Volume 93, Issue 2-3

The presence of amazonitic K-rich feldspar is considered an earmark of evolved granitic pegmatites of NYF type. Elements of an explanation of the “NYF pegmatite-amazonite” connection emerged in a study of the classic Anjanabonoina “hybrid” granitic pegmatite in Madagascar. The NYF assemblages there and at the Sakavalana and Anjahamiary pegmatites contain magmatic U- and Th-rich accessory phases and an amazonitic microcline perthite in miarolitic cavities. The early miarolitic minerals are overgrown by a Li- and Cs-enriched assemblage, depleted in HFSE, containing pale-gray microcline perthite. The amazonitic microcline perthite at Anjanabonoina has a δ 18 O value above 15‰, and that at Sakavalana, close to 13‰, indicative of formation of the NYF-pegmatite-forming magma by the melting of crustal rocks. Locally, the source rocks had become slightly alkaline and enriched in HFSE during periods of metasomatism of the hot granulitic crust accompanying episodes of distension over the course of the protracted Pan-African orogeny. The fertilized crust underwent local anatexis after the culmination of Gondwana re-assembly. The proximity of pyrochlore and other U- and Th-bearing accessory phases in the NYF assemblage provided the ionizing radiation required to interact with H 2 O trapped in the vacancy next to the Pb ions in the structure. The presence of an amazonitic K-feldspar ultimately signals net additions of Pb, U, Th, and the alkalis to the source prior to anatexis during a period of distension after a major orogenic disturbance. Plumbian K-feldspar also occurs in granitic pegmatite bodies formed by anatexis in the vicinity of a galena-bearing orebody (e.g., at Broken Hill, Australia); in this case, the resulting granitic pegmatite is neither of NYF type nor enriched in rare elements, however.

The Wilson Cove Member of the Cushing Formation is a thin (up to 120 m thick) metamorphosed Fe- and Mn-rich unit exposed discontinuously over a distance of >75 km in southern Maine. Cathodoluminescence imaging of zircon grains from the unit reveal texturally isolated detrital cores surrounded by distinct metamorphic overgrowths. U-Pb SHRIMP core ages range from 463 to 2058 Ma with a strong peak in the Neoproterozoic-Early Cambrian. This distribution of ages is consistent with a peri-Gondwanan source region and a Middle to Late Ordovician depositional age. Zircon rims have an age of 373 ± 4 Ma, consistent with growth during late Acadian metamorphism. Whole-rock geochemistry reveals considerable major- and trace-element variability with generally elevated abundances of Fe 2 O 3 (t) (15-43 wt%), MnO (0.1-12.1 wt%), Ba (4-2503 ppm), and As (7-1161 ppm). Geochemical discrimination diagrams suggest the protoliths were mixtures of hydrothermal exhalatives and terrigenous clastic sediment, with these materials most likely having been deposited in a marine back-arc basin proximal to a peri-Gondwanan continental source region. Late Devonian low-pressure, amphibolite-facies metamorphism of these bulk compositions produced assemblages dominated by grunerite + garnet + biotite + quartz. These and other mineral assemblages found in the Wilson Cove unit in south-central Maine are consistent with peak metamorphic conditions of 550 to 600 °C and 3-4 kbar previously determined from nearby metapelites. Mineral assemblages, mineral modes, and mineral compositions in these Fe- and Mn-rich rocks are strongly influenced by whole-rock bulk compositional variability. In particular, the compositions of co-existing garnet and grunerite vary systematically as a function of whole-rock MnO concentration.

Boralsilite, the only natural anhydrous ternary B 2 O 3 -Al 2 O 3 -SiO 2 (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B 2 O 3 and H 2 O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V = 1245.8(2) Å 3 . Boron replaces 14% of the Si at the Si site, and Si or Al replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase (“boron-mullite”) synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) Å 3 . “Boron-mullite” also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar. A possible natural analogue of “boron-mullite” is replacing the Fe-dominant analogue of werdingite in B-rich metapelites at Mount Stafford, central Australia; its composition extends from close to stoichiometric Al 2 SiO 5 to Al 2.06 B 0.26 Si 0.76 O 5 , i.e., almost halfway to Al 5 BO 9 . Boralsilite is a minor constituent of pegmatites cutting granulite-facies rocks in the Larsemann Hills, Prydz Bay, East Antarctica, and at Almgotheii, Rogaland, Norway. Electron-microprobe analyses (including B) gave two distinct types: (1) a limited solid solution in which Si varies inversely with B over a narrow range, and (2) a more extensive solid solution containing up to 30% (Mg,Fe) 2 Al 14 B 4 Si 4 O 37 (werdingite). Boralsilite in the Larsemann Hills is commonly associated with graphic tourmaline-quartz intergrowths, which could be the products of rapid growth due to oversaturation, leaving a residual melt thoroughly depleted in Fe and Mg, but not in Al and B. The combination of a B-rich source and relatively low water content, together with limited fractionation, resulted in an unusual buildup of B, but not of Li, Be, and other elements normally concentrated in pegmatites. The resulting conditions are favorable in the late stages of pegmatite crystallization for precipitation of boralsilite, werdingite, and grandidierite instead of elbaite and B minerals characteristic of the later stages in more fractionated pegmatites.

During metamorphism, evidence of the prograde path is commonly obliterated by continued recrystallization as temperatures increase. However, prograde pseudomorphs that are common in many terrains may provide insight into this portion of the metamorphic path if their mineralogy can be accurately interpreted. Seventeen sillimanite-zone metapelitic samples, each containing 2-5 muscovite-rich pseudomorphs, from the Farmington Quadrangle, Maine, U.S.A., were investigated to evaluate their use as indicators of conditions during prograde metamorphism. SEM-CL images, X-ray maps, image analysis, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorph and the surrounding matrix. Based on modal mineralogy determined from image analyses, the muscovite-rich pseudomorphs are divided into four major types: muscovite-rich (>70% muscovite), quartz-muscovite (60-70% muscovite with 10-25% quartz), plagioclase-muscovite (58-72% muscovite with 10-20% plagioclase), and sillimaniteplagioclase- muscovite (50-60% muscovite with 10-20% each plagioclase and sillimanite). A biotiterich, muscovite-poor mantle surrounds many pseudomorphs. All pseudomorphs are interpreted to be prograde, based on their texture, and are after staurolite because of the partial replacement of staurolite by coarse muscovite at lower grades. Textural modeling of reaction mechanisms required to reproduce the observed mineralogy in the pseudomorphs indicates that each major pseudomorph type holds clues to the prograde path and represents a different mechanism of formation. Muscovite-rich and quartz-muscovite pseudomorphs formed by the breakdown of staurolite containing different modal amounts of poikiloblastic quartz. Quartz-muscovite pseudomorphs likely reflect a quartz-rich initial rock composition. Plagioclasemuscovite pseudomorphs require the infiltration of Na-bearing fluids. Sillimanite-plagioclase-muscovite pseudomorphs require a two-stage process; the infiltration of Na-rich fluids during staurolite breakdown followed by sillimanite growth. The subtle mineralogical differences recorded in the pseudomorphs studied here provide evidence of previously unrecognized controls along the prograde path during metamorphism.

We describe the first occurrence in the Variscan Belt of Western Europe of the relatively rare phosphate wagnerite, ideally Mg 2 PO 4 F. It occurs in albite-rich, cordierite-gedrite-bearing gneisses on the island of Ile d’Yeu, southern Armorican Massif, France. These gneisses are associated with a network of shear zones that crosscut granitoid orthogneisses of calc-alkaline affinity. Wagnerite is zoned and displays a rimward decrease of Fe/(Fe + Mg) from 0.16 to 0.08 and a concomitant increase in F. The F content ranges 0.46-1.05 apfu, but critically depends on the choice of the analytical standard. Based on phase diagrams calculated with THERMOCALC, we infer that the wagnerite-bearing orthoamphibole + cordierite + biotite + chlorite paragenesis equilibrated at ca. 550 °C, and pressures lower than 4 kbar. The presence of staurolite relics requires similar temperatures, but pressures higher than 4 kbar, implying an evolution dominated by decompression. On the basis of whole-rock chemistry and stable isotopes, we suggest that superimposed periods of metasomatic alteration throughout the metamorphic history led to the prograde stabilization of the cordierite-gedrite gneiss at the expense of the orthogneiss. This alteration involved aqueous fluids in isotopic equilibrium with local rocks and caused significant loss of Ca, K, and Si, and gain of Mg and Na. We argue that the Na-enrichment is the most significant difference between wagnerite-bearing and wagnerite-free Mg-rich, Ca-poor rocks on Ile d’Yeu. This emphasizes the possible importance of Na metasomatism for the formation of wagnerite. In light of comparisons with other wagnerite occurrences, we conclude that a long-term fluid-rock interaction, typically associated with shear-zones, may be the rule rather than the exception for the formation of wagnerite in metamorphic rocks unaffected by anatexis.

Precise chemical composition, including Fe 3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O 12-(x+y+z) (OH) x Cl y F z ]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H 2 O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe 3+ /Fe tot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe 3+ -free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X Mg , and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2OH - and the “fluorination” Ti 4+ + 2O 2- = (Fe,Mg) 2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Biotites from metapelitic xenoliths included within trachytes from the Euganean Hills (Italy) were analyzed by single-crystal X-ray diffraction (XRD), electron microprobe (EMP), scanning electron microscope (SEM), secondary ion mass spectrometry (SIMS), and Mössbauer spectroscopy. These biotites are Ti-rich and occur in gneissic xenoliths that underwent regional high-T/low-P metamorphism, at about 750 °C, followed by pyrometamorphism during incorporation in the melt at temperatures close to 950 °C. Biotites are zoned, with TiO 2 content ranging from 6.79 (cores) to 8.14 wt% (rims). SIMS measurements show that the H 2 O content is in the range 2.88-4.08 wt%. The simultaneous occurrence of high-Ti and high-H 2 O contents, and the main cation substitutions based on EMP analyses suggest that the role of Ti-oxy in these biotites is less important than Ti-vacancy and Ti-Tschermak substitutions. Single-crystal XRD confirms that the Ti-oxy exchange was indeed effective but not the dominant substitution mechanism. Based on our data and those taken from literature on petrologically well-constrained systems, we propose that there is a petrologic control on the type of Ti-substitution mechanisms. We consider two types of petrologic groupings for biotites: (1) group A consisting of biotites from H 2 O-free or H 2 O-poor petrologic environments (e.g., volcanic rocks, ultrabasic xenoliths, and crustal xenoliths in which biotite underwent incongruent melting): Ti substitution in these biotites occurs via Ti-oxy predominantly, or more specifically Fe 3+ -Ti-oxy; and (2) group B consisting of biotites from H 2 O-rich petrologic environments (e.g., metamorphic rocks and crustal granitoids): Ti-vacancy, or more specifically Fe 3+ -Ti-vacancy, is the dominant mechanism in them. It is concluded that during high-grade metamorphism the dominating type of Ti substitution in biotite is controlled by H 2 O activity.

Proto-polymorphs of jimthompsonite and chesterite occur in metamorphosed serpentinites from two Japanese ultramafic complexes. The lattice constants of the proto triple-chain silicate, measured by X-ray diffraction, are a = 0.93605(208), b = 2.72560(588), and c = 0.53160(89) nm, whereas those of the mixed double- and triple-chain silicate are a = 0.94202(78), b = 4.54402(392), c = 0.53440(45) nm, and β = 90.026(18)°. The lattice constants and systematic extinctions revealed by selected-area diffraction patterns are consistent with proto-triple-chain silicate (Pbcn) and mixed double- and triplechain silicate (A2/m, Am), but not with the ortho- and clino-polymorphs. High-resolution transmission electron microscopy [016] images of the triple-chain silicate and [0 1 15] images of the mixed-chain silicate indicate they have a (X) configuration. Proto forms of wide-chain pyriboles might be geologically widespread.

Silicate garnets, general formula E 3 G 2 Si 3 O 12 , form an important class of rock-forming minerals and, in nature, most are solid solutions. Their crystal-chemical and solid-solution properties are sometimes interpreted in terms of the widely used Pyralspite-Ugrandite classification scheme, and this can lead to erroneous conclusions. In this study, published data are reviewed and analyzed to achieve a synthesis of relevant experimental and computational results and to construct a working “crystal-chemical model” for describing aluminosilicate garnet, E 3 Al 2 Si 3 O 12 , over different length scales. The pyrope-grossular (Py-Gr) solid solution is given special attention, because it has received a great deal of study. It also shows interesting crystal-chemical and thermodynamic mixing behavior. Computational and experimental investigations made on Py-Gr garnets indicate that the shorter Ca/Mg-O2 bond lengths appear to remain roughly constant in length across the binary and can be described as showing “Pauling limit-type” behavior. The longer Ca/Mg-O4 bonds behave differently, because they lengthen with increasing Gr component in the solid solution. Bond behavior in almandine-spessartine (Al-Sp) garnets appears to be partly different, because both Fe/Mn-O2 and Fe/ Mn-O4 bonds show “Pauling limit-type” behavior. E-O bond-length variations are continuous. The bonding type in all aluminosilicate garnet end-members is similar. An analysis shows that various computational simulations on Py-Gr solid solutions are consistent with each other with respect to E-O bond behavior and also with experimental IR, Raman, NMR spectroscopic, and X-ray diffraction results, but not completely with XAS studies made at the Ca edge. Ca/Mg-O4 bond behavior can be used to explain, partly, the nature of various micro/nanoscopic crystal-chemical and strain properties and macroscopic excess thermodynamic mixing behavior of Py-Gr garnets. Micro/nanostrain for the Py-Gr binary is asymmetric in nature, as are the various thermodynamic mixing functions ΔH ex , ΔS ex , and ΔV ex . The widely cited Pyralspite-Ugrandite classification scheme has limited use in terms of explaining many physical and chemical properties of garnet and it should not be used to predict or describe, for example, solid-solution behavior.

In-situ electron-microprobe dating of monazite holds the promise of being an effective technique for obtaining chronologic data. Our research focuses on I- and S-type granitoids of the Lachlan Fold Belt, Australia, whose petrology and zircon chronology have been thoroughly characterized. This study documents the textural relationships, morphology, zoning, and ages of monazite in these granitoid rocks. The I-type granitoids that lack monazite usually contain other mineral phases enriched in rare earth elements, such as allanite, titanite, and bastnasite. Only silica-rich, highly evolved I-type granitoids contain monazite. This preference in I-type rocks for phases other than monazite to host the REE and Th limit the applicability of monazite dating for this group of igneous rocks. On the other hand, monazite is ubiquitous in S-type rocks, both as interstitial and included grains. High-resolution X-ray maps of individual monazites reveal complex patterns of chemical zoning. Weighted averages of multiple analyses for individual chemical domains show small but systematic differences in age. These weighted averages of the chemical domains are considered the best estimate of the age of the monazite. Monazites from 8 different S-type samples range in age from 405 to 759 Ma, with the majority being 490 Ma and older. These are premagmatic ages for these granitoids, which have crystallization ages of 400 to 430 Ma. These premagmatic ages are similar to Cambro-Ordovician ages obtained from inherited zircon cores in the same granitoids, indicating that monazite can survive anatexis in peraluminous rocks. Thus, monazite dating in peraluminous rocks may illuminate characteristics (composition and age) of source rocks and anatectic processes.

An exposure near Gassetts, Vermont, contains lithologies varying from staurolite-kyanite grade aluminous schists with paragonitic muscovite to potassic gneiss with phengitic muscovite. Singlecrystal laser fusion 40Ar/39Ar ages for paragonitic and phengitic muscovite yield similar distributions with ranges between 366 ± 4 and 326 ± 4 Ma. Intracrystalline ages vary from ca. 394 ± 4 to 330 ± 4 Ma. Thus, we find that the intracrystalline (core-rim) age distribution of relatively large, single crystals essentially encompasses the range of ages obtained through total fusion of smaller crystals, consistent with models for development of diffusion profiles and 40 Ar-closure during cooling with a diffusion dimension controlled by the physical grain size. However, some of the larger crystals studied, particularly those with prominent microscopic defects (features readily evident such as internal grain boundaries and twin planes), yield relatively young ages and lack significant core-rim age discordance. Furthermore, the overall distribution of single-crystal ages in the two samples is bimodal, and we suggest that this age distribution reflects metamorphic deformation and recrystallization event(s) superimposed on early generation muscovite. Thus, the mean age of muscovite in these samples (typical of K/Ar and 40 Ar/ 39 Ar incremental heating analysis of bulk mineral separates) has little relationship to any single, hypothetical closure temperature. In view of the similar results we obtain for muscovite of contrasting composition, the net effects of variations in grain size, deformational character, and growth history are interpreted to be more important in forming the observed variations in age than are the chemical substitutions in these samples.

A crystal-chemical study of thirteen biotite (twelve of 1M polytype and one of 2M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VI Mg 2+ / VI (Mg 2+ + Fe 2+ ) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VI Al ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VI Al ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters (T and f O₂ ) on mica crystal structure, and not just chemical composition.

Based on a large database of tourmaline crystal-structure refinements and associated electron microprobe analyses, the <B-O> bond length in the triangular (BO 3 ) groups was found to be reasonably constant for all tourmaline species, but the B-O2 and B-O8 are variable as a function of the crystal chemistry. Tourmalines, such as elbaite, tend to have B-O2 distances significantly shorter than the B-O8 distances, whereas others, like povondraite, tend to have B-O2 distances that are longer than their B-O8 bonds. Statistical analysis of the bond-valence contributions for the nine-coordinated X-O2, and the six-coordinated Y-O2, Z-O8, and Z′-O8 bonds-as they relate to the B-O2/B-O8 bond valence-suggest that the Z′-O8 bond has the most influence over the geometry of the (BO 3 ) triangle. This study highlights variations in what has otherwise been assumed to be a static site in the crystal structure of tourmaline-group minerals.

Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2M 1 , and its composition is linked to rock-compositional constraints. This work investigates the octahedral sheet crystal-chemical differences between the two phengite types. Seven dioctahedral micas were studied: (1) one 3T phengite from an ultrahigh-pressure metagranitoid in the Dora Maira massif, Italy (P ~ 4.3 GPa, T ~ 730 °C); (2) five 2M1 phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P ≤ 0.7 GPa, T ~ 500-600 °C); and (3) one 2M1 ferroan muscovite from pegmatite in Antarctica (P ≤ 0.2 GPa, T ~500 °C). All micas display significant extents of celadonite substitution. In particular, the 2M1-phengite formulae (calculated on the basis of 11 O) have 0.68 < IVAl < 0.82 atoms per formula unit (apfu); octahedral atoms are dominated by Al (1.6-1.8 apfu), with minor and variable Fe (0.20-0.35 apfu) and Mg (0.05-0.17 apfu), and very minor Ti, Mn, and Cr. Total octahedral occupancies are slightly above 2.00 apfu, i.e., there seems to be partial occupancy of the third M site. For all micas, we recorded XAFS spectra on mosaics of carefully separated flakes oriented flat on a plastic support that could be rotated so as to account for the polarization of the synchrotron radiation beam, and we processed them on the basis of the AXANES theory. Spectra show angle-dependent absorption variations for Al and Fe, which can be deconvoluted and fitted by dichroic effects. Pre-edges consistently show most Fe to be Fe 3+ and little angle-dependent intensity variations. The 2M 1 -ferroan muscovite from Antarctica displays the same AXANES behavior as 2M 1 -phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having IV Al = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the IV Al feature strongly reduced, in agreement with the increased Si content, but also Fe XAFS spectra show one broad feature only, indicating that all Fe is Fe 2+ in a fully disordered distribution with no angle-dependent variations. We conclude that this 3T-phengite is actually contaminated by exsolved Fe-bearing pyrope platelets, which cannot be resolved under SEM examination; by contrast, the 2M 1 -phengites, unrelated to high-pressure, suggest Al/Fe 3+ order over the M1 and (M2, M3) sites, as also does the 2M 1 pegmatitic muscovite.

Volcanic activity at Mt. Vulture lasted about 750 ka and produced SiO 2 -undersaturated volcanic rocks that can be classified as old (~700 ka), intermediate (~600-550 ka), and young (~130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), haüyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mössbauer) methods. EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): SiO 3 = 33.14-38.01, Al 2 O 3 = 15.56-20.45, MgO = 13.02-20.81, FeO tot = 6.34-14.08, TiO 2 = 2.34-6.02, K 2 O = 6.03-9.48, Na 2 O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded H 2 O = 2.86 ± 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT191_2 and VUT191_19, which yielded values of 3.81 ± 0.12 and 1.72 ± 0.08 wt% H 2 O, respectively. Mössbauer investigation showed that all the iron in VUT191 mica is octahedral with Fe 2+ = 25.5% and Fe 3+ = 74.5%, confirming that Vulture micas are particularly Fe 3+ -rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 ≤ R ≤ 3.17, 2.09 ≤ Rw ≤ 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) Å. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [ VI M 2+ + 2(OH) - ↔ VI Ti 4+ + 2O 2- + H 2 ] and M 3+ -oxy [VIM 2+ + (OH)- ↔ VI M 3+ + O 2- + ½H 2 , with M 3+ = Fe 3+ , Al 3+ ] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 VI M 2+ ↔ VI Ti 4+ + VI □ and 3 VI M 2+ ↔ 2 VI M 3+ + VI □ occur.

A 50 m drill core (core no. 383-334) collected from the contact metamorphic aureole in the Mouat Ni-Cu prospect in the Mountain View area of Stillwater County, Montana, exhibits evidence of in situ fluid-absent biotite dehydration melting and local back reaction of melts derived from metasediments and metavolcanics. The drill core was investigated to characterize the mineral assemblages and their textures, to search for evidence of partial melting, and to determine mineral and whole-rock geochemistry to provide an understanding of the petrogenesis of the hornfels close to the contact with the Stillwater Complex. The rocks investigated are predominantly quartz-plagioclase-cordieriteorthopyroxene- biotite ± spinel hornfels, Fe-Cu-Ni sulfides, and Fe-Ti oxides. One sample contains garnet (Alm 73-77 Pyr 20-17 Sps 3 Grs 4 ) in addition to cordierite. Three samples contain cordierite-hercynite symplectites surrounded by a narrow mantle devoid of orthopyroxene, plagioclase, and biotite that are roughly ovoid to lenticular in shape. Partially melted samples show back reaction between crystallizing melt and restite such that retrograde skeletal intergrowths of biotite and quartz in the mesosome were developed. Average T-P conditions of hornfels and melt genesis are 786 °C and 3.7 kbar. Back reactions that produced biotite occurred at somewhat lower temperatures. Whole-rock geochemistry suggests that the protolith of the hornfelsic rocks was likely a mixture of greywacke and additional intermediate to mafic volcanogenic components.

Linear correlations of lnP vs. the thickness, t, of the TOT mica module and vs. the ditrigonal distortion, α, have been established by analyzing a population of 43 natural phengites for which estimates of the crystallization pressure P independent of the Si content are available. Synthetic phengites are not included because they behave as a separate population. The equations for the resultant regression lines are lnP = -0.35(3)α + 5.0(3) (R = 0.90, 26 observations) and lnP = -29(2)t + 293(22) (R = 0.90, 43 observations). These linear correlations are evidence of the links involving P, chemical composition, structural parameter c (i.e., t), and α. In turn, there is a linear correlation between t and α: t = 0.0110(9)α + 9.859(7) (R = 0.92, 26 observations). The linear correlations of lnP, t, and α vs. Si content have equations: lnP = 4.2(4)Si - 12(1) (R = 0.86, 43 observations), t = -0.13(1)Si + 10.37(4) (R = 0.87, 43 observations), and α = -12.7(9)Si + 50(3) (R = 0.94, 26 observations). The observed dispersion of the data are mainly due to the uncertainty in the pressures estimated from mineral assemblages. The possibility of calculating P from only a measurement of c neither overcomes this type of uncertainty nor pretends to be competitive with petrologic geobarometers, but is does open the possibility of estimating the crystallization pressure of nanoscale phengite relics (e.g., by transmission electron diffraction).

The average structure, space group Pnnm [subcell: A = 5.666(16), B = 18.844(5), C = 3.728(11) Å, V = 398.0(2) Å 3 , Z = 1], of the new mineral dovyrenite Ca 6 Zr[Si 2 O 7 ] 2 (OH) 4 has been refined from single-crystal X-ray data to R = 7.97%. The modular structure of dovyrenite is build by alternate stacking of Ca-polyhedral layers characteristic of the tobermorite structure and octahedral layers with attached disilicate groups known from the rosenbuschite group of minerals. No indications of ordered polytypes were detected for the potential OD-structure. Either the small crystal size producing only weak diffraction intensities did not allow detecting diffuse diffraction features (or “super-structure” reflections) or the structure is build by disordered stacks of OD layers. Nevertheless, the resolved average structure allowed unraveling the possible order patterns within the rosenbuschite-like octahedral layers. The key for understanding the polytypic character of this structure is the short periodicity of the tobermorite-like Ca polyhedral layer of only 3.73 Å along c, whereas the periodicity of the attached rosenbuschite-like octahedral layer is doubled. In dovyrenite Ca occurs in sixfold-, sevenfold-, and eightfold-coordination. The octahedral Ca site is only half occupied and may reveal additional vacancies, which must be charge balanced by disordered OH-groups replacing O. A corresponding modular structure with the same subunits but different composition and without octahedral vacancies exists for rinkite (Ti,Nb,Al,Zr)(Na,Ca) 3 (Ca,Ce) 4 [Si 2 O 7 ] 2 (O,F) 4 , which has hitherto been considered as heterophyllosilicate.

Magnetotactic bacteria are known to mediate the formation of intracellular magnetic nanoparticles in organelles called magnetosomes. These magnetite crystals are formed through a process called biologically controlled mineralization, in which the microorganisms exert a strict control over the formation and development of the mineral phase. By inducing magnetite nucleation and growth in resting, Fe-starved cells of Magnetospirillum gryphiswaldense, we have followed the dynamics of magnetosome development. By studying the properties of the crystals at several steps of maturity, we observed that freshly induced particles lacked a well-defined morphology. More surprisingly, although the mean particle size of mature magnetosomes is similar to that of magnetosomes formed by constantly growing and Fe-supplemented bacteria, we found that other physical properties such as crystal-size distribution, aspect ratio, and morphology significantly differ. Correlating these results with measurements of Fe uptake rates, we suggest that the expression of different faces is favored for different growth conditions. These results imply that the biological control over magnetite biomineralization by magnetotactic bacteria can be disturbed by environmental parameters. Specifically, the morphology of magnetite crystals is not exclusively determined by biological intervention through vectorial regulation at the organic boundaries or by molecular interaction with the magnetosome membrane, but also by the rates of Fe uptake. This insight may contribute to better define biomarkers and to an improved understanding of biomineralizing systems.

In-situ Raman measurements using a gated spectroscopy system revealed irreversible changes at 800-1000 °C in a natural red spinel (with 2 cation mol% Cr and 1 cation mol% Zn) and at 1100-1200 °C in a natural clear spinel (without Cr or Zn). Our observations of rapid broadening of a mode at 409 cm −1 and the appearance of two weak modes at 210 and 520 cm −1 at the transition temperature confirm the association of these features with cation disordering proposed by previous quench studies. Furthermore, we found that the frequencies of modes at 313 and 666 cm −1 change at the transition temperature. The discontinuous frequency decrease of the mode at 313 cm −1 and the increase in the frequency of the mode at 666 cm −1 can be explained by the entrance of heavier Al atoms into the tetrahedral sites and the entrance of lighter Mg atoms into the octahedral sites, respectively. Our study demonstrates that in-situ Raman spectroscopy is a powerful tool for studying cation disordering in spinel-structured minerals at high temperature.

We have performed first-principles calculations to investigate the behavior of the hydrogen bond in δ-AlOOH under pressure. The highest OH-stretching A1 and B2 mode frequencies decrease under pressure leading to hydrogen bond symmetrization. After hydrogen bond symmetrization, the corresponding frequencies gradually increase. This softening and subsequent hardening of the OH bonds is a good spectroscopic indicator of hydrogen bond symmetrization and is observed in our GGA static calculations at ~30 GPa without considering tunneling effects. We have also carried out calculations of Raman peak intensities in several supercells with various hydrogen orderings to investigate the potential effect of H-disorder on the Raman spectrum of δ-AlOOH. Our results suggest that the four broad Raman bands observed experimentally in the range of OH-stretching mode frequencies could originate in H-disorder in this phase.

A Mössbauer study of synthetic leucophosphite, KFe 2 (PO 4 ) 2 (OH)·2H 2 O is reported. The sample was prepared by the reaction of synthetic goethite (α-FeOOH) with a K-phosphate solution of pH 2 at 373 K for 20 days. The obtained cell parameters are a = 9.771(1), b = 9.675(2), c = 9.747(2) Å, β = 102.45(1)°. The Mössbauer spectra from 295 K down to 40 K show the existence of two Fe 3+ doublets with ΔE Q1 ≈ 0.62 mm/s and ΔE Q2 ≈ 0.91 mm/s (at 80 K), respectively, whereas for lower temperatures the spectra are composed of two sextets with H hf1 ≈ 48.3 T and H hf2 ≈ 48.8 T at 4.2 K. The magnetic transition temperature was determined to be 36 K from a Mössbauer thermoscan experiment. The magnetic ordering is presumed to be antiferromagnetic. The temperature variations of the hyperfine fields cannot be explained by the simple molecular field approximation. Using a model that takes into account the exchange magnetostriction associated to a non-second-order transition, an excellent reproduction of the H hf (T) curves was obtained.

Grains of rhönite have been discovered in magmatic inclusions in augite grains of the lunar regolith from Mare Crisium, returned to Earth by the Russian Luna 24 spacecraft. These rhönite grains are up to 8 μm long, pleochroic from tan to dark brown, and associated with ulvöspinel and silica-rich glass. Electron microprobe analysis gives a composition near end-member ferroan rhönite: (Ca 1.9 Mn 0.0 Na 0.1 ) (Fe 2+ 4.5 Mg 0.1 Al 0.3 Cr 0.0 )Ti 1.0 (Si 4.0 Al 2.0 )O 20 . The Raman spectrum of these grains is like those of terrestrial rhönites, and distinct from titanian amphiboles. Compositionally, rhönite plus silica plus water or halogens (F, Cl) is equivalent to titanian amphibole plus pyroxene, so the presence of rhönite in lunar basaltic rock is consistent with the known low abundance of volatiles in the Moon. When calibrated, mineral reactions involving rhönite and titanian amphibole may provide quantitative constraints on fugacities of water, F, and Cl in basaltic magmas from the Moon and other planetary bodies.

FeS exhibits extensive polymorphism at high pressure and temperature. All with NiAs-type (B8) or closely related structures. Here we report a new phase transition from FeS VI to CsCl-type (B2) phase (FeS VII) above 180 GPa based on the synchrotron X-ray diffraction (XRD) measurements. A significant volume reduction by 3.0% was observed at the phase transition, due to an increase in the coordination number from six to eight. Present results suggest that a substantial amount of sulfur may be incorporated into an Fe-Ni alloy with bcc structure in the Earth’s inner core.

Direct structural characterization of the changes in the local environment of Mg occurring in the garnet structure as a function of the Ca content are determined by Mg K-edge X-ray absorption fine structure on synthetic samples along the pyrope-grossular join. With increasing Ca content, the short Mg-O2 distance of the dodecahedron slightly decreases, while the long Mg-O4 distance tends to increase, so that the dodecahedron is more distorted in grossular-rich garnets than in end-member pyrope. This quantitative direct description of the changes in the local environment of Mg in the pyrope-grossular solid solution confirms and better defines previous experimental and recent computational results.