Stability of Cave Phosphates: Case Study from Liliecilor Cave (Trascău Mountains, Romania)

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Bat Guano, cave phosphates, cave sediments, Romania, Trascau Mountains

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In this paper we emphasize some of the variables that control the precipitation of one phosphate species over another in caves. We have explored the effects of humidity, pH, available cations, and position within the guano deposit. Forty-nine samples were collected from walls, paleokarst infilling, fallen limestone blocks, and sediment in a cave that hosts a bat colony; thirteen of these were collected at 15 cm intervals in a profile dug in the thickest part of the sedimentary deposit (ca. 2 m). Measurements for pH and moisture were carried out, together with subsequent X-ray diffraction analyses. Additionally, S stable isotopic ratios in 8 gypsum samples were measured and found to be comparable to values in igneous rocks in the area. The phosphate assemblage is composed of hydroxylapatite, brushite, ardealite, taranakite, monetite and whitlockite, accompanied by calcite and gypsum. Bedrock-derived and allochthonous minerals are represented by quartz, muscovite, several clay minerals, hematite, and ulvöspinel. The amorphous material content decreases from the top of the sediment deposit toward its bottom, consistent with increasing decay of organic mater and uptake of compounds into newly formed mineral phases. Moisture in the sediment is positively correlated with depth and inversely correlated with pH, with some of the phosphates showing no pH or moisture preference (e.g., hydroxylapatite), whereas others (e.g., brushite and taranakite) cluster around relatively well defined boundaries.

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Journal of Mineralogy and Geochemistry, v. 191, issue 2, p. 157-168