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Petrologic, chemical, and polytype data are presented for dioctahedral potassic micas from Kfeldspar-bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as [2M.sub.1], notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3 T with pressure. Keywords: Muscovite, phengite, celadonitic substitution, polytypism, [2M.sub.1], 3 T, pressure-polytype relationship, mica chemistry, polytype relationship, petrologic mineralogy DOI: 10.2138/am.2010.3362

We recorded angle-dependent XANES (AXANES) spectra at the potassium K-edge for three compositionally intermediate polylithionite-siderophyllite trioctahedral 1M-micas using polarized synchrotron radiation. We evaluated the experimental spectra for both their in-plane and out-of-plane component fractions of the electric dipole contribution using the analytical formulae of Brouder (1990), referring to theory to extract the origin of their multiple-scattering pathways of Natoli et al. (2003). This analysis was extended to a fourth lithian mica studied previously and allowed characterization of the local environment and ordering around the potassium atoms in the interlayer of the entire set of micas. The AXANES in-plane components are notably similar to the XANES spectra recorded on randomly oriented powders, provided these are oriented at the 'magic angle' (Pettifer et al. 1992). Most observed contributions arise from multiple-scattering interactions of the photoelectron ejected from the potassium absorber colliding with atoms located in the interlayer itself. Note that this includes not only interactions with other coplanar potassium and/or alkali atoms distributed along the interlayer plane, but also with their near- and next-nearest neighboring oxygen atoms which lie on the basal planes of the tetrahedral sheets facing the interlayer. By contrast, the AXANES out-of-plane component suggests that several multiple-scattering pathways cross the energetic and structural barrier represented by the tetrahedral sheets. They reach not only the X anions that are located on the upper level of the octahedral sheets, at the center of the open cavity in the tetrahedral sheet, but also the metal cations centering the octahedral sheet itself. Therefore, the out-of-plane components provide indirect information on the number of independent octahedral sites, the cation oxidation state, and the trans- vs. cis-orientation of the anionic sites. Keywords: Polylithionite, siderophyllite, X-ray absorption pleochroism, XAFS, XANES, MXAN DOI: 10.2138/am.2010.3344

A detailed description of the interlayer site in trioctahedral true micas is presented based on a statistical appraisal ofcrystal-chemical, structural, and spectroscopic data determined on two sets of trioctahedral micas extensively studied by both X-ray diffraction refinement on single crystals (SCXRD) and X-ray absorption fine spectroscopy (XAFS) at the potassium K-edge. Spectroscopy was carried out on both random powders and oriented cleavage flakes, the latter setting taking advantage of the polarized character of synchrotron radiation. Such an approach (AXANES) is shown to be complementary to crystal-chemical investigation based on SC-XRD refinement. However, the results are not definitive as they focus on few samples having extreme features only (e.g., end-members, unusual compositions, and samples with extreme and well-identified substitution mechanisms). The experimental absorption K-edge (XANES) for potassium was decomposed by calculation and extrapolated into a full in-plane absorption component ([[sigma].sub.[parallel]]) and a full out-of-plane absorption component ([[sigma].sub.[perpendicular to]). These two patterns reflect different structural features: [[sigma].sub.[parallel]] represents the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets; [[sigma].sub.[perpendicular to] is associated with multiple-scattering interactions entering deep into the mica structure, thus also reflecting interactions with the heavy atoms (essentially Fe) located in the octahedral sheet. The out-of-plane patterns also provide insights into the electronic properties of the octahedral cations, such as their oxidation states (e.g., [Fe.sup.2+] and [Fe.sup.3+]) and their ordering (e.g., trans- vs. cis-setting). It is also possible to distinguish between F- and OH-rich micas due to peculiar absorption features originating from the F vs. OH occupancy of the 04 octahedral site. Thus, combining crystal-chemical, structural, and spectroscopic information is shown to be a practical method that allows, on one hand, assignment of the observed spectroscopic features to precise structural pathways followed by the photoelectron within the mica structure and, on the other hand, clarification of the amount of electronic interactions and forces acting onto the individual atoms at the various structural sites. Keywords: Crystal structure, X-ray absorption spectroscopy, XAFS, XRD, correlation, assignment

The crystal chemistry of three Li-, Fe-, and Mn-rich trioctahedral micas has been characterized by single-crystal X-ray diffraction. The samples ate from Hirukawa mine, Japan: ([Si.sub.3.43][Al.sub.0.57])([Al.sub.1.00][Fe.sub.0.38][Mg.sub.0.01] [Mn.sub.0.17][Li.sub.1.44])([Na.sub.0.05] [K.sub.0.95])[O.sub.10][F.sub.1.88][(OH).sub.0.12]; from Mokrusha mine, Russia: ([Si.sub.3.30][Al.sub.0.70])([Al.sub.1.00][Fe.sub.0.36][Mg.sub.0.01] [Mn.sub.0.31][Li.sub.1.32])([Ca.sub.0.01] [Na.sub.0.04][K.sub.0.94])[O.sub.10][F.sub.1.91][(OH).sub.0.09]; and from Sawtooth Mountains, Boise County, Idaho, U.S.A.: [Si.sub.3.11][Al.sub.0.89])([Al.sub.0.91][Ti.sub.0.02][Fe.sub.0.46] [Mg.sub.0.03][Mn.sub.0.52][Li.sub.1.06]) ([Na.sub.0.05][K.sub.0.92][Rb.sub.0.02])[O.sub.10][F.sub.1.89] [(OH).sub.0.l1]. 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 TI 1 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. Keywords: Masutomilite, polylithionite, siderophyllite, pegmatite, crystal structure, crystal chemistry

This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tufts, 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.sup.3+], [Ti.sup.4+], and, only in a few samples, to [Fe.sup.3+]. The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [[sup.VI][Ti.sup.4+VI][(Mg, Fe).sup.2+.sub.-1] [(OH).sup.-.sub.-2][O.sup.2-.sub.2]] and Ti-vacancy [[sup.VI] [Ti.sup.4+VI][not less than]Vi][(Mg, Fe).sup.2+.sub.-2]] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly 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.sub.tot] diagram, possibly as a consequence of a variation in intensive parameters (T, P, [MATHEMATICAL EXPRESSIONS NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSIONS NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSIONS NOT REPRODUCIBLE IN ASCII]) during crystallization in the magmatic chamber. In micas with significant [Fe.SUP.3+] contents, the layer charge balance is accomplished by the following mechanisms. [sup.VI] [Fe.sup.2+.sub.-3][Fe.sup.3+VI.sub.2][not less than], [sup.VI] [Fe.sup.2+VI.sub.-1][Fe.sup.3+][(OH).sup.-.sub.-1][O.sup.2-], and [sup.VI][Fe.sup.2+VI.sub.-1][Si.sup.4+VI.sub.-1][Al.sup.3+]. These micas show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters significantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The [Fe.sup.3+]-vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effects related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration). Keywords: Volcanic micas, central Italy, mica oxidation, hydrothermalism, crystal chemistry, Ti-substitutions, hydrothermalism

The X-ray absorption angle-dependent behavior at the potassium K-edge has been determined for two end-members and two intermediate trioctahedral micas, which are representative of the [Fe.sup.2+] [Mg.sub.-1], [Fe.sup.3+][Al.sub.-1], Si[Al.sub.-1]Fe[Li.sub.-1], and [F.sub.-1]OH exchange vectors, using horizontally polarized synchrotron radiation. Experimental spectra are interpreted by the multiple scattering theory following decomposition according to the angular dependence approach of Brouder (1990). The experimental spectra sum up anisotropic effects deriving from in-plane and out-of-plane photoelectron interactions with the potassium near- and next-near neighbors up to the fifth/sixth coordination sphere. For the first time, the absorption edge of a low Z atom is decomposed so as to produce two partial patterns giving, respectively, the full in-plane absorption spectrum ([[sigma].sub.[parallel]]) and the full out-of-plane spectrum ([[sigma].sub.[perpendicular to]]). They fully describe the complete X-ray absorption dichroic behavior of layered compounds, of which mica is a prototype. [[sigma].sub.[parallel]], essentially reflects the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets. Differences among the four micas depend on structural changes induced by the adaptation of their tetrahedral sheets to the increasing sizes of their octahedral sheet due to chemical substitutions (i.e., the [Fe.sup.2+][Mg.sub.-1] vector, mainly) and to chemical changes at the octahedral anion site ([F.sub.-1]OH vector). By contrast, [[sigma].sub.[perpendicular to]] reflects multiple-scattering interactions entering deep into the mica structure, beyond the tetrahedral sheet and well into the octahedral sheet. Such contributions reflect both changes in the anions coordinating the octahedral cations and increasing total amount of heavy atoms (essentially Fe), and differ based on electronic properties such as oxidation states (e.g., [Fe.sup.2+] and [Fe.sup.3+]). Therefore, angle-dependent XANES spectroscopy is one of the rare spectroscopic techniques able not only to detect, but also to quantify structural effects in the atoms at the medium-to long-range order. Keywords: Mica, XAS, polarized XANES, potassium K-edge, trioctahedral micas, crystal structure

Poppiite, a new mineral from Gambatesa mine (Val Graveglia, Genova, Northern Italy), is the [V.sup.3+] end-member of the pumpellyite group [[sup.[VII]][([Ca.sub.7.68][Na.sub.0.27][K.sub.0.03] [Rb.sub.0.02]).sub.[summation of]8.00] [sup.[VI]][([V.sup.3+.sub.1.26][Fe.sup.3+.sub.1.02] [Mg.sub.0.78][Mn.sup.2+.sub.0.59][Al.sub.0.31] [Cu.sup.2+.sub.0.04]).sub.[summation of].4.00] [sup.[VI]][([V.sup.3+.sub.6.89][Al.sub.1.07][Ti.sub.0.04]).sub.[summation of]8.00] [sup.[IV]][([Si.sub.11.69][Al.sub.0.31]).sub.[summation of]12.00] [O.sub.42][(OH).sub.14]; C2/m, a = 19.2889(6), b = 6.0444(2), c = 8.8783(3) [Angstrom], [beta] = 97.328(2)[degrees], V = 1026.66(6) [[Angstrom].sup.3], [D.sub.meas] = 3.36(2) g/[cm.sup.3], and [D.sub.calc] = 3.44 g/[cm.sup.3]]. Poppiite crystals, with size varying from 0.1 to 0.6 mm, are minute, greenish-brown, and prismatic, and are associated with roscoelite, ganophyllite, manganaxinite, goldmanite, and calcite. The strongest lines in the X-ray powder diffraction pattern [[d.sub.obs] ([Angstrom]), [I.sub.rel], (hkl)] are: 2.930, 100, (511); 3.817, 70, (202); 2.548, 65, ([bar.3]13); 2.551, 62, (420); 1.612, 57, ([bar.7]31,424); and 2.367, 51, (222, 403). Poppiite is optically negative, with [2V.sub.calc] = 44[degrees], [n.sub.[alpha]] = 1.768(9), [n.sub.[beta]] 1.804(8), [n.sub.[gamma]] 1.810(9). The pleochroic scheme is [alpha] = light yellowish brown, [beta] = deep greenish brown, and [gamma] = brown to reddish brown. The crystal structure was refined using 1918 unique reflections to R = 0.0307. Like the other pumpellyite-group minerals the crystal structure of poppiite consists of chains of edge-sharing octahedra linked by Si[O.sub.4], [Si.sub.2][O.sub.7], and Ca[O.sub.7] polyhedra. Keywords: New mineral, analysis chemical (mineral), crystal structure, optical properties, XRD data

This work provides a crystal-chemical description of the trioctahedral sheet in 1M mica polytypes of C2/m symmetry. Some octahedral parameters were found to be related strictly to octahedral chemical composition, whereas others were related to the overall layer chemistry. As a general rule, the M1 site is more affected by octahedral chemical composition than the M2 site, and octahedral distortions are more affected than octahedral size. This behavior is attributed to the constraints related to edge-sharing octahedra and octahedral-tetrahedral coupling, which involves the sharing of the apical tetrahedral oxygen atom between the tetrahedron and octahedron. A more quantitative explanation was sought by considering expressions that relate octahedral parameters as a function of atomic coordinates and unit-cell parameters. This procedure identified those parameters affected by local distortions and those that are a function of a more global effect and whose variation greatly affects the layer. Local distortions are obviously more likely a result of local chemical composition. The size and distortion of the octahedral sheet seem to be independent for the database of samples considered. The [alpha] angle was found to be a complex parameter required to: (1) fit tetrahedral and octahedral lateral dimensions; (2) fit tetrahedral topology to its local chemical composition, and (3) fit the interlayer cation electrostatic interactions.

Phengitic muscovite-2[M.sub.1] crystals [Mathematical Expression Omitted] from pegmatites and peraluminous granites were refined to investigate the influence of phengitic substitution on the mica structure. Single-crystal X-ray diffraction data were collected for eleven crystals in the C2/c space-group (agreement factor 2.1% [less than or equal to] [R.sub.obs] [less than or equal to] 3.9%). Tetrahedral Si and Al cation disorder was found for each sample, with the mean tetrahedral cation-oxygen distances ranging from 1.639 [Angstrom] [less than or equal to] [less than or equal to] 1.647 [Angstrom] and 1.640 [Angstrom] [less than or equal to] [less than or equal to] 1.646 [Angstrom]. As phengitic substitution increases, the octahedral sheet expands and requires a less distorted (more hexagonal) tetrahedral ring (7.70 [degrees] [less than or equal to] [Alpha] [less than or equal to] 11.38 [degrees]) and low and low corrugation of the basal O plane (0.1796 [Angstrom] [less than or equal to] [Delta]z [less than or equal to] 0.2296 [Angstrom]). The electron density at the M2 site is greater than that required for the ideal muscovite-2[M.sub.1] structure, and a small excess of electron density is found in the M1 site. The inner sixfold coordination of the interlayer (A) cation is elongated along [c.sup.*], which is consistent with the high [Alpha] values and the long A-O11 bond length.

Well-crystallized euhedral crystals of aluminian lizardite-2[H.sub.2] ([Mathematical Expression Omitted]) ([Si.sub.1.41][Al.sub.0.59])[O.sub.5.00][(OH).sub.4.00] were found near Schio (Vicenza, Italy). To gain insight into the role of a high Al content lizardite, chemical analyses and single-crystal X-ray data collection were conducted. Structure refinement, completed in space group [P6.sub.3] (agreement factor R = 0.034), gives mean T-O values of 1.654 A and 1.664 A for T1 and T2 sites, respectively. The ditrigonal distortion of the six-membered tetrahedral ring is positive ([Alpha] = +9.7 [degrees]), as expected for the [2H.sub.2] polytype. The octahedral site has a mean bond length similar to that of the Mg-rich octahedra of amesite and distortion parameters similar to those of Al-rich octahedra.

Crystal-structure refinements were performed on clintonite-1M crystals [ideal composition [6]Ca[6][([Mg.sub.2]Al).sub.[4](Si[Al.sub.3])[O.sub.10][(OH).sub.2]] from skarns of the Predazzo-Monzoni area and Adamello Massif (northern Italy) with the aim of characterizing some aspects of their crystal chemistry and their relationships with closely associated phlogopite-1M. In the clintonite samples examined, the tetrahedral composition ranges from [Si.sub.1.19][Al.sub.2.78][Fe.sub.0.03] to [Si.sub.1.28][Al.sub.2.70][Fe.sub.0.02] indicating that the extent of the exchange vector [4][Al.sub.-1][6][Mg.sub.-2][4]Si[6](Al, ??), which links trioctahedral with dioctahedral Ca-bearing brittle micas, was very limited. Single-crystal X-ray diffraction data were collected and structure refinements completed in space group C2/m converging to [R.sub.obs] from 0.027 to 0.037 for six samples. The [4][Al.sup.3+] for [4][Si.sup.4+] substitution, which is close to 70%, produces more regular and flatter tetrahedra than in the case of phlogopite, together with an increase in the thickness and in the lateral dimensions of the sheet; the presence of [Al.sup.3+] in octahedral coordination, on the other hand, reduces the dimensions of both M1 and M2 sites with a consequent decrease in the thickness of the sheet. The volume, the flattening angle [Psi], and the central cation off-center shift (BLD) of the trans M1 octahedral site are greater than those of the cis M2 site, thus indicating a normal octahedral ordering. The high misfit value (from 1.187 to 1.326 [Angstrom]) between tetrahedral and octahedral sheets is mostly compensated by the distortion of the tetrahedral ring (tetrahedral rotation angle [Alpha]: 23.1 [less than or equal to] [Alpha] [less than or equal to] 24.9[degrees]). Relative to phlogopite, the interlayer separation in clintonite is reduced by about 0.6 [Angstrom].

This contribution deals with the crystal chemistry of phlogopite and [Fe.sup.3+]-rich phlogopite from the Tapira alkaline-carbonatite complex (Brazil) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion composed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebedourite, garnet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of phlogopite is related to the variation of [f.sub.[O.sub.2]], [a.sub.[H.sub.2]O], and [a.sub.C[O.sub.2]], as well as magma bulk-chemical composition during fractional crystallization. Phlogopite from alkaline-silicate rocks (ranging from dunite to bebedourite) is characterized by fairly constant Al content, moderate [4][Fe.sup.3+] substitution, and variable amounts of Ti. The [4][Fe.sup.3+] substitution, accompanied by crystals showing reverse pleochroism, increases during fractional crystallization. These features correspond to crystallization at low pressure and high [f.sub.[O.sub.2]], and [a.sub.[H.sub.2]O] in the presence of moderate saturation in Ti-bearing phases, [Al.sub.2][O.sub.3] in the magma, or both. Phlogopite from silicate-carbonatite rocks, classified as ferriphlogopite on the basis of strong reverse pleochroism related to [4][Fe.sup.3+] tetrahedral substitution, also presents low to very low Al, [Fe.sup.2+], and Ti contents. These features suggest very high [f.sub.[O.sub.2]], [H.sub.2]O, and C[O.sub.2] conditions in the presence of strong saturation in Ti-bearing phases as well as very low [Al.sub.2][O.sub.3] content in the liquid. The crystal-structure refinements of Tapira phlogopite show that [Fe.sup.3+] substitutes for Si in tetrahedral sites; Fe distribution is completely disordered, so the resulting space group is C2/m. The octahedral-site composition is similar to that of phlogopite, the octahedral sites being preferentially occupied by Mg. The presence of [Fe.sup.3+] in the tetrahedral sheet enlarges the whole structure. This enlargement is reflected by an increase in cell-edge lengths and a decrease in [Beta]-angle values. The increase in distortion of the tetrahedral ring ([Alpha] angle up to [approximately]11 [degrees]) is necessary for the tetrahedral and octahedral sheets to fit together.