Atoms Pfu Research Articles

Lead-tellurium oxysalts from Otto Mountain near Baker, California: X. Bairdite, Pb2Cu42+Te26+O10(OH)2(SO4)(H2O), a new mineral with thick HCP layers

Bairdite, Pb2Cu42+Te26+O10(OH)2(SO4)(H2O), is a new tellurate-sulfate from Otto Mountain near Baker, California, U.S.A. It occurs in vugs in quartz associated with khinite, cerussite, goethite, and hematite. It is interpreted as having formed from the partial oxidation of primary sulfides and tellurides during or following brecciation of quartz veins. Bairdite is monoclinic, space group P21/c, with unit-cell dimensions a = 14.3126(10), b = 5.2267(3), c = 9.4878(5) Å, b = 106.815(7)°, V = 679.41(7) Å3, and Z = 2. Bairdite occurs as diamond-shaped tabular crystals up to about 250 mm long and 5 mm thick, in subparallel and fan-shaped aggregates. The color is lime green, the streak is pale lime green, and the luster is adamantine. The Mohs hardness is estimated at between 2 and 3. Bairdite is brittle with an irregular fracture and one perfect cleavage on {100}. The calculated density based on the empirical formula is 6.062 g/cm3. Bairdite is biaxial (+), with calculated indices of refraction of a = 1.953, b = 1.966, and g = 2.039. The measured 2V is 47(2)°, dispersion is r < v, strong and the optical orientation is Y = b; Z ^ a = 34° in obtuse angle b. The pleochroism is strong: Z (pale green) <<< X (green) < Y (green). Electron microprobe analyses (average of 4) provided: PbO 34.22, CaO 0.06, CuO 23.80, TeO3 26.34, SO3 5.74, H2O 2.81 (structure), total 92.97 wt%. The empirical formula (based on 17 O atoms pfu) is: Pb2.05Ca0.01Cu2+3.99Te6+2.00S0.96O17.00H4.16. The eight strongest powder X-ray diffraction lines are [dobs in Å (hkl) I]: 4.77 (110,102) 50, 4.522 (002,011,111) 66, 3.48 (multiple) 62, 2.999 (311,411) 97, 2.701 (502,113,213) 79, 2.614 (013,020) 100, 1.727 (multiple) 65, and 1.509 (911,033,324) 83. The crystal structure of bairdite (R1 = 0.072 for 1406 reflections with Fo > 4σF) contains edge-sharing chains of Te6+O6 and Cu2+O6 octahedra parallel to b that are joined by corner-sharing in the a direction, forming thick stair-step-like hexagonal close packed layers parallel to {100}. The polyhedral sheet has similarities to those in the structures of timroseite and paratimroseite. The thick interlayer region contains PbO10 polyhedra and half-occupied SO4 groups. Raman and infrared spectral data are presented

Rapidcreekite in the sulfuric acid weathering environment of Diana Cave, Romania

The Diana Cave in SW Romania develops along a fault line and hosts a spring of hot ( T avg = 51 °C), sulfate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H 2 S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulfuric acid produced by H 2 S oxidation/hydrolysis causes a strong acid-sulfate weathering of the cave bedrock generating a sulfate-dominated mineral assemblage that includes rapidcreekite, Ca 2 (SO 4 )(CO 3 )·4H 2 O closely associated with gypsum and halotrichite group minerals. Rapidcreekite forms bundles of colorless tabular orthorhombic crystals elongated along [001] and reaching up to 1.5 mm in length. For verifying the hydrogen bond scheme and obtaining crystal-chemical details of the carbonate group a single-crystal structure refinement of rapidcreekite was performed. Its unit-cell parameters are: a = 15.524(2), b = 19.218(3), c = 6.161(1) A; V = 1838.1(5) A 3 , Z = 8, space group Pcnb . Chemical composition (wt%): CaO 35.65, SO 3 24.97, CO 2 13.7, H 2 O 23.9, Na 2 O 0.291, MgO 0.173, Al 2 O 3 0.07, total 98.75%. The empirical formula, based on 7 non-water O atoms pfu, is: Ca 1.98 Na 0.029 Mg 0.013 Al 0.004 (S 0.971 O 4 )(C 0.97 O 3 )·4.13H 2 O. The δ 34 S and δ 18 O values of rapidcreekite and other cave sulfates range from 18 to 19.5‰ CDT and from −9.7 to 7.8‰ SMOW, respectively, indicating that the source of sulfur is a marine evaporite and that during hydration of the minerals it has been an abundant 18 O exchange with percolating water but almost no oxygen is derived from O 2(aq) . This is the first description of rapidcreekite from a cave environment and one of the very few natural occurrences worldwide. We also report on the mineral stability and solubility, parameters considered critical to understand the co-precipitation of carbonates and sulfates, a process that has wide applications in cement industry and scaling prevention.

Paseroite, PbMn2+(Mn2+,Fe2+)2(V5+,Ti,Fe3+,〈)18O38, a new member of the crichtonite group

Paseroite, ideally PbMn 2þ (Mn 2þ ,Fe 2þ )2(V 5þ ,Ti,Fe 3þ ,&)18O38, is a new mineral (IMA2011-069) from fossil wood in the upper part of the Molinello mine, Val Graveglia, Italy. Paseroite occurs in direct association with quartz, chalcocite, volborthite, metatyuyamunite and pyrophanite, and was also found as zones within V-rich senaite crystals with which it forms a solid-solution series. Paseroite forms as isolated submetallic, dark grey to black, elongated scalenohedral crystals between 50 and 100mm in length, with the forms {001} and {102} present. The tenacity is brittle, the fracture conchoidal, and the streak is black. The Vickers hardness is 847 kg mm � 2 (load 500g), which is equivalent to 6-6.5 on the Mohs scale. The calculated density is 4.315 g/cm 3 (on the basis of the empirical formula). In plane-polarised incident light, paseroite is greyish in colour, weakly bireflectant and non-pleochroic. Internal reflections are absent. Between crossed polars, paseroite is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) are: 18.4 %, 18.2 % (471.1 nm); 17.9 %, 17.7 % (548.3 nm); 17.6 %, 17.3 % (586.6 nm); and 17.0 %, 16.8 % (652.3 nm), respectively. The empirical formula, calculated on the basis of 38 O atoms pfu is: (Pb0.61Sr0.39)1.00 (V 5þ 7.78Ti 4þ 7.03Mn 2þ 1.86Fe 2þ 0.67Fe 3þ 0.37Zn0.24Na0.19U0.02Mg0.02&2.82)21.00O38. According to the structural results, the simplified formula is: PbMn 2þ (Mn 2þ ,Fe 2þ )2(V 5þ ,Ti,Fe 3þ ,&)18O38. Structurally, paseroite crystallises in the space group R3, with the unit-cell parameters a ¼ 10.3894(5), c ¼ 20.8709(8) A u , V ¼ 1950.98(15) A u 3 and Z ¼ 3. The crystal structure was refined to R ¼ 0.0234 for 632 reflections with Io . 2s(Io) and is isostructural with senaite and all other members of the crichtonite group. The eight strongest X-ray powder-diffraction lines (d in A u (I/I0 )( hkl)) are: 3.417 (100) (024), 3.012 (21) (300), 2.896 (61) (216), 2.858 (36) (214), 2.765 (27) (303), 2.260 (85) (144), 2.149 (65) (415) and 1.809 (57) (418). The name is after Marco Pasero (b. 1958), Professor of Mineralogy at the University of Pisa.

Chemistry, microstructure, and alpha decay damage of natural brannerite

To investigate the long-term stability of brannerite with respect to alpha decay damage and interaction with aqueous fluids, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually located within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. Based on a total of 3 metal cations, the Ti and U contents of unaltered brannerite range from 1.8 to 2.1 and 0.4 to 0.9 atoms per formula unit (apfu), respectively. Maximum amounts of the other major cations on the A-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. Maximum values of other cations on the B-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from an aqueous fluid phase, and up to 40–90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO 2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nm, indicating that brannerite is rendered completely amorphous by alpha decay damage. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have accumulated alpha decay doses in the range of 0.7–200 × 10 16 α mg −1. Although imprecise, the U–Th–Pb chemical ages determined by microanalysis are generally consistent with the known ages and geological histories of the brannerite host rocks. To a first approximation, it is possible to understand the results of chemical and isotopic dating of brannerite by treating each sample as a complex system composed of nominally unaltered, altered, and recrystallised areas. The dominant effects are U loss from altered areas and Pb loss from unaltered brannerite and, to a lesser extent, altered brannerite, the structure of which approximates to a Ti–Si–O glass-like network wherein Pb is more comfortable as a network modifier.

Early Mesozoic unroofing pattern of the Dabie Mountains (China): Constraints from the U-Pb detrital zircon geochronology and Si-in-white mica analysis of synorogenic sediments in the Jianghan Basin

This paper presents the results of an integrated U-Pb detrital zircon geochronology and Si-in-white mica analysis for synorogenic sediments in the Jianghan Basin to the south of the Dabie Orogen. The results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early Mesozoic Dabie Mountain. Si contents of detrital white micas range from 3.09 to 3.34 atoms pfu for the upper Triassic sandstones whereas 3.06 to 3.59 atoms pfu for the lower and middle Jurassic sandstones. The majority of detrital white micas in the lower Jurassic sandstones is phengitic and originated exclusively from the Dabie high- to ultrahigh- pressure rocks. The U-Pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late Triassic to early and middle Jurassic. For the upper Triassic sandstone, the U-Pb age clusters of these zircons are characterized by ~ 420-450 Ma, ~ 750-820 Ma, ~ 1050-1200 Ma and ~ 2500 Ma with minor Luliangian (~ 1700–2000 Ma) components. In contrast, the zircon ages of the Jurassic sandstones are dominated by the Luliangian (~ 1700–2000 Ma) ages with only minor Caledonian (~ 420-450 Ma) and Greenville (~ 1050-1200 Ma) ages. In combination with other available geological data, it can be concluded that the Dabie HP-UHP rocks might initially be exposed to the surface at the beginning of early Jurassic (~ 190 Ma). The Jiangnan terrain (also named “Jiangnan old continental in Chinese) to the south of the Jianghan basin provided the predominant supply of upper Triassic sediments, whereas the Paleoproterozoic Yangtze crustal materials (overlying the present Dabie Complex at the time) were the important provenance of the Jurassic sediments in the Jianghan basin.

Skorpionite, Ca3Zn2(PO4)2CO3(OH)2H2O, a new mineral from Namibia: description and crystal structure

Skorpionite, Ca 3 Zn 2 (PO 4 ) 2 CO 3 (OH) 2 ·H 2 O, is a new secondary mineral from the Skorpion zinc deposit in south-western Namibia. Associated minerals are tarbuttite, hydrozincite and gypsum. Skorpionite forms colourless needle-like crystals of monoclinic symmetry that are elongated parallel to [001] with dominant {100} and {110}; additional forms are {111}, {221}, {131} and {−101}. D (meas.) = 3.15(3) g/cm 3 ; D (calc.) = 3.17 g/cm 3 . Skorpionite is optically biaxial negative, α = 1.5884(10), β = 1.6445 (calc.), γ = 1.6455(10), 2V = 15.0° (5) (589 nm); no dispersion. Optical orientation is Y ∧ c = 26° (in acute β); Z = b . Means of 17 sets of microprobe analyses gave CaO 30.89, ZnO 28.83, P 2 O 5 25.49, CO 2 (calc.) 7.96, H 2 O (calc.) 6.52, total 99.69 wt.%. The empirical formula (based on 14 oxygen atoms pfu ) is Ca 3.05 Zn 1.96 (PO 4 ) 1.99 (CO 3 ) 1.00 (OH) 2.06 ·0.98H 2 O. Single-crystal X-ray investigations gave space group C 2/ c , with unit-cell parameters a = 19.045(3), b = 9.320(2), c = 6.525(1) A, β = 92.73(2)°, V = 1156.9(4) A 3 , Z = 4. ZnO 4 and PO 4 tetrahedra are corner linked to corrugated sheets. The crystal structure forms a three-dimensional network of [Ca2 2 Zn 2 (OH) 2 (PO 4 ) 2 ] 0 and [Ca1(CO 3 )(H 2 O)] 0 layers that are linked by hydrogen bonds and by Ca–O bonds.

Single-crystal high-pressure electronic absorption spectroscopic study of natural orthopyroxenes

Unpolarized, (X, Y)-mixed electronic absorption spectra were measured on (001) plates of two natural orthopyroxenes with 24 and 45 mole % ferrosilite and with a Ca content ≤0.07 atoms pfu, at pressures up to 10.3 and 8.83 GPa, using DAC-techniques. Polarized spectra could not be obtained as the plane of linearly polarized light is lost due to strain-induced birefringence in the diamond anvils at pressures above 2.5 GPa. The (X, Y)-mixed electronic spectra exhibit a slightly pressure-dependent UV-edge at around 25000 cm −1 . There are also two strong bands a and d at ∼11000 and ∼5000 cm −1 , respectively, caused by spin-allowed transitions 5 A 1 → 5 A 1 and → 5 B 1 of Fe 2+ in the highly distorted (M2)O 6 polyhedra with site symmetry 1 of the orthopyroxene structure. The corresponding Fe 2+ (M1) transitions expected to be observed in mixed spectra (Y, Z) of the (100) plates were not found due to their low intensity and smaller energy difference. Increasing pressure induces two effects in the electronic spectra of Fe 2+ in M2 sites: (i) A high energy shift of the two bands a and d , such that the mean energy, representing the crystal field parameter 10 Dq , is increasing which indicates local compression of the Fe 2+ (M2)O 6 polyhedra. The shift with pressure is +88 cm −1 GPa −1 for band a in the entire pressure range and +210 cm −1 .GPa −1 up to ∼7 GPa or +80 cm −1 .GPa −1 above that pressure for band d. This indicates a decrease in M2 polyhedral distortion for the Fe 2+ -centered M2 sites up to ∼7 GPa, but nearly unchanged distortion at higher pressures, (ii) This change of distortion is accompanied by a strong reduction of the linear as well as integral intensities in the range of applied pressures, of the two Fe 2+ (M2) bands to about 70 % of the values at ambient conditions. This effect is ascribed to a gradual reduction of the odd crystal field components of Fe 2+ (M2), i.e. to a gradual development of the polyhedral geometry towards a regular octahedron or — at least — a centrosymmetric one. Distinct breaks in both dependences, (i) and (ii), at pressures of 6–7 GPa, are interpreted as a change in the compression mechanism of the orthopyroxene structure.

When did the ultrahigh-pressure rocks reach the surface? A 207Pb/ 206Pb zircon, 40Ar/ 39Ar white mica, Si-in-white mica, single-grain provenance study of Dabie Shan synorogenic foreland sediments

A single-grain study of synorogenic, Triassic–Jurassic quartz arenites of the eastern and southern forelands of the ultrahigh-pressure (UHP) Hong'an–Dabie orogen illustrates the utility and sensitivity of three provenance analysis techniques. 40Ar/ 39Ar detrital white mica ages indicate that the Middle Triassic stratigraphic level reached ∼400 °C at ∼125 Ma, probably heated by latest Jurassic–Early Cretaceous intrusions. Un-reheated single grains are as young as the depositional age of their sedimentary hosts and yield a Triassic–Jurassic age maximum that indicates a Hong'an–Dabie source, and some Paleozoic and Proterozoic ages that are mostly younger than 207Pb/ 206Pb detrital zircon ages. Si contents of detrital white micas from the same samples range from 3.0 to 3.6 atoms pfu, with ∼70% of the micas having <3.3 Si atoms pfu. The grains with 3.3–3.6 Si atoms pfu probably originated from Hong'an–Dabie, indicating initial exposure of high-pressure (HP) and even ultrahigh-pressure rocks during the Middle Jurassic. The single-grain evaporation 207Pb/ 206Pb ages of detrital zircons provide little indication that one of Earth's most spectacular unroofing processes was active in the adjacent orogen. Proterozoic and Silurian zircon ages abound, indicating that the detritus originated from the northern margin of the Yangtze craton and regions to the southeast, in particular the South China fold belt and the Cathaysia block, but only a few Triassic zircon ages, indicating a Hong'an–Dabie source, were measured. The Triassic signal is smallest because growth of metamorphic zircon is rare and is mostly limited to rim formation on pre-existing zircons, and because neither a magmatic arc nor syn- to post-collisional magmatism have been mapped in the Hong'an–Dabie orogen. Based on the white mica ages and the Si-in-white mica analysis, the Hong'an–Dabie orogen provided as much as 80% of the foreland detritus. The ages of the oldest micas suggest that exhumation of Hong'an–Dabie began at 240±5 Ma and thus the average exhumation rates exceeded 2 mm/year during the Middle Triassic to Middle Jurassic.

Another step toward understanding the true nature of sartorite: Determination and refinement of a ninefold superstructure

Single-crystal X-ray diffraction data, collected from a sartorite crystal from Lengenbach (Binntal, Valais, Switzerland), yielded a ninefold superstructure: space group P2 1 /c; a = 37.71(2), b = 7.898(3), c = 20.106(8) Å, β = 101.993(7)°; R 1 = 6.08% for 6293 reflections with I &gt; 2σ I . The sample is an N 1,2 = 3,3 sartorite homologue and the refined formula Pb 8 Tl 1.5 As 17.5 S 35 compares very well with the empirical formula Pb 8.2 Tl 1.4 As 17.5 Sb 0.5 S 35 obtained from electron microprobe analyses. It has a high Tl content, up to 6.5 wt%. In a coupled substitution approximately 1.5 Tl + replace about 0.5 As 3+ and 1 PbS. The refined structure has 35 S atoms pfu instead of the expected 36 S (= 9 × 4 S, from PbAs 2 S 4 ). The incorporation of substantial amounts of Tl + into PbAs 2 S 4 is essential for the type and periodicity of superstructures in sartorite. The refined superstructure can be interpreted as a socalled “lock-in” structure with a composition that yields a commensurate lattice for a mineral that usually has an incommensurate lattice. No commensurate periodicity could be found for a second crystal with about 0.5 Tl apfu.

Enstatite–sapphirine crack-related assemblages in ultrahigh-pressure pyrope megablasts, Dora-Maira massif, western Alps

Throughout the ultrahigh-pressure (UHP) metamorphic unit of the Dora-Maira massif, western Alps, pyrope megablasts contain the typical assemblage clinochlore–kyanite–talc–rutile ± phlogopite ± ellenbergerite as prograde inclusions. In the upper part of the UHP unit in Val Gilba, some megablasts (XMg=0.89–0.98) contain in addition polymineralic inclusions consisting of various combinations of enstatite, gedrite, sapphirine, clinochlore, talc, magnesiostaurolite and rare corundum or spinel. We present evidence that these assemblages developed from cracks running across the megablasts, and are therefore of late origin, post-dating the highest-pressure stage. Enstatite (XMg=0.94–0.99) contains 0.7 to, typically, 3 wt% Al2O3, but up to 8.4 wt% in the presence of sapphirine. Sapphirine (XMg=0.96–0.998, Be-free) shows the largest compositional variations, with Si contents ranging from 1.7 to at least 2.1 atoms pfu, thereby clearly exceeding the 2:2:1 stoichiometry. The late-stage talc contains up to 4 wt% Al2O3, 0.35 wt% Na2O and 0.6 wt% F; gedrite 1.1–2.9 wt% Na2O and up to 0.36 wt% F. The successive development within pyrope of alternative hydrous assemblages involving first enstatite plus an Al-rich phase (kyanite, sapphirine, magnesiostaurolite) ± clinochlore, then a gedrite compositionally close to pyrope, then talc plus an Al-rich phase (sapphirine, corundum), is a clear record of decompression. However, the temperature conditions implied under the assumption of high H2O activity are 100 to 150 °C higher than, and so inconsistent with existing constraints on the decompression path. These constraints are in particular the stability of talc + phengite in the matrix assemblage during decompression, and the absence of regional evidence for a granulite-facies event. This inconsistency can only be alleviated if H2O activity inside the garnet megablast was (or became) considerably reduced with respect to that in the matrix. Fluid influx into an opening fracture in garnet, sealing of the fracture by breakdown products of pyrope and continued evolution under closed-system conditions may have led to increasing solute concentration and such low H2O activity within the garnet megablast, driving the microsystem toward fluid-absent conditions. Micrometre-size inclusions of Ca-sulfate and crandallite-type compounds in minerals of these reactive areas may be evidence for such residual brines and suggest that these were phosphate- and sulfate- rather than halide-dominated. This finding is additional evidence for the very local control that fluid composition and H2O activity may have on the occurrence of granulite-facies assemblages, regardless of temperature. It highlights the role of deformation (here fracturing) in triggering reactions in otherwise unreactive systems. It also shows how carefully inclusion- to-host relationships have to be considered, post-growth reaction within the host being more common than hitherto reported.

Composition, Geochemical Alteration, and Alpha-Decay Damage Effects of Natural Brannerite

To investigate the long-term alteration behavior of brannerite, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. SEMEDX analyses of unaltered areas indicate that the chemical formulae may deviate from the ideal stoichiometry. The U content ranges from 0.45 to 0.88 atoms per formula unit (pfu). Maximum amounts of the other major cations on the U-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. The Ti content ranges from 1.86 to 2.10 atoms pfu. Maximum values of other cations on the Ti-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from the fluid phase, and up to 40-90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO 2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nmi, indicating complete amorphization of the brannerite. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have average accumulated alpha-decay doses of 2-170 × 10 16 alphas/mg. Our results indicate that brannerite is subject to amorphization and may lose U under certain P-T-X conditions, but the overall durability of the titanate matrix remains high.

Mineral chemistry and parageneses of magnetoplumbite from the Filipstad district, Sweden

Electron-microprobe analyses of magnetoplumbite (formally PbFe 12 O 19 ) from La˙ngban-type deposits in the Filipstad district, Sweden, demonstrate that the mineral is highly variable in composition with ranges: PbO 16.1-19.8, BaO 0.1-1.5, Sb 2 O 5 0.0-9.0, ZnO 0.0-2.1, Fe 2 O 3 37.0-64.0, MnO 2.8-9.2, Mn 2 O 3 0.0-22.7, Cr 2 O 3 0.0-1.0, TiO 2 0.0-10.2, Al 2 O 3 0.0-2.6, MgO 0.1-0.8 (all in wt.%). The major mechanisms of substitution are Mn 3+ = Fe 3+ , Mn 2+ + Ti 4+ = 2 Fe 3+ and 2 Mn 2+ + Sb 5+ = 3 Fe 3+ . Ba is the only significant substituent for Pb with up to 0.12 atoms pfu, and the ratio between large and small (interstitial) cations is ideal within the analytical error limits

Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals

A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na2O)-Al2O3-TiO2-SiO2-H2O at PH20 = Ptot between 25 and 30 kbars and 975 and 1000° C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 μm in diameter, were studied by X-ray, electron-microprobe and FTIR-microscope spectrometric techniques. The pyrope crystals were colourless when hem/mt or mt/wu buffers were used during the synthesis, and pale blue with the wu/iron buffer and in unbuffered runs. Sodium was not found in the synthetic crystals, titanium was always near 0.06 Ti atoms pfu, independent on the Ti-excess used in the starting material. A substitution Al2+[6]+Si4+[4]+4O2-= Ti4+[6]+□[4]+[(OH)3O^5-, providing charge balance for octahedral Ti4+-substitution is found to be compatible with all properties (number, widths, position, integrated intensity) of the stretching vibrations of defect hydroxyls, which have energies 3684, 3568, 3525 cm-1.

Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer

There is currently a dearth of reliable thermobarometers for many hornblende and plagioclase-bearing rocks such as granitoids and amphibolites. A semi-empirical thermodynamic evaluation of the available experimental data on amphibole+plagioclase assemblages leads to a new thermometer based on the Aliv content of amphibole coexisting with plagioclase in silica saturated rocks. The principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is \(\left( {Na\square _{ - 1} } \right)^A \left( {AlSi_{ - 1} } \right)^{T1}\). We have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions $$1. Edenite + 4 Quartz = Tremolite + Albite$$ (1.) $$2. Pargasite + 4 Quartz = Hornblende + Albite.$$ (2.) The equilibrium relation for both (1) and (2) leads to the proposed new thermometer $$T = \frac{{0.677P - 48.98 + Y}}{{ - 0.0429 - 0.008314 ln K}} and K = \left( {\frac{{Si - 4}}{{8 - Si}}} \right)X_{Ab}^{Plag} ,$$ where Si is the number of atoms per formula unit in amphiboles, with P in kbar and T in K; the term Y represents plagioclase non-ideality, RTlnγab, from Darken's Quadratic formalism (DQF) with Y=0 for Xab>0.5 and Y=-8.06+25.5(1-Xab)2 for Xab<0.5. The best fits to the data were obtained by assuming complete coupling between Al on the T1 site and Na in the A site of amphibole, and the standard deviation of residuals in the fit is ±38°C. The thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende-plagioclase assemblages with uncertainties of around ±75° C for rocks equilibrated at temperatures in the range 500°–1100° C. The thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than An92 and with amphiboles containing less than 7.8 Si atoms pfu. Good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks. Our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry.