimated activation energy is as low as 0.79 eV. Upon Ba2+ exchange for Cs+, structural Fe2+ is partially oxidized (33%), and the layer charge is reduced significantly, resulting in a coarse-grained but swollen (monolayer hydrate) smectite-type layered silicate. Therefore, further functionalization by pillaring will be feasible resulting in a conducting microporous material. The new material has been characterized by the analysis of chemical composition (wavelength dispersive X-ray spectroscopy and laser ablation-inductively coupled plasma−mass spectrometry), differential thermal analysis, electron microscopy, Mössbauer, UV−vis/NIR, impedance spectroscopy, and single-crystal structure refinement (wR(F2) = 0.0883). The structure is of the 1M polytype (C2/m) with cell parameters a = 5.277(2) Å, b = 9.148(2) Å, c = 10.804(2) Å, and β = 99.19(3)°. The congruent melting point was observed at 1103 (±5) K.

Crystal structure and chemical composition of Li-, Fe-, and Mn-rich micas

The crystal chemistry of three Li-, Fe-, and Mn-rich trioctahedral micas has been characterized by single-crystal X-ray diffraction. The samples are from Hirukawa mine, Japan: (Si3.43Al0.57)(Al1.00 Fe0.38Mg0.01Mn0.17Li1.44)(Na0.05K0.95)O10F1.88(OH)0.12; from Mokrusha mine, Russia: (Si3.30Al0.70)(Al1.00Fe0.36 Mg0.01Mn0.31Li1.32)(Ca0.01Na0.04K0.94)O10F1.91(OH)0.09; and from Sawtooth Mountains, Boise County, Idaho, U.S.A.: (Si3.11Al0.89)(Al0.91Ti0.02Fe0.46Mg0.03Mn0.52Li1.06)(Na0.05K0.92Rb0.02)O10F1.89(OH)0.11. Our crystals belong to the 1M polytype with layer symmetry C121(1) and show M1 and M3 sites much larger in size than M2. Mean electron-count (m.e.c.) values are more variable for the M1 and M3 sites than for M2. With the exception of the sample from Sawtooth Mountains, all tetrahedral mean bond lengths appear to be smaller for T1 than for T11 site. When compared to the Li- and Fe-rich series, crystals show similar crystal-chemical trends, thus suggesting that the layer structure is affected in a similar way by Fe and Mn cations.

Deformation-promoted defects and retrograde chloritization of biotite in slates from a shear zone, Southern Iberian Massif, SE Spain

AbstractNaturally deformed biotite in contact-metamorphosed slates affected by a shear zone of the Southern Iberian Massif near Jaén (SE Spain) were studied by X-ray diffraction, optical microscopy, scanning electron microscopy, electron probe microanalysis and high-resolution transmission and analytical electron microscopy. Biotite is found in the contact metamorphism aureole produced by the intrusion of a granodioritic stock, but shear strain caused its deformation. The southern part of the shear band is strongly deformed, containing thick clay gouge zones. The northern part is less deformed and develops weaker planar-linear fabrics. X-ray diffraction data reveal the predominance of the 2M1 biotite in the undeformed samples whereas the 1M polytype is predominant in the sheared samples. Chemical data and electron images of the biotite from unsheared slates do not show the presence of intercalated phases. This biotite is almost defect-free and electron diffraction patterns have sharp reflections indicating a two-layer polytype (probably 2M1). Back-scattered electron images from the deformed biotite in the moderate deformation part of the shear zone do not reveal intergrown minerals, but the electron microprobe analyses show some Fe- and Mg-enriched compositions. Transmission electron microscopy indicates that disordered polytype packets are predominant (probably 1Md). Their electron diffraction patterns have diffuse streaking along c*. These packets have high dislocation densities, microcavities with ∼5 Å latticefringe regions (probably brucite-like sheets) and interlayering of chlorite-berthierine. Kaolinized biotite can be observed in the clay gouges from the strongly deformed south part of the shear zone. The degree of streaking, as an indication of the intensity of deformation, revealed that the disordered polytype packets are more deformed than the two-layer polytype packets. The microcavities of the disordered polytype packets could act as potential channels for transport of fluids during the shearing stage and serve as sites for chloritization of biotite, producing chlorite-berthierine domains within biotite. Berthierine is an intermediate metastable phase replaced by chlorite with along-layer transitions.

Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)

A detailed investigation of microstructures of authigenic illite has been carried out using highresolution TEM. Near-atomic images of 1M and 2M1 polytypes of illite with incidence [100] and [110] were observed. We unambiguously determine 1M illite in both individual particles and microtwinned domains. Newly formed 2M1 domains are surrounded by former packets of 1Md or 1M illite. Such intergrowth of different illite-polytypic domains on a scale of several layers suggests that they are metastable. The lateral-coherent 1M and 2M1 illite domains are aligned along the same orientation and crystallographically continuous normal to [001]*. This is a new phenomenon observed for polytypic intergrowth and transition. The intralayer-stepwise change of stagger direction in a series of successive layers and consequently the interlayer-stacking angle change resulting in polytypic transition are hypothesized as an “intralayer-stepwise transition” model. It explains the process of illite polytypic transition in the solid state.

Crystal-chemistry of synthetic K-feldspar-buddingtonite and muscovite-tobelite solid solutions

Experiments in the system K 2 O-(NH 4 ) 2 O-Al 2 O 3 -SiO 2 -H 2 O-HCl have been conducted between temperatures of 400 and 600 °C at pressures of 200, 400, 500, and 1500 MPa. The run products consisted of solid solutions of the K-feldspar.-buddingtonite, muscovite- tobelite series, and quartz. The run products were characterized by electron microprobe, powder X-ray diffraction with Rietveld analysis, and infrared spectroscopy. It can be shown that complete solid solution exists for both series for temperatures at least above 400 °C. The grain size of the synthesized phases was usually small and rarely exceeded 10 μm. In addition the micas were quite thin and the thickness was lower than 100 nm. Due to the small grain size and the (NH 4 ) 2 O-content, reliable electron-microprobe results were difficult to obtain. Rietveld analyses did show that the synthesized micas consisted of a complex mixture of 1M, 2M1, 2M2, 3T, and 2Or polytypes. The lattice parameters of the feldspar and mica solid solutions are linear combinations of the end-members and show practically no excess volume. Reasonable compositions of the solid solutions can be derived from the lattice parameters. The mica solid solutions are not binary mixtures but at elevated pressure increasing amounts of pyrophyllite component were observed. By using the NH 4 -deformation mode at around 1430 cm -1 and the OHstretching vibrations ν OH at 3600-3700 cm -1 the NH 4 -contents of the micas could be determined also by IR spectroscopy.

Crystal chemistry and polytypism of tyrolite

The crystal structures of the 1M and 2M polytypes of tyrolite have been solved from single-crystal X-ray diffraction data. The structure of tyrolite-1M [monoclinic, P2/c, a = 27.562(3), b = 5.5682(7), c = 10.4662(15) Å, β = 98.074(11)°, V = 1590.3(3) Å 3 ] has been refined to R 1 = 0.086 on the basis of 2522 unique observed reflections collected using synchrotron radiation at the Swiss-Norwegian beamline BM01 of the European Synchrotron Research Facility (SNBL at the ESRF). The structure of tyrolite-2M [monoclinic, C2/c, a = 54.520(6), b = 5.5638(6), c = 10.4647(10) Å, β = 96.432(9)°, V = 3154.4(6) Å 3 ] has been refined to R 1 = 0.144 on the basis of 2666 unique observed reflections obtained from a non-merohedrally twinned crystal using in-house X-ray radiation and a STOE IPDS II imageplate diffractometer. The structures are based upon complex nanolayers consisting of Cu, As, and Ca coordination polyhedra. The core of the nanolayer is a copper arsenate substructure consisting of A and B sublayers. The B sublayer consists of chains of edge-sharing Cu octahedra running along the b axis. The A sublayer contains trimeric units of Cu octahedra sharing corners with AsO 4 tetrahedra. Two adjacent A sublayers are linked by the octahedral chains of the B sublayer resulting in formation of the 18 Å thick ABA slab. The ABA slab is sandwiched between sublayers of Ca 2+ cations and H 2 O molecules. Adjacent nanolayers are connected by hydrogen bonds to the interlayer species (carbonate anions and H 2 O molecules). The structures of tyrolite-1M and tyrolite-2M differ by the stacking sequence of the nanolayers only. The adjacent nanolayers in tyrolite-2M are shifted by b/2 = 2.8 Å in comparison to the relative position of the nanolayers in tyrolite-1M. The structural formula of tyrolite can be written as [Ca 2 Cu 9 (AsO 4 ) 4 (OH) 8 (CO 3 )(H 2 O) 11 ](H 2 O) x where x = 0-1.

Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

We have used recently developed quantitative crystal chemical models and a simple structural free-energy model to examine and interpret: (1) previously reported powder X-ray diffraction data for several trioctahedral mica solid solution series (64 synthetic powder samples between the Mg, Co, Ni, and Fe end-members, with different degrees of oxidation, vacancy contents, and Al/Si ratios; indexed as 1M polytype, space group C2/m; supplemented here by 57 Fe Mössbauer spectroscopy to obtain accurate iron-site populations of IV Fe 3+ , VI Fe 3+ , and VI Fe 2+ ), and (2) 175 previously published single-crystal refinements comprising 138 natural and 37 synthetic 1M mica samples refined in space group C2/m. The crystal chemical models were validated by comparisons between predicted and measured relations between structural parameters, and needed model parameters and their uncertainties were extracted, using the single-crystal refinements. Two main results arise. First, an observed limit value of the b lattice parameter in certain synthetic solid solution series is shown to correspond to an upper limit value for the tetrahedral rotation angle α of αmax = 9.5° for AlSi 3 tetrahedral sheets in K-rich micas. This upper limit is also clearly seen in the single-crystal refinement data for those Krich single-crystals that have near-AlSi 3 tetrahedral compositions. We argue that the (tetrahedral sheet composition dependent) upper limit of tetrahedral rotation is an intrinsic property of the tetrahedral sheet (presumably corresponding to an intra-tetrahedral-sheet bond-bending limit) rather than arising either from interactions with the interlayer cations or from an octahedral sheet lateral-contraction limit. Second, we find that, except in the extreme cases where one approaches the lower (α = 0°) or upper (α = α max ) tetrahedral rotation limits, the magnitude of the octahedral flattening angle ψ is predominantly determined by octahedral cation stereo-chemical bonding requirements (and other intra-octahedral-sheet properties such as intra-sheet bond bending and intra-sheet electrostatic forces) rather than arising from tetrahedral-octahedral inter-sheet interactions (as generally argued or assumed). In addition, we corroborate a previously reported difference in the crystal chemical behaviors of trivalent octahedral cation (Fe 3+ , Al 3+ ) and vacancy-bearing trioctahedral micas relative to samples that contain only divalent octahedral cations (e.g., Fe-Mg, Fe-Ni, Mg-Ni, and Co-Mg synthetic series); their b vs. average octahedral metal-oxygen bond-length behaviors are dramatically different, a result that is consistent with our proposed dominant stereo-chemical control of ψ.

Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia

Clay minerals associated with the unconformity-type uranium deposits of the East Alligator Rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the Kombolgie unconformity. Alteration develops a similar pattern at regional and smaller scales, with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity (e.g., faults, fractures, breccias). These discontinuities were mechanically active during the hydrothermal activity. The hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the Kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks. The time-space alteration sequence established in this study can be summarized as follows: (1) early crystallization of illite ± illite-smectite (I-S) mixed layer minerals + LREE-Sr–bearing aluminum phosphate-sulfates (APS) in sandstones above the unconformity, illite ± I-S mixed layer minerals + sudoite + APS on both sides of the unconformity and in fault gouges, and illite ± I-S mixed layer minerals + chlorite ± apatite deeper in the basement; and (2) late precipitation of chlorite in the open fracture network mainly centered below the unconformity. The main stage of uranium deposition was associated with chloritization in the basement rocks. Smaller particle size, lath-shaped or flaky morphology, common interstratification with expandable layers, lower crystallinity, and Mg-rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and/or chlorite. Hydrothermal illite can be distinguished from diagenetic and metamorphic illite-phengite by its poor crystallinity along the c-axis, interstratification with minor amounts of smectite layers, and a 1M polytype in which trans-vacant octahedral sites predominate (1 Mt). Chemically, the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher Mg content. The hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size, spherulitic habit, poor crystallinity along the c-axis, and common association with corrensite and/or chlorite-corrensite mixed layers. Most of the hydrothermal chlorite is Mg rich and has a structural formula close to that of clinochlore (XFe <0.20). Near the uranium deposits, the spherules of hydrothermal chlorite are chemically zoned (from XFe <0.10 to XFe <0.40), which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization. The space-time alteration sequence established for clay minerals and the observed transition of APS to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub-basinal brines. In turn, these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron-bearing phyllosilicates (Fe chlorite and biotite) and iron sulfides of metamorphic origin. The ore deposits are surrounded by zones in which the degree of fluid-rock interaction was very high and chloritization is most abundant. The octahedral Mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks, tens to hundreds of meters below. The crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30° to 50°C lower than those expected at the peak diagenesis of the Kombolgie sandstones near the unconformity.

Cation-site partitioning in Ti-rich micas from Black Hill (Australia): A multi-technical approach

AbstractThe crystal chemistry of Ti-rich trioctahedral micas of plutonic origin, cropping out at Black Hill (South Australia) has been investigated by combining electron microprobe analysis, single crystal X-ray diffraction, Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Chemical analyses have shown the samples taken to be quite homogeneous and Ti-rich (TiO2 ≈ 7 wt.%). Mössbauer investigation yielded Fe2+/Fe3+ ≈ 30. X-ray photoelectron spectroscopy analysis seems to suggest the occurrence of three Ti species: octahedral Tr4+(60%), octahedral Ti3+(26%), and tetrahedral Ti4+(14%). The analyzed sample belongs to the 1M polytype and the relevant crystal data from structure analysis are: a = 5.347(1) Å, b = 9.261(2) Å, c = 10.195(2) Å, β = 100.29°(1). Anisotropic structure refinement was performed in space group C2/m, and converged at R = 2.62, Rw = 2.80. Structural details (the c cell parameter, the off-center shift of the M2 cation towards 04, the bond-length distortions of the cis-M2 octahedron, the interlayer sheet thickness, the projection of K−O4 distance along c*, the difference &lt;K−O&gt;outer-&lt;K−O&gt;inner) support the occurrence of the Ti-oxy substitution (VIR2+ + 2(OH)− ⇌ Ti4+ + 202− + H2) in the sample. Analysis of structural distortions as a function of the Ti content revealed that the positions of the oxygens 03 and 04 are displaced in opposite senses along [100]. This produces an enlargement of the M1 site with respect to the M2 site and a shortening of the interlayer distance. This trend seems to be in common with other Ti-rich 1M micas of plutonic origin.

Geometric crystal chemical models for structural analysis of micas and their stacking polytypes

A sequence of progressively more realistic geometric crystal chemical models for TOT layers in mica is developed, starting from the usual main uniform distortions (octahedral flattening, tetrahedral rotation, octahedral counter-rotation) toward additional features as they are shown to be required. These additional features include tetrahedral basal flattening and apical bond adjustment, geometric meso-octahedral sheets (having unequal M1 and M2/M3 site bond lengths), and geometric heterooctahedral sheets (having unequal M1, M2, and M3 site bond lengths). A crystal chemical model for the unit cell of a 1M polytype with C2/m space group is developed from geometric homo-octahedral sheets (having equal M1, M2, and M3 site bond lengths) and is described using a minimal number of independent crystal chemical parameters: octahedral, tetrahedral basal, tetrahedral apical, and interlayer metal-anion bond lengths, and flattening angles of octahedral and tetrahedral sheets. The monoclinic lattice parameters (a, b, c, and β) and the tetrahedral rotation angle (α) follow from these assumed parameters. These models are designed to allow analyses (that are reported elsewhere) of both structural and lattice-parameter refinement data in terms of deviations from various predictions based on specified sets of crystal chemical assumptions. Fractional atomic coordinates are derived in terms of the atomic positions for the 1M unit cell of C2/m symmetry for each known homogeneous mica polytype with highest space group symmetry (polytype, space group = 2M 1 , C2/c; 2M 2 , C2/c; 2O, Ccmm; 3T, P3 1 12). These coordinates allow a structural analysis of diffraction data for different stacking polytype structures using the same 1M-type TOT layer as a modular unit.

Detrital illite crystals identified from crystallite thickness measurements in siliciclastic sediments

Illite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures >70 °C, of superimposed diagenetic modification in the parent sediment. We distinguished the relative proportions of 2M1 detrital illite and possible diagenetic 1Md + 1M illite by a combined analysis of crystal-size distribution and illite polytype quantification. We found that the proportions of 1Md + 1M and 2M1 illite could be determined from crystallite thickness measurements (BWA method, using the MudMaster program) by unmixing measured crystallite thickness distributions using theoretical and calculated log-normal and/or asymptotic distributions. The end-member components that we used to unmix the measured distributions were three asymptotic-shaped distributions (assumed to be the diagenetic component of the mixture, the 1Md + 1M polytypes) calculated using the Galoper program (Phase A was simulated using 500 crystals per cycle of nucleation and growth, Phase B = 333/cycle, and Phase C = 250/cycle), and one theoretical log-normal distribution (Phase D, assumed to approximate the detrital 2M1 component of the mixture). In addition, quantitative polytype analysis was carried out using the RockJock software for comparison. The two techniques gave comparable results ( r 2 = 0.93), which indicates that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M1 detrital illite, from crystallite thickness measurements. The overall illite crystallite thicknesses in the samples were found to be a function of the relative proportions of thick 2M1 and thin 1Md + 1M illite. The percentage of illite layers in I-S mixed layers correlates with the mean crystallite thickness of the 1Md + 1M polytypes, indicating that these polytypes, rather than the 2M1 polytype, participate in I-S mixed layering.

Shirozulite, KMn2+3(Si3Al)O10(OH)2, a new manganese-dominant trioctahedral mica: Description and crystal structure

Shirozulite is a new Mn-dominant trioctahedral mica from the Taguchi mine, Aichi Prefecture, Japan. The mineral occurs in tephroite-rhodochrosite ores in contact with a Ba-bearing, K-feldspar vein. Shirozulite formed during regional low-P/T metamorphism and, thereafter, suffered thermal metamorphism from a local granodiorite. Grains of shirozulite are up to 0.5 mm across and have a typical micaceous habit. Color: dark reddish brown. Cleavage: (001), perfect. Optical properties: biaxial negative, 2V x = very small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, absorption X &lt; Y ≈ Z. Refractive indices: n α = 1.592(2), n β ≈ n γ = 1.635(2). The structural formula is (K 0.90 Ba 0.09 ) (Mn 2+ 1.53Mg 0.94 Fe 2+ 0.20 Ti 0.04 Al 0.29 ) (Si 2.54 Al 1.46 ) O 10 [(OH) 1.97 F 0.03 ], and the end-member composition is KMn 2+ 3 AlSi 3 O 10 (OH) 2 . Density: obs. = 3.20(3) g/cm 3 by pycnometer, calc. = 3.14(2) g/cm 3 . Shirozulite is monoclinic, C2/m, 1M polytype, a = 5.3791(7), b = 9.319(1), c = 10.2918(9) Å, β = 100.186(9)°, V = 507.8(1) Å 3 . The six strongest lines in the powder X-ray diffraction pattern are as follows: d (Å), l (%), (hkl): 10.16, 100, (001); 2.654, 96, (1̅31); 3.386, 51, (003); 1.556, 48, (3̅̅13); 2.467, 46, (1̅32); 2.202, 36, (1̅33). The crystal structure has been refined to an R value of 4.1% based on 663 observed reflections collected with MoKα X-radiation from a single crystal. The mean bond lengths, tetrahedral rotation, and octahedral flattening angles are as follows: &lt;T-O&gt; = 1.668, &lt;M1-O&gt; = 2.118, &lt;M2-O&gt; = 2.103, &lt;K-O&gt;(inner) = 2.995, and &lt;K-O&gt;(outer) = 3.376 Å, α = 8.36°, ψ M1 = 58.5°, ψ M2 = 58.2°. The apparent element distribution coefficient analyses among coexisting manganese or manganoan silicate minerals indicate that the trioctahedral mica structure cannot contain larger amounts of Mn 2+ relative to Mg and Fe 2+ than in olivine, pyroxenoid, and garnet.

The Crystal Structure of Roscoelite-1M

AbstractSingle-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) Å, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, &lt;T−O&gt; = 1.641 Å, is close to the mean &lt;T−O&gt; value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance &lt;M2−O&gt; = 2.020 Å which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Hydrogen deficiency in Ti-rich biotite from anatectic metapelites (El Joyazo, SE Spain): Crystal-chemical aspects and implications for high-temperature petrogenesis

Titanium-rich biotites from graphite-bearing metapelitic xenoliths, equilibrated at ca. 850 °C and 7 kbar in the presence of a granitic melt, have been studied through complete chemical analysis and single-crystal XRD refinements. The chemical study combines EMP analyses, hydrogen determination by both SIMS and C-H-N elemental analysis, and Mössbauer spectroscopy. Biotites in the analyzed xenoliths have TiO 2 contents ranging from 4.5 to 4.9 wt% and an XFe of 0.67. Their F and Cl contents are negligible, and Fe 3+ /Fetot ranges from 10 to 16%. The H 2 O content of the biotites ranges from 2.4 to 2.8 wt%, and a small fraction of H is accommodated in the lattice as NH 4 . Based on these full chemical analyses, the calculated OH occupancy is 1.26 to 1.30 groups per formula unit, more than one third less than the stoichiometric value. The entrance of Ti in the octahedral site of biotite is consistent with the Ti-oxy exchange, whereas Ti-Tschermak or Ti-vacancy substitutions play a very minor role. The Fe 3+ -oxy exchange cannot account for the observed OH deficiency. From single-crystal XRD, biotites belong to the 1M polytype and contain variable amounts of stacking faults. The c cell parameter, K-O 4 and &lt;K-O&gt;outer distances provide an independent estimate of the OH content, which agrees with SIMS determinations. The linear relationship between VITi 4+ and the bond-length distortion of the cis-M2 octahedron reveals the partitioning of Ti 4+ in M2, and the Ti 4+ off-center shift toward O 4 supports the occurrence of the Ti-oxy exchange. The ordering of Ti 4+ over two non-equivalent M2 sites, which would be favored energetically, is in agreement with the evidence for a third octahedral site suggested by Mössbauer spectroscopy. The biotite dehydrogenation combined with the partitioning of Ti 4+ in M2 and the low thermal expansion of Ti 4+ - containing octahedra, are the keys to understanding the thermal stabilization of Ti substitution in biotites.

Norrishite, K(Mn23+Li)Si4O10(O)2, an oxymica associated with sugilite from the Wessels Mine, South Africa: Crystal chemistry and40Ar-39Ar dating

Norrishite, K(Mn 2 3+ Li)Si 4 O 10 (O) 2 , an OH-free mica, is found in a silicate-rich sample from the Wessels mine, Kalahari manganese field, South Africa. The mineral forms centimeter-sized black aggregates in dark-purple sugilite, and is associated with kornite and Mn-rich pectolite. Single-crystal X-ray data, collected on norrishite of end-member composition, yielded a monoclinic C2/m lattice with a = 5.291(2), b = 8.904(2), c = 10.049(4) Å, β = 98.15(5)°, V = 920.6 Å 3 , and Z = 2, characteristic of the 1M polytype. 40 Ar- 39 Ar analyses on a gem-quality sugilite yielded an age of 1048.1 ± 5.9 Ma interpreted as a mineral crystallization age that dates the main hydrothermal event that led to enrichment of the Mn ore. The norrishite age data are more complex and suggest a crystallization age ≈ 1010 Ma and a subsequent alteration at ca. 850 Ma. The age results obtained suggest that the hydrothermal activity is related to the collision and tectonism affecting the edges of the Kalahari craton, and show that sugilite, and probably also unaltered norrishite, are suitable for dating metamorphism.

Synthesis, properties and stability of end member boromuscovite, KAl2[BSi3O10](OH)2

End member boromuscovite, with nearly the ideal composition, was synthesized as a single phase from mixtures of its own composition, or with excess boron and water, at high pressures of between 15 and 30 kbar at 700 °C. The mica synthesized consists of a mixture of 2M1 and 1M polytypes with the cell dimensions of 2M1: a=5.071(4), b=8.786(4), c=19.830(89) A, β=95.84(12)°, V=878.5(1.4) A3; and 1M: a=5.059(5), b=8.819(6), c=10.025(17) A, β=101.39(57)°, V=438.4(1.3) A. The IR spectrum shows characteristic differences relative to that of muscovite. DTA registers an endothermic peak due to dehydration breakdown above 680 °C. Seeded experiments indicate that boromuscovite is a high-pressure phase requiring minimum pressures of 3 to 10 kbar at temperatures that concomitantly increase from 300 to 750 °C. At lower pressures, the anhydrous solid assemblage K-feldspar + Al-borate (probably Al4B2O9) coexists with a vapor rich in boric acid. The conversion of this assemblage to boromuscovite is connected with increases in the coordination number of B from [3] to [4], and of Al from [4] to [6]. Above 10 kbar, the boromuscovite stability field is limited along its high-temperature side by congruent (or incongruent?) melting of the mica, starting near 750 °C and 10 kbar and increasing to about 900 °C at 50 kbar, although, at such very high pressures a supercritical fluid may be present. Because, even in the presence of excess-boron fluid, the synthetic mica shows invariable X-ray properties, it is concluded that one B atom per formula unit represents the maximum, and – contrary to olenitic tourmalines – no further substitution of Si by B linked with addition of hydrogen takes place. In contrast to muscovite, KAl2[AlSi3O10](OH)2, end member boromuscovite is not stable under normal P–T conditions of the Continental Crust along a 30 °C/km geotherm, and especially not during the intrusion of acidic igneous rocks including their pegmatites, which may explain its scarcity in nature. However, it may form in the waning hydrothermal stages of deep-seated (>10.5 km) pegmatites and – providing sufficient boron is available – in HP/LT subduction zone environments, where it may carry boron to considerable depths.

Transmission Electron Microscopy Study of Smectite Illitization During Hydrothermal Alteration of a Rhyolitic Hyaloclastite From Ponza, Italy

AbstractDioctahedral phyllosilicates from an altered rhyolitic hyaloclastite located at Ponza Island, Italy, were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples are from a sequence previously characterized by X-ray diffraction (XRD) methods, indicating that a complete range of illitization accompanies alteration. Backscattered electron (BSE) images, obtained from ion-milled samples, show that samples partly retain the original textures since clay minerals pseudomorph lapilli fragments and preserve vesicular texture. The lowest-grade sample studied contains obsidian clasts partially replaced by smectite. As the alteration grade increases, illitization proceeds with formation of interstratified illite-smectite (I-S), zeolites, illitic phases, feldspars and quartz. The most altered sample contains illite, mica and quartz. Lattice-fringe images show that following the formation of smectite, illitization takes place through the formation of (R=1) I-S, highly illitic I-S and illite with mica; (R=1) I-S is the only ordered interstratified I-S. The BSE and TEM images of Ponza samples show irregular cavities filled with euhedral dioctahedral clay minerals and the zeolite mordenite, providing direct evidence for neocrystallization from a fluid. Chemical compositions by analytical electron microscopy (AEM) support the sequence described. Selected area electron diffraction (SAED) patterns indicate the predominance of 1 Md polytypism both in I-S and illitic phases, and the coexistence in the more altered samples of 1Md illite and a 2-layer mica polytype (probably 2M1), without the intermediate 1M polytype generally assumed to exist in prograde sequences. Previous XRD studies indicated progressive change from cis-vacant, turbostratically stacked smectite, to interstratified cis- and trans-vacant, 1Md I-S, to trans-vacant, 1M illite, and then to 2M1 illite in Ponza Island samples. We observed a clear correlation between the chemical compositions as determined by AEM and the proportion of cis-vacant determined by XRD, suggesting that the octahedral cation distributions change in the studied samples with increasing degree of illitization.

Clay Mineralogy of Phyllic and Intermediate Argillic Alteration at Bingham, Utah

The earliest alteration mineral zones in the porphyry Cu-Mo-Au ore deposit at Bingham, Utah, include the outermost propylitic and inner potassic alteration of igneous rocks. Innermost phyllic and intermediate argillic alteration is superimposed on propylitic and potassic alteration. Petrographic, X-ray diffraction, and electron microprobe analyses are used to characterize the clay mineralogy of phyllic and intermediate argillic alteration. Phyllic alteration affects monzonite near the contact with the quartz monzonite porphyry in the southwest and northeast portions of the deposit and consists primarily of veins and vein selvages of illite interstratified with an average of 7 percent smectite (R >3 ordering). The veins and selvages also contain small amounts of smectite and kaolinite. The illite/smectite interstratification is a mixture of 1M and 2M polytypes. Structural formulas for the illite show less K and Al and more Si than muscovite, octahedral Mg, and both a tetrahhedral and an octahedral layer charge. The Kubler crystallinity index and smectite content increase systematically as particle size decreases, consistent with an increase in smectite in the interstratification in the smaller size fractions. Intermediate argillic alteration consists of smectite, illite, and kaolinite that occur primarily as a replacement of plagioclase phenocrysts. A few veins of intermediate argillic minerals are present in the quartz monzonite porphyry. Illite is interstratified with an average of 6 percent smectite (R >3 ordering), but a few samples contain a regular interstratification of illite with 40 to 50 percent smectite (R1 ordering). The smectite replacing plagioclase is montmorillonite, and smectite that occurs in one vein is beidellite. Statistical trends in intermediate argillic alteration mineralogy within the quartz monzonite porphyry show that illite and kaolinite abundances increase generally upward and toward the northeast. Smectite abundance increases generally downward and toward the southwest. In a cross section near the center of the deposit, abundances of kaolinite and illite decrease as abundance of smectite increases with increasing depth. The occurrence of interstratified illite/smectite averaging 6 to 7 percent smectite together with discreet smectite suggest formation near 200°C. A temperature difference between phyllic and intermediate argillic alteration could not be resolved from the mineralogy. Temperature estimates from fluid inclusions for phyllic alteration at other porphyry copper deposits generally range from 200° to 350°C. The activity ratio of Na + /K + for the fluid calculated from Na + /K + in illite averages 5.0 for a phyllic vein that crosscuts propylitic alteration, 1.4 for a phyllic vein that crosscuts potassic alteration, and 2.8 for intermediate argillic alteration superimposed on potassic alteration. Log f H 2 O / f HF of the hydrothermal fluid computed from the fluorine content of illite averages 6.1 for a phyllic vein that crosscuts propylitic alteration, 5.8 for a phyllic vein that crosscuts potassic alteration, and 5.7 for intermediate argillic alteration. Log f H 2 O / f HF of fluids forming late-stage biotite is about 5.0. Intermediate argillic alteration could not be correlated with sulfides in the quartz monzonite porphyry. An increase in illite upward and toward the northeast may correlate with massive sulfides in sediments recently discovered in that region.

Chloritization and polytypism of biotite in the Łomnica granite, Karkonosze Massif, Sudetes, Poland: stable isotope evidence

Biotites from core samples of the Łomnica granite (Karkonosze Mts., Sudetes) are chloritized. Two generations of chlorite occur: an earlier one that filled interstices, reducing granite permeability, and a later, major one that replaced biotite crystals. The degree of chloritization of biotite varies from 0% to 93%. The rate of potassium removal from biotite during the progress of chloritization is not constant. The rapid removal of K is observed at initial stages of chloritization. After the replacement of 20% of the biotite, K removal is much slower. The degree of chloritization of biotite correlates with isotopic signals: (i) the change in δD is semi-logarithmic starting at −85‰ for unaltered biotite, and achieving −50‰ in the chloritized one, while some biotitic hydrogen underwent replacement during or even prior to chloritization; and (ii) δ 18O correlates linearly, changing from about 4.9‰ to 2.8‰. Quartz observed in cold cathodoluminescence (CCL) shows internal zoning, which might be indicative of the hydrothermal remobilization. The temperature of chloritization, estimated from the isotopic fractionation of oxygen between quartz and chlorite, was about 300 °C, which agrees with chlorite geothermometer data. Water from within the chloritization environment had higher δD and lower δ 18O than magmatic water. The preservation of magmatic hydrogen and oxygen isotopic ratios in unaltered biotite indicates only limited water–rock interaction in a subsolidus environment. The polytypic composition of biotite, expressed as the percent ratio of 2M 1 polytype in a mixture with 1M (and 3T) varies between 30% and 60% of 2M 1 polytype. An isotopic effect was observed in coexisting polytypes of biotite. The 2M 1 polytype tends to concentrate the lighter isotope of oxygen relative to the 1M (and 3T) polytypes.

First structure determination of an MDO-2O mica polytype associated with a 1M polytype

The structure refinement of associated phlogopite-2 O and phlogopite-1 M from the Khibiny massif (Kola Peninsula, Russia) is reported. Crystal data are: α = 5.2781(5), b = 9.141(1), c = 20.124(4) A, Ccmm (20); a = 5.305(2), b = 9.199(2), c = 10.232(4) A, β = 100.03(2)°, C 2/m (1 M ). Least-squares refinement of single-crystal X-ray diffraction data converged to R1 = 0.034 (2 O , 926 independent reflections) and 0.037 (1 M , 677 independent reflections). This is the first structure refinement of an MDO (standard) mica-2 O : two previous structure reports concerned anandite-2 O , which was not a true polytype, having an oP (primitive orthorhombic) lattice not compatible with the C -centred cell common to all mica polytypes. The two phlogopite polytypes show practically the same chemical composition (K 0 . 95Na 0.01 ) (Mg 2.16 Fe 0.34 Ti 0.04 Mn 0.04 Li 0.40 )[Si 3.40 Al 0.60 O 10 ][(OH) 1.35 F 0 .65] and no cation ordering. Both polytypes are affected by stacking disorder, which broadens the non-family reflections ( k ≠ 3n). As a consequence, the diffracted intensities of the two types are measured at different scales and large residues in the difference Fourier maps were observed at ± b /3 along [010], [310] and [310]. These residues ( uroviceffect ) disappear by carrying out the refinement with separate scale factors for the two types of reflections. On the basis of structural, morphology and zoning considerations the formation of the two associated polytypes is attributed to chemical oscillation in the crystallization mean.