Ulrike Troitzsch

High-PT study of solid solutions in the system Zr02-Ti02: The stability of srilankite

The ZrO 2 -TiO 2 phase diagram was studied with synthesis experiments between 1200°C and 1650°C, 1 atm and 28 kbar, investigating the effect of pressure on the compositions of rutile [TiO 2 ], zirconia [ZrO 2 ], and zirconium titanate [(Zr,Ti) 2 O 4 ] solid solutions. All three phases become more Ti-rich with increasing pressure, which is in good agreement with the smaller ionic radius of Ti 4+ compared to Zr 4+ , and consistent with the observed unit-cell volume of (Zr,Ti) 2 O 4 . The range of zirconium titanate solid solution, which is limited to X Ti Ti = 0.68 at 28 kbar and 1500°C. Thus the compounds (Zr,Ti) 2 O 4 [also referred to ZrTiO 4 when X Ti = 0.5] and (Zr,Ti) 2 O 6 (srilankite), which were previously regarded as the respective high- and low-temperature forms of zirconium-titanate, are both stable at high temperatures ( Ti = 0.67) formed at 1440°C and 28 kbar in equilibrium with rutile. This is the first synthesis of the phase at high pressures under equilibrium conditions. The stability of srilankite at high pressures and temperatures in our experiments contrasts with earlier studies that proposed a hydrothermal origin for the mineral, with an upper stability limit of 900°C. Our results, however, are consistent with the natural occurrence of srilankite in high-grade rocks such as eclogites, granulites, lamprophyres and chromitites. Our experiments show that the stability of zirconium titanate is also strongly dependent on oxygen fugacity.

Greenhouse conditions induce mineralogical changes and dolomite accumulation in coralline algae on tropical reefs

Human-induced ocean acidification and warming alter seawater carbonate chemistry reducing the calcification of reef-building crustose coralline algae (CCA), which has implications for reef stability. However, due to the presence of multiple carbonate minerals with different solubilities in seawater, the algal mineralogical responses to changes in carbonate chemistry are poorly understood. Here we demonstrate a 200% increase in dolomite concentration in living CCA under greenhouse conditions of high pCO2 (1,225 μatm) and warming (30 °C). Aragonite, in contrast, increases with lower pCO2 (296 μatm) and low temperature (28 °C). Mineral changes in the surface pigmented skeleton are minor and dolomite and aragonite formation largely occurs in the white crust beneath. Dissolution of high-Mg-calcite and particularly the erosive activities of endolithic algae living inside skeletons play key roles in concentrating dolomite in greenhouse treatments. As oceans acidify and warm in the future, the relative abundance of dolomite in CCA will increase.

Stevensite in the modern thrombolites of Lake Clifton, Western Australia: A missing link in microbialite mineralization?

Microbialites form the earliest macroscopic evidence of life, and have always been important in particular aquatic ecosystems. They demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size. Microbialites are generally assumed to form by microbial trapping and binding of detrital grains, by carbonate organomineralization of microbial biofi lms, or by inorganic mineralization around microbial templates. Here we present a signifi cant discovery that modern thrombolitic microbialites in Lake Clifton, Western Australia, gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite. This nucleates in and around microbial fi lament walls when biological processes suppress carbon and Ca activities, leaving Mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web. After microbial materials are entombed, local carbon and Ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely, with eradication of biogenic textural features. This may explain why many ancient microbialite carbonates lack clear evidence for biogenicity. Stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation.

THE SYNTHESIS AND CRYSTAL STRUCTURE OF CAALFSIO4, THE AL-F ANALOG OF TITANITE

Abstract Aluminum-rich titanites [Ca(Ti,Al)(O,F)SiO4] with XAl > 0.53 [XAl = Al/(Al+Ti)], including the pure end-member CaAlFSiO4, were synthesized for the first time in a high-pressure experimental study. The crystal structure of CaAlFSiO4 was determined by Rietveld analysis of an X-ray powder diffraction pattern. CaAlFSiO4 is monoclinic, belongs to the space group A2/a, and has the unit-cell dimensions a = 6.9149(2) Å, β = 8.5064(1) Å, c = 6.4384(2) Å, and b = 114.684(2)°. The unit-cell volume is less than 93% of CaTiOSiO4, which is consistent with the natural occurrence of Al-rich titanite in high-P rocks. Although previous studies suggested that titanite with XAl > 0.5 is possibly not stable, this study demonstrates that complete solid solution occurs between CaTiOSiO4 and CaAlFSiO4. The similarity of the crystal structures of titanite and CaAlFSiO4 explains why in natural Al-rich titanite the end-member CaAlFSiO4 generally dominates over the hypothetical end-member CaAlOHSiO4, which under geological conditions is stable in a different crystal structure.

Dolomite rich coral reef coralline algae resist dissolution in acidified conditions

Wave-resistant algal rims—chiefly composed of carbonate from crustose coralline algae—form critical structures for the survival of many shallow coral reefs, raising concerns about the susceptibility of these protective structures to ocean acidification. Research now shows that dolomite-rich frameworks—common in shallow coral reefs globally—are likely to persist as carbon dioxide increases.

Characterization of mineral coatings associated with a Pleistocene‐Holocene rock art style: The Northern Running Figures of the East Alligator River region, western Arnhem Land, Australia

This data article contains mineralogic and chemical data from mineral coatings associated with rock art from the East Alligator River region. The coatings were collected adjacent to a rock art style known as the “Northern Running Figures” for the purposes of radiocarbon dating (doi:10.1016/j.jasrep.2016.11.016; (T. Jones, V. Levchenko, P.L. King, U. Troitzsch, D. Wesley, 2017) [1]). This contribution includes raw and processed powder X-ray Diffraction data, Scanning Electron Microscopy energy dispersive spectroscopy data, and Fourier Transform infrared spectral data.

Heat capacity measurements of CaAlSiO4F from 5 to 850 K and its standard entropy

Abstract Heat capacity (CP) data of Al-F-bearing titanite are presented that yield the standard entropy S298.15∘

Single-crystal structure and Raman spectroscopy of synthetic titanite analog CaAlSiO 4 F

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The ZrO2-TiO2 phase diagram

of F-Al-titanite CaAlFSiO4 (FAT). CP of synthetic FAT was measured with relaxation calorimetry and differential scanning calorimetry between 5 and 764 K. The results constrain S298.15∘

Dolomite-rich coralline algae in reefs resist dissolution in acidified conditions

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