Volume 103, Issue 8

We address the problem of unpolarized light spectroscopy of geological materials. Using infrared radiation, the aim of this technique is to learn about the absorbing species, such as hydroxyl. The use of unoriented samples leads to the need to perform a rigorous statistical analysis, so that the three principal absorbances of the crystal can be retrieved. We present here such an analysis based on a derivation of the probability density function for a single random measurement. Previous methods for retrieval of the absorbances are shown to be suboptimal, producing biased results that are sometimes even unphysical (e.g., negative estimates for an inherently positive quantity). The mathematical structure of the problem is developed to use the maximum likelihood estimation method, and we show how to optimize for the three absorbance parameters. This leads to good parameter retrieval on both synthetic and real data sets.

Sulfides in mantle rocks sometimes have unradiogenic Pb isotopic compositions and, assuming specific conditions, may represent a low U/Pb reservoir that might balance the radiogenic Pb isotope reservoirs of the silicate Earth. A critical requirement to test this hypothesis is knowledge of typical Pb contents in sulfides from different types of mantle rocks and estimates of their contribution to the Pb budget of the mantle rocks. However, data on the distribution of Pb between mantle minerals in mantle rocks from different geologic settings are scarce. In this study, new Pb and Tl concentration data from well-characterized unserpentinized spinel-facies peridotites and pyroxenites from the Balmuccia mantle tectonite (Ivrea-Verbano Zone, Italian Alps) are presented as an example to better understand the Pb distribution in minerals and rocks of the upper mantle. Most peridotites display variable bulk-rock Pb contents (13–97 ng/g), which tend to be lower than Pb contents in the websterites (60–254 ng/g) and clinopyroxenites (174–657 ng/g). The pyroxenites show broadly positive correlations of Pb with Al 2 O 3 , Ce, and also S contents. In situ laser ablation-ICP-MS data indicate low Pb contents in olivine, orthopyroxene, and spinel (mostly below the detection limits of 50 ng/g); whereas Pb contents are higher in clinopyroxene (from <50 to 920 ng/g) and in sulfides (typically a few micrograms per gram and sometimes higher in chalcopyrites). Mass-balance calculations indicate that silicates predominantly control Pb contents in bulk rocks (>70–80% of the Pb), with a minor role for sulfides (mostly <20%). This result from Phanerozoic subcontinental lithosphere mantle rocks is consistent with data on abyssal peridotites. As in some previous studies, bulk-rock Pb contents calculated from constituent phases of peridotites are often lower than the measured values. This imbalance mainly reflects trapped fluid inclusions in silicate minerals and, perhaps also fine-grained crystallization products of trapped melt along grain boundaries. Bulk-rock Tl contents in peridotites (0.05–3.5 ng/g) are lower than in pyroxenites (0.66–7.9 ng/g) and display no correlations with Al 2 O 3 and S contents. The bulk-rock data probably reflect the highly heterogeneous distribution of Tl in sulfides (<0.01–110 μg/g), and, as for Pb, the effect of trapped fluid inclusion. Because the Pb budget in mantle rocks is mainly controlled by silicates, mantle sulfides with unradiogenic Pb isotopic compositions likely cannot balance radiogenic Pb isotopic compositions of oceanic basalts.

Speckling of sphalerite with micrometer-sized blebs of chalcopyrite is usually referred to as “chalcopyrite disease.” Fe-rich sphalerites are particularly prone to chalcopyrite disease. Considering the low degree of solid solution between sphalerite and chalcopyrite, exsolution is discarded as a process to explain the development of chalcopyrite disease. Diffusion-controlled replacement of Fe by Cu, and sphalerite-chalcopyrite co-precipitation are invoked as the most probable mechanisms. Although metamorphism is expected to dispel inhomogeneities through recrystallization, chalcopyrite disease interestingly appears unaffected and to be quite common in metamorphosed sulfide ores. We have conducted experiments on different bulk compositions in the system ZnS-PbS-FeS-Cu 2 S-As 2 S 3 at 600 °C and annealed the run products containing melt at 350 °C to evaluate the role of sulfide partial melting, if any, in the development of chalcopyrite disease. The results indicate that chalcopyrite blebs developed only in those sphalerites that contained Fe and in which S atoms were in excess over Fe + Zn atoms. Also it was observed that the occurrence of Fe-bearing sphalerite and the sulfide partial melt (that invariably was S-deficient and Cu-enriched) in direct contact with each other was necessary for the chalcopyrite blebs to form. We propose nonstoichiometry-driven diffusion of Cu as the mechanism and sulfide partial melting as the principal causative factor behind the development of chalcopyrite disease in sphalerite. Chalcopyrite disease thus may be used as an easily identifiable potential indicator of sulfide partial melting in metamorphosed base metal sulfide deposits.

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe) 9 S 8 ], pyrite (FeS 2 ), and chalcopyrite (CuFeS 2 ). These BMS were found forming composite inclusions (<60 μm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayarí-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt%, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt%. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt% of Ru and <3.5 wt% of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt% and Os <0.2 wt%. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite.

We determined the stability and chemical composition of Al-bearing superhydrous phase B at 20–24 GPa and 1400–2000 °C to discuss the mechanism of water transport in the mantle transition zone and uppermost lower mantle at temperatures close to the mantle geotherm. Superhydrous phase B contained significant amounts of Al 2 O 3 , from 14 to 32 wt%, and Al-bearing superhydrous phase B remained stable, even at 2000 °C and pressures of approximately 20–24 GPa. Moreover, two types of superhydrous phase B with different chemical compositions coexisted at 20–24 GPa and 1600 °C. The Al 2 O 3 and H 2 O contents increased, and the MgO and SiO 2 contents decreased as the pressure and temperature increased up to 1600 °C. Above 1600 °C, the MgO and Al 2 O 3 contents increased, and the SiO 2 and H 2 O contents decreased as the temperature increased. We found two substitution mechanisms: (1) 2Mg 2+ + Si 4+ ⇄ 2Al 3+ + 2H + + □ Mg (Mg site vacancy) (2Mg 2+ = Al 3+ + H + + □ Mg ):(Si 4+ = Al 3+ + H + ) = 1:1, (2) Si 4+ + 16H + ⇆ 4Mg 2+ + 4Al 3+ . The maximum H 2 O content of Al-bearing superhydrous phase B is 11.1(3) wt%, which is ~ 1.9 times larger than that of the Mg-end-member. The crystal structures of the two coexisting superhydrous phase B values are expected to be slightly different from each other. The present results indicate that Al-bearing superhydrous phase B can be stable in a subducted slab with a high Al content compared to pyrolite (e.g, chlorite) at temperatures typical of the mantle transition zone and the lower mantle. Thus, water can be transported to the lower mantle by Al-bearing superhydrous phase B in the subducting slab, even at the typical mantle geotherm.

The Miocene rhyolites of the Spor Mountain Formation host Earth’s largest beryllium deposit, which produced 85% of the world’s beryllium in 2010. The fresh lava is extremely enriched in Be (up to 75 ppm in matrix glass). We have examined the rhyolite to better understand the Be enrichment. The Spor Mountain rhyolite contains ~40% quartz, ~40% sanidine, ~10% biotite, and ~10% plagioclase, along with accessory fluorite, columbite, euxenite, fergusonite, monazite, thorite, and zircon. Two types of rhyolite are present within the Spor Mountain Formation, a less-evolved magma (1150 ppm Rb, 42 ppm Be, 0.7 wt% F in glass) and a more-evolved magma (1710 ppm Rb, 75 ppm Be, 1.6 wt% F in glass). Eruption temperatures estimated using two-feldspar (Elkins and Grove 1990; Putirka 2008; Benisek et al. 2010), plagioclase-liquid and alkali feldspar-liquid (Putirka 2008), Ti-in-quartz (Thomas et al. 2010, 2015; Huang and Audétat 2012), biotite (Righter and Carmichael 1996), and zircon saturation (Boehnke et al. 2013) geothermometers converge on 718 °C for the less-evolved magma and 682 °C for the more-evolved magma. Using the Ti-in-Qz equation of Huang and Audétat (2012), the pre-eruptive pressure of the Spor Mountain rhyolite system is estimated to be about 2 kbar at 700 °C. Water content of the rhyolite melt was less than <5 wt%, based on the co-crystallization of all four major mineral phases at 700 °C, and the magma was water undersaturated (Webster et al. 1987). Viscosity of the rhyolite was about 10 6.2 Pa·s for the less-evolved rhyolite and 10 5.8 Pa·s for the more-evolved rhyolite. Fluorine lowered the melt viscosity, though not by a large amount (less than 0.5 log units at 1.6 wt% F). Partition coefficients for Be and other trace elements were determined for biotite, sanidine, plagioclase, and quartz from laser ablation-inductively coupled plasma-mass spectrometry analyses. Partition coefficients for trace elements in feldspars from the Spor Mountain rhyolite are generally higher than for feldspars from other silicic magmas and lower for biotite. The enrichment of beryllium in the Spor Mountain rhyolite was aided by its high incompatibility in the major mineral phases, with a bulk partition coefficient <0.1. Trace element models using the measured partition coefficients are inconsistent with accumulation of increments of melt formed by different degrees of partial melting and cannot explain the great depletion of compatible elements. Rather, the trace element abundances and Nd and Sr isotopic compositions are consistent with derivation of rhyolite by ~25% partial melting of crust hybridized with mantle-derived components, followed by extensive fractional crystallization (75%). The combination of these magmatic processes set the stage for the formation of a world-class beryllium deposit.

We estimate the mantle source compositions for mantle plumes and, by implication, Earth’s lower mantle by: (1) measuring trace (e.g, Sc, V, Cu) and minor (e.g., Ca, Mn, Ni) element concentrations of high-forsterite olivine grains from several plume localities, (2) estimating the parent liquid compositions from which they crystallized, (3) calculating mantle potential temperatures and degrees of partial melting, and (4) estimating trace element compositions of depleted and enriched mantle sources. Our sample set includes two continental flood basalt provinces (Emeishan and Deccan), a flood basalt that erupted in a continental rift setting (Baffin Island), our type example of a thermal mantle plume (Hawaii), and lavas from the Siqueiros Transform at the East Pacific Rise, which represent the mid-ocean ridge system. We also present olivine (Ol) compositions for peridotite xenoliths from Kilbourne Hole, New Mexico, U.S.A., which are commonly used as primary and secondary analytical standards. We find that trace elements in lava-hosted olivine grains are too far removed from their mantle source to provide anything but greatly hindered views of such. Olivine compositions reflect not only evolving liquid compositions (including partial melting conditions and later fractionation), but also evolving O1+liq partition coefficients, which mostly increase with decreasing T during crystallization. Mantle compositions, delimited by maximum forsterite contents and estimates of parental magmas (and experimentally determined partition coefficients) indicate that our selected plumes reflect some combination of (1) a depleted mantle source that is quite similar to that obtained by other methods and (2) a variably enriched plume source that is more enriched than current estimates of pyrolite. The enriched plume mantle sources can be explained remarkably well as a mixture of subducted mid-ocean ridge basalt (MORB; Gale et al. 2013) and depleted MORB mantle (DM; Salters and Stracke 2004), with MORB:DMM ratios of 1:5 to 1:4. These ratios are most sensitive to estimates of melt fraction where plume parental magmas are last equilibrated with their mantle source, but are nonetheless consistent across a wide range of chemically very different elements, and estimates of MORB and DM obtained by very different means. Baffin Island is of particular interest. Like prior studies, we verify a high mantle potential temperature ( T p ) of 1630 °C (compared to T p = 1320–1420 °C for MORB from Cottrell and Kelley 2011 for O1 of Fo 89.3–91.4 ). The Baffin source is also within error the same as DM with respect to trace elements, although still isotopically distinct; Baffin appears to be sourced in something that is akin to DMM that lies at the base of the mantle, where plumes acquire their excess heat. Thus while part of our analysis supports the concept of a “slab graveyard” at the bottom of the lower mantle (e.g., Wyession 1996), that cemetery is by no means ubiquitous at the CMB: subducted slabs are either unevenly interred, or efficiently excavated by later upwellings.

Hydrogen has been considered as an important candidate of light elements in the Earth’s core. Because iron hydrides are unquenchable, hydrogen content is usually estimated from in situ X-ray diffraction measurements that assume the following linear relation: x = ( V FeHx – V Fe )/Δ V H , where x is the hydrogen content, Δ V H is the volume expansion caused by unit concentration of hydrogen, and V FeHx and V Fe are volumes of FeH x and pure iron, respectively. To verify the linear relationship, we computed the equation of states of hexagonal iron with interstitial hydrogen by using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). The results indicate a discontinuous volume change at the magnetic transition and almost no compositional ( x ) dependence in the ferromagnetic phase at 20 GPa, whereas the linearity is confirmed in the non-magnetic phase. In addition to their effect on the density-composition relationship in the Fe-FeH x system, which is important for estimating the hydrogen incorporation in planetary cores, the magnetism and interstitial hydrogen also affect the electrical resistivity of FeH x . The thermal conductivity can be calculated from the electrical resistivity by using the Wiedemann-Franz law, which is a critical parameter for modeling the thermal evolution of the Earth. Assuming an Fe 1–y Si y H x ternary outer core model (0.0 ≤ x ≤ 0.7), we calculated the thermal conductivity and the age of the inner core. The resultant thermal conductivity is ~100 W/m/K and the maximum inner core age ranges from 0.49 to 0.86 Gyr.

Because magmatic oxygen fugacity ( f O 2 ) exerts a primary control on the discrete vanadium (V) valence states that will exist in quenched melts, V valence proxies for f O 2 , measured using X-ray absorption near-edge spectroscopy (XANES), can provide highly sensitive measurements of the redox conditions in basaltic melts. However, published calibrations for basaltic glasses primarily relate measured intensities of specific spectral features to V valence or oxygen fugacity. These models have not exploited information contained within the entire XANES spectrum, which also provide a measure of changes in V chemical state as a function of f O 2 . Multivariate analysis (MVA) holds significant promise for the development of calibration models that employ the full XANES spectral range. In this study, new calibration models are developed using MVA partial least-squares (PLS) regression and least absolute shrinkage and selection operator (Lasso) regression to predict the f O 2 of equilibration in glasses of basaltic composition directly. The models are then tested on a suite of natural glasses from mid-ocean ridge basalts and from Kilauea. The models relate the measured XANES spectral features directly to buffer-relative f O 2 as the predicted variable, avoiding the need for an external measure of the V valence in the experimental glasses used to train the models. It is also shown that by predicting buffer-relative f O 2 directly, these models also minimize temperature-relative uncertainties in the calibration. The calibration developed using the Lasso regression model, using a Lasso hyperparameter value of α = 0.0008, yields nickel-nickel oxide (NNO) relative f O 2 predictions with a root-mean-square-error of ±0.33 log units. When applied to natural basaltic glasses, the V MVA calibration model generally yields predicted NNO-relative f O 2 values that are within the analytical uncertainty of what is calculated using Fe XANES to predict Fe 3+ /ΣFe. When applied to samples of natural basaltic glass collected in 2014 from an active lava flow at Kilauea, a mean f O 2 of NNO-1.15 ± 0.19 (1σ) is calculated, which is generally consistent with other published f O 2 estimates for subaerial Kilauea lavas. When applied to a sample of pillow-rim basaltic glass dredged from the East Pacific Rise, calculated f O 2 varies from NNO-2.67 (±0.33) to NNO-3.72 (±0.33) with distance from the quenched pillow rim. Fe oxybarometry in this sample provides an f O 2 of NNO-2.54 ± 0.19 (1σ), which is in good agreement with that provided by the V oxybarometry within the uncertainties of the modeling. However, the data may indicate that V XANES oxybarometry has greater sensitivity to small changes in f O 2 at these more reduced redox conditions than can be detected using Fe XANES.

Electrical conductivity experiments under pressure and temperature conditions relevant to planetary interiors are a powerful tool to probe the transport properties of Earth and planetary materials as well as to interpret field-based electrical data. To promote repeatability and reproducibility of electrical experiments among multi-anvil facilities that use this technique, we designed and developed an electrical conductivity cell for multi-anvil experiments based on the 14/8 assembly that was developed to allow access to high temperatures. Here we present the details of design and parts developed for this cell that is available via the Consortium for Material Properties Research in Earth Sciences (COMPRES). The electrical cell has been tested up to 10 GPa and about 2000 °C on different materials (silicates and metals, both in the solid and liquid state).

The Raman and infrared spectra of kernite [Na 2 B 4 O 6 (OH) 2 ·3H 2 O] have been characterized up to ~25 and 23 GPa, respectively, to explore pressure-induced changes in a structurally novel mineral that contains mixed coordination borate groups (threefold and fourfold), as well as both hydroxyl groups and water molecules. BO 3 and BO 4 vibrational modes are characterized in both the Raman and infrared spectra of kernite, and reassignments of some modes are made based on observed pressure shifts. Under compression to ~25 GPa, kernite undergoes three phase transitions: one initiates near ~2.5 GPa, the second occurs at ~7.0 GPa, and the third near 11.0 GPa. The first transition is characterized by a loss of both a subset of the Na-associated modes and a sharp OH peak, and it is fully reversible. The second transition is characterized by the loss of most of the BO 3 modes, some of the BO 4 modes at ~7 GPa, and further broadening of the H 2 O and OH peaks. This transition is partially reversible on decompression, but the Raman spectra indicate that the high-pressure structure and its reversion products are likely disordered. The third transition is characterized by the loss of most of the Raman and infrared modes, and it is not reversible on decompression. The decompression products from ~25 GPa have markedly different infrared and Raman spectra from the starting material, and appear to involve different bonding environments of boron than are present in the starting materials: thus, pressure treatment of boron-rich materials may generate novel quenchable structures. Notably, the BO 3 groups in kernite are destabilized under compression, with the BO 4 groups appearing to be more stable under compression. This shift in coordination may be endemic among borate glasses and crystalline borates under compression. The coordination conversion (and its products on decompression) is likely to highly depend on the detailed topology of the starting materials. Commonalities and differences between the threefold to fourfold transitions in borate systems relative to those in carbonate systems are presented. Our results, combined with prior shock data, indicate that kernite, which may be representative of low-temperature borate minerals, can persist during impact events with peak shock pressures less than ~33 GPa, and the primary polyborate structural units involved in early ribose stabilization would thus survive most impact conditions.

Bobdownsite, IMA number 2008-037, was approved as a new mineral by the Commission on New Minerals, Nomenclature and Classification (CNMNC) as the fluorine end-member of the mineral whitlockite. The type locality of bobdownsite is in Big Fish River, Yukon, Canada, and bobdownsite was reported to be the first mineral with essential monofluorophosphate (PO 3 F 2– ). The type specimen of bobdownsite has been reinvestigated by electron probe microanalysis (EPMA), and our data indicate that fluorine abundances are below detection in the mineral. In addition, we conducted detailed analysis of bobdownsite from the type locality by gas chromatography isotope ratio mass spectrometry, Raman spectroscopy, EPMA, and NMR spectroscopy. These data were compared with previously published data on synthetic monofluorophosphate salts. Collectively, these data indicate that bobdownsite is indistinguishable from whitlockite with a composition along the whitlockite-merrillite solid solution. Bobdownsite is therefore discredited as a valid mineral species. An additional mineral, krásnoite, has been purported to have monofluorophosphate components in its structure, but reexamination of those data indicate that F – in krásnoite forms bonds with Al, similar to OH – bonded to Al in perhamite. Consequently, krásnoite also lacks monofluorophosphate groups, and there are currently no valid mineral species with monofluorophosphate in their structure. We recommend that any future reports of new minerals that contain essential monofluorophosphate anions be vetted by abundance measurements of fluorine, vibrational spectroscopy (both Raman and FTIR), and where paramagnetic components are permissibly low, NMR spectroscopy. Furthermore, we emphasize the importance of using synthetic compounds containing monofluorophosphate anions as a point of comparison in the identification of minerals with essential monofluorophosphate. Structural data that yield satisfactory P-F bond lengths determined by X-ray crystallography, coupled with direct chemical analyses of fluorine in a material do not constitute sufficient evidence alone to identify a new mineral with essential monofluorophosphate.

Adrianite (IMA 2014-028), Ca 12 (Al 4 Mg 3 Si 7 )O 32 Cl 6 , is a new Cl-rich silicate mineral and the Si,Mg analog of wadalite. It occurs with monticellite, grossular, wadalite, and hutcheonite in altered areas along some veins between primary melilite, spinel, and Ti, Al-diopside in a Type B1 FUN (Fractionation and Unidentified Nuclear effects) Ca-Al-rich inclusion (CAI), Egg -3, from the Allende CV3 carbonaceous chondrite. The mean chemical composition of type adrianite by electron probe microanalysis is (wt%) CaO 41.5, SiO 2 27.5, Al 2 O 3 12.4, MgO 7.3, Na 2 O 0.41, Cl 13.0, O=Cl –2.94, total 99.2, giving rise to an empirical formula of (Ca 11.69 Na 0.21 )(Al 3.85 Mg 2.88 Si 7.23 )O 32 Cl 5.80 . The end-member formula is Ca 12 (Mg 5 Si 9 )O 32 Cl 6 . Adrianite has the I 4 3 d wadalite structure with a = 11.981 Å, V = 1719.8 Å 3 , and Z = 2, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.03 g/cm 3 . Adrianite is a new secondary mineral in Allende, apparently formed by alkali-halogen metasomatic alteration of primary CAI minerals such as melilite, anorthite, perovskite, and Ti, Al-diopside on the CV chondrite parent asteroid. Formation of secondary Cl-rich minerals sodalite, adrianite, and wadalite during metasomatic alteration of the Allende CAIs suggests that the metasomatic fluids had Cl-rich compositions. The mineral name is in honor of Adrian J. Brearley, mineralogist at the University of New Mexico, U.S.A., in recognition of his many contributions to the understanding of secondary mineralization in chondritic meteorites.

As the only known mineral with confirmed ages >4 Ga, zircon is unmatched in the field of early Earth research. In the past two decades, researchers have continued to establish connections between zircon chemistry and the physical/chemical processes that shaped the early crust. This connection has benefited greatly from the application of high-temperature and high-pressure laboratory experiments. This study presents: (1) new zircon U-Pb geochronology and strategies for characterizing and identifying ancient terrestrial material from the Inukjuak Domain in northern Québec, and the Jack Hills, Western Australia; and (2) a blend of new laboratory experiments and measurements of isotope ratios and trace impurities of natural zircon. Research directions in need of future exploration, with emphasis on early Earth studies, are also explored. Topics include Hadean hydrous magmatism and the structural accommodation of “water” into the zircon lattice, Hadean subaerial crust and the identification of peraluminous or metaluminous source melts, methods to characterize the oxidation state of magmas and fluids, and the complementarity of the Si- and O-isotopic systems as proxies for crustal weathering. Finally, the implications of this work are discussed in the context of a possible transition from prebiotic to biotic chemistry on the early Earth.

POLARIO is a computer program to calculate total electronic polarizabilities, refractive energies, and refractive indices of transparent minerals and synthetic crystalline compounds from their chemical composition and molar volume. If measured refractive indices or observed polarizabilities are entered, POLARIO also allows calculation of the deviation between observed and calculated values and determines the compatibility index as a measure of agreement. The density of the compound is calculated from the chemical composition and the molar volume of a formula unit. Atom parameters can be read in cif format to determine possible coordination numbers of cations and to compose the entire input necessary to do the calculations. It displays a table of interatomic distances and angles, and it shows the chemical composition with superscripted coordination numbers and valencies. The program is written in Delphi XE6 for WINDOWS operating systems and contains 5300 constants and parameters to do the calculations.