The inherent displacive flexibility of the hexacelsian tetrahedral framework and its relationship to polymorphism in Ba-hexacelsian

 The results of a rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the tetrahedral framework of the ideal hexacelsian structure type are presented. One of the three types of RUM found to exist is characterized by modulation wave vectors perpendicular to <110> and atomic displacement patterns involving tetrahedral rotation around the parent c axis while a second type of RUM is found to be soft at any modulation wave vector and to involve tetrahedral rotation about in-plane rotation axes. It is shown how a combination of these two types of RUM motion associated with the same q=1/2<1ˉ101>^* modulation wave vector enables the outstanding crystal chemical problems and apparently mutually contradictory results as regards polymorphism in Ba-hexacelsian to be resolved. Received: 30 December 1999 / Accepted: 16 May 2000     

Plastic deformation of metamorphic pyrite: new evidence from electron-backscatter diffraction and forescatter orientation-contrast imaging

Application of new scanning electron microscope techniques to the study of deformed metamorphic pyrite reveals evidence for plastic deformation not readily recognised by more traditional methods. Specifically, use of forescatter solid-state detectors in conjunction with tilted polished specimens of pyritic ore produces high quality crystallographic orientation contrast images, which map the distribution of deformation domains within grains. Use of electron-backscatter diffraction allows quantification of the crystallographic misorientations shown by the orientation contrast images. Combination of these techniques shows that the pyrite studied deforms by slip on {100} and more rarely {110} systems. Slip is often associated with distributed rotation of up to 20° about <100> and more rarely <110> axes. Pyrites may have simple histories involving rotation about a single <100> axis, or more complex histories involving rotation about different <100> axes, and more rarely <110>, in different domains of the same pyrite grain, or sequential rotations about quite different systems, typically distributed rotation about <100> followed by discrete rotation about a non-crystallographic axis. Received: 25 June 1997 / Accepted: 14 May 1998     

Thermal expansion and structural transformations of stuffed derivatives of quartz along the LiAlSiO4–SiO2 join: a variable-temperature powder synchrotron XRD study

 The structural behavior of stuffed derivatives of quartz within the Li_{1− x} Al_{1− x} Si_{1+ x} O₄ system (0 ≤ x ≤ 1) has been studied in the temperature range 20 to 873 K using high-resolution powder synchrotron X-ray diffraction (XRD). Rietveld analysis reveals three distinct regimes whose boundaries are defined by an Al/Si order-disorder transition at x=∼0.3 and a β–α displacive transformation at x=∼0.65. Compounds that are topologically identical to β-quartz (0 ≤ x < ∼0.65) expand within the (0 0 1) plane and contract along c with increasing temperature; however, this thermal anisotropy is significantly higher for structures within the regime 0 ≤ x < ∼0.3 than for those with compositions ∼0.3 ≤ x < ∼0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structures (0 ≤ x < ∼0.3). The phases with ∼0.65 ≤ x ≤ 1 adopt the α-quartz structure at room temperature, and they display positive thermal expansion along both a and c from 20 K to their α–β transition temperatures. This behavior arises mainly from a rotation of rigid Si(Al)-tetrahedra about the <100> axes. Landau analysis provides quantitative evidence that the charge-coupled substitution of Li+Al for Si in quartz dampens the α–β transition. With increasing Li+Al content, the low-temperature modifications exhibit a marked decrease in spontaneous strain; this behavior reflects a weakening of the first-order character of the transition. In addition, we observe a linear decrease in the α–β critical temperature from 846 K to near 0 K as the Li+Al content increases from x=0 to x=∼0.5. Received: 26 June 2000 / Accepted: 1 December 2000     

Phase relations of calc-silicate assemblagesin the Auerbach marble, OdenwaldCrystalline Complex, Germany

Summary In the Odenwald Crystalline Complex, calc-silicate rocks are concentrated at the margins of the marble layer of Auerbach. They were presumably formed by metasomatic exchange between the calcite marble and the neighbouring granodioritic and quartz-dioritic intrusives. The investigated samples contain the characteristic mineral assemblages: garnet + clinopyroxene + epidote/clinozoisite + calcite + quartz ± titanite (1) and wollastonite + clinopyroxene + garnet + calcite ± quartz ± epidote/clinozoisite ± titanite (2). Microprobe analyses revealed the following compositional ranges: garnet grs_40–98adr_2–55alm_<5.5sps_<5.5pyp_<1; clinopyroxene di_46–88hed_9–47joh_0–5cats_0–6; epidote/clinozoisite ps_20–80. Different phase diagrams were calculated in the system CaO-MgO-Al₂O₃-TiO₂-SiO₂-CO₂-H₂O (CMATSCH) to decipher the P-T-X_CO2 parameters of metamorphism: isobaric T-X_CO2 sections and a P-T projection with mixed volatiles. The phase diagrams illustrate that the observed assemblages can only form in the presence of an H₂O-rich volatile phase. The assemblages are stable over a large range of temperatures, from 580 °C to < 400 °C (at 4 kbar) and at X_CO2 values of less than 0.055 (at 4 kbar). Higher temperatures can be inferred from reaction textures which indicate that garnet + plagioclase (T > 580 °C, at 4 kbar) and wollastonite + plagioclase (T > 660 °C, at 4 kbar) coexisted during an early metamorphic stage. A minimum pressure of 3.5 kbar can be inferred for the early high-temperature stage. Furthermore, on the basis of the calculated phase diagrams, combined with modal abundances in thin sections, it is possible to evaluate fluid behaviour; in the investigated specimens, infiltration of fluids from an external reservoir occurred. A minimum fluid:rock ratio of 3.6:1 can be estimated.

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Zusammenfassung Phasenbeziehungen in Kalksilikat-Paragenesen des Marmorzuges von Auerbach, Odenwald-Kristallin-Komplex, Deutschland Im kristallinen Odenwald konzentrieren sich kalksilikatische Gesteine in den Randbereichen des Marmorzuges von Auerbach. Die kalksilikatischen Partien wurden vermutlich duch metasomatischen Austausch zwischen dem Calcit-Marmor und benachbarten Granodioriten und Quarzdioriten gebildet. Die untersuchten Proben enthalten die charakteristischen Mineralparagenesen: Granat + Klinopyroxen + Epidot/Klinozoisit + Calcit + Quarz + Titanit (1) und Wollastonit + Klinopyroxen + Granat + Calcit ± Quarz ± Epidot/Klinozoisit ± Titanit (2). Mikrosondenanalysen ergaben folgendes Zusammensetzungsspektrum: Granat grs_40–98adr_2–55alm_<5.5sps_<5.5pyp_<1; Klinopyroxen di_46–88hed_9–47joh_0–5cats_0–6s; Epidot/Klinozoisit ps_20–80. Verschiedene Phasendiagramme wurden für das Modellsystem CaO-MgO-Al₂O₃-TiO₂-SiO₂-CO₂-H₂O (CMATSCH) berechnet, um die P-T-X_CO2-Parameter der Metamorphose einzugrenzen: Isobare T-X_CO2-Schnitte und eine P-T-Projektion mit einer Fluid-Mischphase. Die Phasendiagramme verdeutlichen, da? die beobachteten Paragenesen nur in Anwesenheit eines H₂O-reichen Fluids gebildet werden k?nnen. Die Paragenesen sind über einen gro?en Temperaturbereich hinweg stabil, von 580 °C bis < 400 °C (bei 4 kbar) und bei X_CO2-Gehalten von < 0.055 (bei 4 kbar). Ursprünglich h?here Temperaturen k?nnen anhand von Reaktions-Texturen rekonstruiert werden, die zeigen, da? Granat + Plagioklas (T > 580 °C, bei 4 kbar) und Wollastonit + Plagioklas (T > 660 °C, bei 4 kbar) w?hrend eines früheren Metamorphosestadiums koexistierten. Ein Minimaldruck von 3.5 kbar kann für dieses frühe Hochtemperatur-Stadium abgeleitet werden. Mit Hilfe der berechneten Phasendiagramme, in Kombination mit beobachteten Modalgehalten, ist es m?glich, das Verhalten der fluiden Phase abzusch?tzen. Die untersuchten Gesteine implizieren Fluidinfiltration, wobei ein minimales Verh?ltnis Fluid:Gestein von 3.6:1 abgesch?tzt werden kann.

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Received July 29, 1999; accepted March 28, 2000     

Rare earth and trace elements in igneous and high-temperature metamorphic minerals of oceanic gabbros (MARK area, Mid-Atlantic Ridge)

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amphiboles of six gabbroic samples from Holes 921–924 of the Ocean Drilling Program Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge–transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions. The metagabbros underwent granulite to subgreenschist facies conditions through three main tectono-metamorphic phases: (1) ductile regime (750 < T < 1000 °C and P ≈ 0.3 GPa); (2) transitional regime (600 < T < 700 °C and P ≈ 0.2 GPa); (3) brittle regime (350 ≤ T < 600 °C and P < 0.2 GPa). Igneous clinopyroxenes show C1-chondrite normalized patterns depleted in LREE, and nearly flat for HREE. The rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as REE reservoirs, and comprise the main contribution to the overall rock content. The abundances in igneous minerals reflect the degree of fractionation of the parent liquids. In metamorphic clinopyroxenes recrystallized in anhydrous assemblages, the REE and trace elements patterns mimic those of the primary ones. Conversely, clinopyroxene re-equilibrated in amphibole-bearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr and Ba. An overall increase of REE and some trace elements is evident in hydrous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196–22844 ppm), LREE depleted patterns and La_N/Sm_N=0.10–0.33, La_N/Yb_N=0.10–0.30. Amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for Ti_N and negative anomalies for Sr_N and Zr_N. Volatiles in amphibole are low, with Cl/F < 1; H₂O% is significantly lower than the stoichiometric ratio (1.33–1.53%). The composition of the clinopyroxene and amphibole recrystallized in low-strain domains records evidence of incomplete re-equilibration, and element diffusion and partitioning is in part controlled by the textural site. The possible origins of the fluids involved in the metamorphic recrystallization are discussed: (1) remobilization from igneous amphibole; (2) exsolution from evolved melts; (3) introduction of seawater-derived fluids modified in rock-dominated systems; (4) injection of highly evolved hydrous melts during the metamorphic process. Received: 6 September 1999 / Accepted: 6 March 2000     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

TEM evidence of perovskite–brownmillerite coexistence in the Ca(AlxFe1−x)O2.5 system with minor amounts of titanium and silicon

Observations by transmission electron microscopy (TEM) of the submicrometer phases present in calcium aluminate cements have shown that Ca-Al-Fe oxides coexist in two forms with brownmillerite (b) and perovskite (p) structures, respectively. Homogeneous single crystals of both brownmillerite and perovskite have been observed but exsolved lamellae also occur on the scale of tens of nanometers. Perovskite lamellae in brownmillerite exhibit coherent interfaces with an almost perfect [1 0 1]_b = {1 0 0}_p topotactic relationship. Energy-dispersive X-ray spectroscopy (EDXS) measurements show that perovskite lamellae are enriched in Ti and Si relative to the brownmillerite lamellae. The perovskite phase may accommodate up to 0.17 Si atoms per formula unit, but the exsolution process seems mainly to concern the Ti content. It is estimated that the solvus width ranges between concentrations of 0.06 < Ti < 0.13 atoms per formula unit. O K and Fe L _2,3 edges collected by electron energy loss spectrometry (EELS) confirm that both phases are mainly composed of Fe³⁺, requiring that the perovskite is highly oxygen-deficient. Al K and Si K EELS spectra have features comparable with those of fourfold-co-ordinated Al and Si sites, suggesting that they are probably located close to oxygen-vacant sites. Received: 23 June 1999 / Accepted: 18 February 2000     

Melt percolation and reaction atop a plume: evidence from the poikiloblastic peridotite xenoliths from Borée (Massif Central, France)

Poikiloblastic harzburgite xenoliths (P-type) from Borée, France are characterised by large (>1 cm), essentially unstrained olivines and high equilibrium temperatures (>1200 °C). Mineralogical data, trace element abundances and Sr-Nd-O isotopes of the constituent minerals are consistent with formation as a result of melt percolation-reactions in a lherzolite precursor during lithospheric erosion by an upwelling plume. This petrogenetic model contrasts with previous models involving isochemical recrystallisation from a granular lherzolite precursor (G-type) or derivation as metacumulates from tholeiitic magmas. Numerical simulation of percolation reactions at the lithosphere-plume boundary using the plate model of Vernières et al. (1997) indicates that the different textured xenoliths may represent mantle from different levels in a percolation-reaction column. If correct then the P-type harzburgites resulted from pyroxene-dissolving and olivine-producing reactions at increasing melt fraction (>3%) at the lower part of column (base of the lithosphere), whereas the G-type lherzolites were located within the low-porosity domain (<0.1%) above a permeability barrier, and are formed through a melt-rock reaction at decreasing melt mass. Given the very low melt fraction, the REE fractionation in this zone is controlled by chromatographic effects coupled with source effects of reaction. The variations in porosity, melt/rock ratio and melt-rock reaction mechanism are believed to be responsible for the diversity of REE patterns and striking correlation between REE abundance and texture in Borée xenoliths. Received: 15 June 1997 / Accepted: 7 January 1998     

An electron diffraction study of the polymorphs of siO2-tridymite

The results of a detailed TEM study of the low frequency modes of distortion of high temperature SiO₂-tridymite and their relationship to the extensive structural polymorphism of tridymite are presented. It is found that low energy modes of distortion of the ideal tridymite tetrahedral framework structure give rise to a strong and extremely characteristic diffuse intensity distribution (which can be broken into two component types) for the two highest temperature polymorphs of tridymite. Experimental results strongly suggest that this observed diffuse distribution is not a result of irreversible beam damage but is rather an intrinsic property of the ideal tridymite tetrahedral framework structure. The diffuse intensity distribution is closely related to the lower temperature polymorphs of tridymite — in particular, the primary modulation wave-vectors of these low temperature polymorphs always fall on the higher temperature diffuse distribution. The first type of diffuse distribution appears to result from coupled tetrahedral edge rotation of 〈110〉 columns of corner-connected SiO₄ tetrahedra (uncorrelated from column to column as a result of the tetrahedral connectivity of the ideal tridymite framework structure). The real space structural origin of the second curved type of diffuse distribution, however, remains unclear.     

Mineralogical and fluid inclusion studies of low-sulfidation epithermal veins at Osilo (Sardinia), Italy

Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from <0.1 to <20 ppm, with silver-gold ratios of around 4 to 7. Mineralogical studies show a systematic distribution of three hydrothermal mineral assemblages. At distances >50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances <10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (<4.1 equivalent wt.% NaCl and <4 wt.% CO₂), near neutral pH, reduced and, at times, boiling. Received: 19 May 1998 / Accepted: 8 March 1999     

Rigid Unit Modes in disordered nepheline: a study of a displacive incommensurate phase transition

 Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline structure are used to explain the diffuse scattering previously seen in electron diffraction experiments. The RUM calculations also show that the modulation wavelength for incommensurate nephelines is essentially determined by the framework topology. X-ray diffraction is used to measure the intensity of the diffuse scattering as a function of temperature. The diffuse intensity increases sharply at 308 K. This effect is interpreted as being due to the softening of a phonon mode, indicating a phase transition. Measurements of this phase transition below the transition temperature are made using hard mode infrared spectroscopy. Received: 17 February 1999 / Revised, accepted. 15 October 1999     

Grain growth in CaTiO3-perovskite + FeO-wüstite aggregates

Grain growth kinetics in CaTiO₃-perovskite + FeO-wüstite aggregates were studied at the conditions of T = 1223–1623 K, P = 0.1 MPa and P = 200 MPa. Starting samples were fabricated by hot-pressing mechanically mixed powders of CaTiO₃ + FeO with FeO = 0%, 1%, 3%, 6%, 10%, 20% and 100% by weight in a gas-medium apparatus at 1323 K and 300 MPa for 5 h. The increase of grain size (G) of CaTiO₃ with time (t) follows a growth law: G ⁿ −G ⁿ ₀ = κ·t(κ=κ₀exp(−(Q/RT)). Two grain growth regimes are observed at T < 1523 K and T ≥ 1523 K. For T < 1523 K, the best fits of the data to the growth law yield growth exponents of n = 2.2 ± 0.2, 3.0 ± 0.3 and 3.5 ± 0.3 for samples with FeO = 0%, 3% and 10% respectively. Under these conditions the rate constants, κ, obey an Arrhenius relation with Q = 206 ± 35 kJ/mol and 385 ± 65 kJ/mol for samples with FeO = 3% and 10%. Grain growth of CaTiO₃ becomes sluggish when FeO content exceeds 6%. For T ≥ 1523 K, the best fits of the data to the growth law yield n = 2.5 ± 0.2 for both samples with FeO = 3% and 10%. The activation energies (Q ) were determined as 71 ± 30 kJ/mol and 229 ± 45 kJ/mol for samples with FeO = 3% and 10%, respectively. The TEM observations show a remarkable difference in the distribution and geometry of FeO below and above 1523 K: nanometer-sized particles of FeO were observed along CaTiO₃ grain boundaries in samples annealed at T < 1523 K. No FeO particles were detected along CaTiO₃ grain boundaries in samples annealed at T ≥ 1523 K, but large clusters of FeO particles are observed locally indicating a fast separation of FeO from CaTiO₃. Thus we conclude that the slow growth rate of CaTiO₃ at T < 1523 K is due to the pinning by FeO particles at grain boundary, and that the change of grain growth kinetics in CaTiO₃ at T ≥ 1523 K may relate to the separation of FeO from CaTiO₃, which we interpret as due to the phase transformation of CaTiO₃ at around 1523 K. Received: 19 June 1998 / Revised, accepted: 24 March 1999     

Elasticity of the pyrope (Mg3Al2Si3O12)-majorite (MgSiO3) garnets solid solution

Dense isotropic polycrystalline specimens of majorite-rich garnets (Py₁₀₀, Py₆₂Mj₃₈, Py₅₀Mj₅₀, Py₂₁Mj₇₉ and Mj₁₀₀) along the pyrope (Mg₃Al₂Si₃O₁₂ = Py₁₀₀)-majorite (MgSiO₃ = Mj₁₀₀) join were fabricated in a 2000-ton uniaxial split-sphere anvil apparatus (USSA-2000) at pressures from 10 to 18.5 GPa and temperatures from 1200 to 1850 °C, within their stability fields in runs of 2–4-h duration, using hot-pressing techniques developed by Gwanmesia et al. (1993). These specimens are single-phased, fine-grained (≤5 mm), free of microcracks, and have bulk densities greater than 99% of the corresponding single-crystal X-ray density. Elastic compressional (P) and shear (S) wave velocities were determined at room pressure and temperature for these polycrystalline garnet specimens by phase comparison ultrasonic interferometry. For Mj₁₀₀, the P and S wave velocities are within 1% of the Hashin-Shtrikman averages calculated from the single crystal elastic moduli measured by Brillouin spectroscopy. Both the elastic bulk modulus (K) and the shear modulus (G) decrease continuously with increasing majorite content from pyrope garnet (Py₁₀₀) to pure majorite garnet (Mj₁₀₀). The compositional dependence of K and G are given by K = 172.3 (40) − 0.085X, and G = 91.6 (10) − 0.038X, where X = mol% majorite), respectively, indicating that substitution of Si for Mg and Al decreases both K and G by about 5% along the solid solution series. Received: 25 March 1999 / Accepted: 12 July 1999     

Synthetic MgGa2O4-Mg2GeO4 spinel solid solution and β-Mg3Ga2GeO8: chemistry, crystal structures, cation ordering, and comparison to Mg2GeO4 olivine

 Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa₂O₄-Mg₂GeO₄ spinel solid solution (0, 8, 15 and 23 mol% Mg₂GeO₄; 1400 °C, 1 bar) and for spinelloid β-Mg₃Ga₂GeO₈ (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by XRF and EPMA. Addition of Mg₂GeO₄ causes the cation distribution of the MgGa₂O₄ component to change from a disordered inverse distribution in end member MgGa₂O₄, ^[4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg₂GeO₄. An increase in a_o with increasing Mg₂GeO₄ component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga³⁺⇄ Mg²⁺+Ge⁴⁺. The spinel exhibits high configurational entropy, reaching 20.2 J mol⁻¹ (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive enthalpy of disordering over the solid solution, where ΔH _D  = αx + βx ², α = 22(3), β = −21(3) kJ mol⁻¹. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no longer maintained. Spinelloid β-Mg₃Ga₂GeO₈ has cation distribution ^M1[Mg_0.50(2)Ga_0.50(2)] ^M2[Mg_0.96(2)Ga_0.04(2)] ^M3[Mg_0.77(2) Ga_0.23(2)]₂ (Ge_0.5Ga_0.5)₂O₈ (tetrahedral site occupancies are assumed). Octahedral site size is correlated to Mg distribution, where site volume, site distortion, and Mg content follow the relation M1<M3<M2. The disordered cation distribution provides local electrical neutrality in the structure, and stabilization through increased configurational entropy (27.6 J mol⁻¹; eight oxygen basis). Comparison of the crystal structures of Mg_{1+ N} Ga_{2−2 N} Ge _N O₄ spinel, β-Mg₃Ga₂GeO₈, and Mg₂GeO₄ olivine reveals β-Mg₃Ga₂GeO₈ to be a true structural intermediate. Phase transitions across the pseudobinary are necessary to accommodate an increasing divergence of cation size and valence, with addition of Mg₂GeO₄ component. Octahedral volume increases while tetrahedral volume decreases from spinel to β-Mg₃Ga₂GeO₈ to olivine, with addition of Mg and Ge, respectively. Furthermore, M-M distances increase regularly across the join, suggesting that changes in topology reduce cation-cation repulsion. Received: 9 November 1998 / Revised, accepted: 3 August 1999     

Heat capacities of Tschermak substituted Fe-biotite

Six members of the annite–siderophyllite join were synthesized in a three step process – crystallization of biotite from gels, decomposition of the fine-grained biotite under oxidizing conditions and resynthesis of Fe-Al biotite with planned compositions from these products – producing biotite crystals with thicknesses of up to 10 μm. The biotite was characterized by microprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the data were fitted to a c_p-polynomial, c _p =k ₀+k ₁ T ^−0.5+ k ₂ T ⁻²+k ₃ T ⁻³. In the temperature range 143 to 250 K, heat capacities of the different annite–siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the c_p-polynomial of biotites with intermediate composition (except Ann₇₉Sid₂₁) exhibit a steeper slope than those of other biotites. This produces positive excess heat capacities in the annite–siderophyllite join at higher temperatures. The activity-composition data of the same binary derived from phase equilibrium experiments (Benisek et al. 1996) and the data of this study suggest two compositional regions along this join, with different extent of deviation from ideality. One at X _Sid < 0.3, characterized by a small deviation, one at X _Sid > 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according to the vibrational model of Kieffer (1979). The comparison of the vibrational function with the c_p-polynomials shows that the vibrational function reproduces well the DSC-data of the siderophyllite-poor and -rich members, but deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle α increases from 0 to 13° for annite to siderophyllite, respectively. At the composition of the discontinuity, this rotation angle α reaches a value of ∼8^∘. The processing of ∼300 chemical data of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7 and 9°. It would appear that biotites with these structural characteristics are most stable. Received: 27 August 1998 / Accepted: 10 November 1998     

Assessment of heavy-metal contamination of floodplain soils due to mining and mineral processing in the Harz Mountains, Germany

2 study area was assessed with respect to its heavy-metal load on the basis of the current guideline values. The heavy-metal loads of the soils in the study area have ranges of <0.2–200 mg kg⁻¹ for Cd, <10–30,000 mg kg⁻¹ for Pb, 7–10,000 mg kg⁻¹ for Cu and 50–55,000 mg kg⁻¹ for Zn. Mobility of the heavy metals was determined by extraction at different pH values. The acid neutralisation capacity (ANC_x) at these pH values was also determined to estimate the probability that the pH can drop to pH=x. The ANC values in the study area ranged from 6 to 3000 mmol H⁺ kg⁻¹, from −33 to 800 mmol H⁺ kg⁻¹ and from −74 to 160 mmol H⁺ kg⁻¹ for ANC_3.5, ANC_5.0 and ANC_6.2, respectively. Together with pedological data, the extraction experiments permit differentiation between soil units that have been placed in the same environmental hazard class on the basis of total heavy-metal loads. Received: 10 August 1998 · Accepted: 14 August 1999     

Strong tin enrichment in a pegmatite-forming melt

To investigate processes of magmatic tin enrichment and cassiterite deposition, we studied the abundances of major, trace, and volatile elements in a large number of rehomogenized silicate melt inclusions in quartz and topaz from a pegmatite body at the Ehrenfriedersdorf Sn–W deposit. This deposit is associated with evolved Variscan granites of the central Erzgebirge, southeast Germany. The melt inclusions are peraluminous; the molar aluminum saturation index (ASI) ranges from 1.15 to 2.0, and many inclusions are characterized by a very high content of fluxing components and volatiles. Some inclusions contain more than 20 wt% of H₂O, F, Cl, and P₂O₅, plus Li as well as very high levels of Sn. Some rare, highly evolved fractions of late-stage pegmatite-forming liquid at Ehrenfriedersdorf contained up to 7000 ppm Sn. The presence of hydrogen and methane in addition to water and carbon dioxide in the vapor phase of the melt inclusions suggests a very low oxygen fugacity for some fractions of magma. The extreme levels of tin, volatiles, and fluxing components in this magma had an important influence on processes of melt movement and cassiterite precipitation. Melts, like these, that are high in volatiles and alkalis (sum of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O is >8 wt%) have low densities (≤1.8 g/cm³), low viscosities (<10 Pa.s at 700 °C), facilitate relatively rapid diffusion of ions through melts, and hence are excellent solvents for extracting and transporting ore-forming elements. Received: 1 February 1999 / Accepted: 19 January 2000     

Structural thermal behavior of paragonite and its dehydroxylate: a high-temperature single-crystal study

 The lattice constants of paragonite-2M₁, NaAl₂(AlSi₃)O₁₀(OH)₂, were determined to 800 °C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25–600 °C, were: α_a = 1.51(8) × 10⁻⁵, α_b = 1.94(6) × 10⁻⁵, α_c = 2.15(7) ×  10⁻⁵ °C⁻¹, and α_V = 5.9(2) × 10⁻⁵ °C⁻¹. At T higher than 600 °C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements of natural paragonite, carried out at 25, 210, 450 and 600 °C, before dehydroxylation, showed that the larger thermal expansion along the c parameter was mainly due to interlayer thickness dilatation. In the 25–600 °C range, Si,Al tetrahedra remained quite unchanged, whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the interlayer cation Na became more regular with temperature. Tetrahedral rotation angle α changed from 16.2 to 12.9°. The structure of the new phase, nominally NaAl₂ (AlSi₃)O₁₁, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room temperature and its expansion coefficients determined in the range 25–800 °C. The most significant structural difference from paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V ₀ = 1 + 5.9(2) × 10⁻⁵ T(°C) − 0.00153(4) P(kbar). Received: 12 July 1999 / Revised, accepted: 7 December 1999     

Defect structure of the low temperature α-cristobalite phase and the cristobalite ⇆ tridymite transformation in (Si-Ge)O2

Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase of (Si_0.9 Ge_0.1)O₂, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking faults on (1 1 1)_β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases. Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation mediated synchro-shear process on (1 1 1)_β planes; the geometry of this process is analyzed. Received: 13 June 1999 / Accepted: 30 October 1999     

The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation

Potholes represent areas where the normally planar PGE-rich Merensky Reef of the upper Critical Zone of the Bushveld Complex transgresses its footwall, such geometric relationships being unusual in layered intrusions. The recognition of vertical dykes of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tensional deformation due to the loading effects of major new magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement. The Merensky Reef and its footwall have distinct initial Sr-isotope ratios (R ₀ > 0.7066 and <0.7066, respectively), which may be used to constrain these processes related to pothole formation. Merensky Reef in potholes (R ₀ = 0.7069−0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discordant replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall. The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R ₀ = 0.7066), and immediately adjacent stratiform lithologies are slightly modified towards footwall values. In contrast, Neptunian pyroxenitic (Merensky) dykes cross-cutting the footwall lithologies, with a large surface area to volume ratio, and low Sr content, do not display footwall-like Sr-isotope initial-ratios (R ₀ = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement pyroxenite (“replacement pegmatoid”), at the base of the Merensky Reef within potholes, has a high initial-ratio (R ₀ > 0.7071), and so models of pervasive metasomatism by footwall material are not applicable. This isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during, or after, the formation of Merensky Reef potholes, a basic tenet of existing pothole formation hypotheses involving footwall mass-transfer. In contrast, the isotopic data are entirely consistent with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating laterally to fill extensional zones in the footwall layers. Received: 11 October 1997 / Accepted: 21 December 1998