The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Summary The crystal structure of Ca₅(PO₄)₂SiO₄ (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR _w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm^–3. The crystal contains about 2.5% V₂O₅ as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO₄ ^3–, SiO₄ ^4– and possibly VO₄ ^3– occurs among the anion sites. The structure has some relationship to that of Ca₅(PO₄)₃OH, the predominant inorganic phase in the human body, but suggests that the Ca₅(PO₄)₃OH type structure may not be stable without some of the OH positions being filled. Ca₅(PO₄)₂SiO₄ is more closely related to K₃Na(SO₄)₂ (glaserite) if it is considered that there are systematic cation vacancies in Ca₅(PO₄)₂SiO₄.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca₂SiO₄ and Ca₃(PO₄)₂ and between Ca₃(PO₄)₂ and CaNaPO₄.

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Die Kristallstruktur vonCa ₅(PO ₄)₂ SiO ₄ (Silicocarnotit)

Zusammenfassung Die Kristallstruktur von Ca₅(PO₄)₂SiO₄ (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR _w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; D_obs.=3,06 g · cm^–3, D_exp.=3,03 g · cm^–3. Der Kristall enthält etwa 2,5% V₂O₅ als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO₄ ^3–, SiO₄ ^4– und möglicherweise VO₄ ^3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca₅(PO₄)₃OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca₅(PO₄)₃OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca₅(PO₄)₂SiO₄ zeigt engere Beziehungen zu K₃Na(SO₄)₂ (Glaserit), wenn man berücksichtigt, daß in Ca₅(PO₄)₃SiO₄ systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca₂SiO₄ und Ca₃(PO₄)₂ und zwischen Ca₃(PO₄)₂ und CaNaPO₄ verantwortlich sind.

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With 9 Figures     

Use of in situ-produced Be in carbonate-rich environments: A first attempt

We have determined the production rate of in situ-produced ¹⁰Be in calcite through measurements of ¹⁰Be and ³⁶Cl concentrations in the same calcite samples and of ¹⁰Be concentrations in depth profiles of flint from the same erosional surface. This yields a ¹⁰Be production rate in calcite of 37.9 ± 6.0 at/g/yr at sea level and high latitude, approximately sixfold higher than production in the coexisting flint. We propose that this higher rate of production may be due to high production cross sections for C spallation by cosmic rays with energies below 50 MeV. These results also open the possibility of dating burial events in carbonate-rich environments by differential radioactive decay of ¹⁰Be and ³⁶Cl.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—I. Electronic structure of H6Si2O7, H6AlSiO71−, and H6Al2O72−

As part of a study of the effect of geologically common network modifiers on polymerization in silicate melts, glasses, and silica-rich aqueous solutions, we have studied the energies, electronic structures, and inferred chemical properties of ^IVT-O-^IVT linkages in the tetrahedral dimers H₆,Si₂O₇, H₆AlSiO₇^1?, and H₆Al₂O₇^2? using semi-empirical molecular orbital theory (CNDO/2). Our results indicate that the electron donating character of the bridging oxygen, O(br), linking two tetrahedra increases with increasing T-O(br) bond length but decreases with decreasing T-O(br)-T angles and increasing O-T-O(br) angles. This increase or decrease of the donor character of O(br) coincides with an increase or decrease of the affinity of O(br) for hard acceptors. The calculated electronic structure for the H₆Si₂O₇ molecule is compared with the observed X-ray emission, absorption, and photoelectron spectra of quartz and vitreous silica; the reasonable match between calculated and observed oxygen Kα emission spectra of vitreous silica supports our assertion that non-bonded O(br) electron density energetically at the top of the valence band controls the chemical reactivity of ^IVT-O-^IVT linkages in polymerized tetrahedral environments.     

Woolshed Creek fossil site: a key part of Canberra’s scientific and cultural heritage

The Woolshed Creek fossil site near the Royal Military College, Duntroon, Canberra, contains brachiopods Atrypa duntroonensis (early Homerian, early Silurian, ca 430.5?Ma) within a mudstone of the Canberra Formation. Their discovery in 1844 by the Reverend William B. Clarke (“the Father of Australian Geology”), and subsequent comparison with other fossil collections from around the world, contributed significantly to the nineteenth century debate about the oldest rocks in Australia. The fossil site is now on the ACT Government Heritage List and recent site improvements make it readily accessible via a pathway from the sports grounds of the Royal Military College.     

Structure and viscosity of rhyolitic composition melts

The average local structure of a rhyolitic composition glass has been determined at 25°C using X-ray radial distribution analysis (RDA) and quasi-crystalline modelling and is best described as similar to that in a stuffed framework composed principally of six-membered rings of Si and Al tetrahedra (basically a stuffed tridymite-like model). Using this model it is possible to calculate a density (2.41 g/cm³) which compares well with the measured density (2.40 g/cm³); a structural model based on four-membered rings (an albite-like model) results in a substantially higher calculated density (2.60 g/cm³). We suggest that the rhyolite glass structural model is appropriate for rhyolitic melts, based on evidence from the recent literature. New viscosity data for an anhydrous rhyolite composition measured between 1200°C and 1500°C are presented and interpreted in terms of our proposed structural model and previous melt structure models for the major normative components of rhyolite. A mechanism for diffusion and viscous flow in framework silicate melts (including rhyolite composition) is proposed on the basis of recent molecular orbital calculations and molecular dynamics simulations of silicate and fluoride melts.     

Application of stress mapping in cross‐section to understanding ore geometry,predicting ore zones and development of drilling strategies

Stress mapping is a numerical modelling technique used to determine the distribution and relative magnitude of stress during deformation in a mineralised terrane. It is based on the general principle that fluid flow in the Earth's crust is primarily related to pressure gradients. It is best applied to epigenetic hydrothermal mineral deposits, where fluid flow and fluid flux are enhanced in dilational sections of structures and in sites of enhanced rock permeability due to high fracture density. These are defined by sites of low minimum principal stress (σ₃). Most stress mapping is carried out in two dimensions in plan view using geological maps. This is suitable for terranes with steeply dipping lithostratigraphy and structures in which the distribution of mineral deposits is largely controlled by fault structures portrayed on the maps. However, for terranes with gently dipping sequences and structures, and for situations where deposits are sited in and near the hinges of complex fold structures, stress mapping in cross‐section is preferable. The effectiveness of stress mapping is maximised if mineralisation was late in the evolutionary history of the host terrane, and hence the structural geometry of the terrane and contained deposits were essentially that expressed today. The orientation of syn‐mineralisation far‐field stresses must also be inferred. Two examples of orogenic gold deposits, which meet the above criteria, are used to illustrate the potential of stress mapping in cross‐section. Sunrise Dam, located in the Archaean Yilgarn Craton, is a lode‐gold deposit sited in a thrust‐fold belt. Stress mapping illustrates the heterogeneity of stress distribution in the complex structural geometry of the deposit, and predicts the preferential siting of ore zones around the intersections of more steeply dipping, linking thrusts and banded iron‐formation units, and below the controlling more gently dipping basal thrust, the Sunrise Shear. The Howley Anticline in the Pine Creek block hosts several Palaeoproterozoic gold deposits, sited in complex anticlinal structures in greywacke sequences. Stress mapping indicates that gold ores should develop in the hinge zones of symmetrical anticlines, in the hinge zones and more steeply dipping to overturned limbs of asymmetric anticlines, and in and around thrusts in both anticlines and parasitic synclines. The strong correlation between the predictions of the stress mapping, based on the distribution of low σ₃, and the location of gold ores emphasises the potential of stress mapping in cross‐section, not only as an exploration tool for the discovery of additional resources or deposits, but also as a test of geological models. Knowledge of the potential siting of gold ores and their probable orientations also provides a guide to drilling strategies in both mine‐ and regional‐scale exploration.     

Low temperature,low pressure deformation and metamorphism of the Vangorda massive sulphide orebody,Yukon, Canada

The Vangorda orebody is a small stratiform massive sulphide orebody located in Anvil District, Yukon, Canada. The orebody consists of fineto medium-grained semi-massive and massive sulphides with a common sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena, and minor chalcopyrite. The host rocks and the sulphide lithofacies have been complexly deformed during two phases of deformation (D₁ and D₂) and associated metamorphism (M₁ and M₂). The effects of d₁ and M₁ are penetratively overprinted by D₂ and M₂. D₂ and M₂ resulted in tight to isoclinal F₂ folding of the orebody, remobilisation of the sulphides, recrystallisation and development of shear zones along the limbs of the F₂ folds. Chlorite thermometry and sulphide thermobarometry have been carried out on the host phyllites and on the sulphides. Chlorite was analysed from the S₁ and S₂ foliations in the phyllites to determine M₁ and M₂ temperatures, respectively. However, no difference was found between chlorite compositions in these foliations and a mean temperature of 363 °C was calculated from the tetrahedral A1_IV occupancy. Arsenopyrite thermometry yielded a comparable mean temperature of 336 °C. Sphalerite inclusions in M₂ pyrite porphyroblasts from D₂ shear zones were analysed for pressure using the sphalerite + hexagonal pyrrhotite + pyrite barometer. Inclusions were analysed in an attempt to determine if relic m₁ sphalerite, and hence pressure signature, was preserved. Inclusion compositions appear to reflect only M₂ conditions and yielded a mean pressure of 4.0 kb. Sphalerite + hexagonal pyrrhotite assemblages were analysed from D₂ shear zones to determine the M₂ pressure using the sphalerite + hexagonal pyrrhotite barometer. These calculations yielded a mean pressure of 6.1 kb. The M₂ temperatures and pressures calculated using these calibrations are in good agreement with those estimated from petrogenetic relationships.