Sequential dehydration of the phosphate–sulfate association from Gura Dobrogei Cave, Dobrogea, Romania

Delia-Georgeta Dumitraş,Ştefan Marincea

doi:10.5194/ejm-33-329-2021

Delia-Georgeta Dumitraş, Ştefan Marincea

Open Access

https://doi.org/10.5194/ejm-33-329-2021

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Abstract

Abstract. A rich association of primary guano minerals, including taranakite, hydroxylapatite, brushite and gypsum with relicts of illite, kaolinite, alpha (low) quartz and calcite, was identified in the fossil bat guano deposit from Gura Dobrogei Cave, Dobrogea County, Romania. Gypsum and Ca phosphates developed preferentially on the carbonate bedrock or on fallen carbonate blocks in the guano mass, whereas taranakite was identified in the clay-rich, detritic sequences. The mineral species from the cave were characterized by optical methods, scanning electron microscopy, X-ray powder diffraction, Fourier-transform infrared and inductively coupled plasma–atomic emission spectrometry analysis. Chemically induced local dehydration of primary minerals, characterized by low temperatures (up to 100 ∘C or even lower) and critically depending on exothermal reactions in the guano mass, prompted the formation of a secondary association, consisting of francoanellite, bassanite and monetite. Topotactic substitutions were observed in the cases of francoanellite on taranakite, bassanite on gypsum and monetite on brushite. In its turn, ardealite was partially replaced by monetite and bassanite. The sequential dehydration process seems driven by the degradation of organic matter by microbial action and also, presumably, by other exothermic reactions at local scale (e.g., oxidation of ammonia, allogenic pyrite or other organic compounds).

Highlights

The sequential dehydration affecting guano masses in dry caves exposed to air circulation was described or supposed by a number of authors (e.g., Hill and Forti, 1997; Karkanas et al, 2000; Onac and Veres, 2003; Frost and Palmer, 2011; McFarlane and Lundberg, 2018; Audra et al, 2019, and references therein) in relation to the thermal or dehydration reactions in the guano mass
Local dehydration of the very stable authigenic mineral taranakite to francoanellite due to a local decrease in the water vapor pressure was, for example, described in the Magurici Cave (Onac and Veres, 2003) and expresses a path of the early diagenesis
In tropical caves where complete sequences of fossil bat guano minerals were subjected to desiccation, the microbial activity seems responsible of the breakdown by dehydration of variscite in berlinite or gibbsite (e.g., McFarlane and Lundberg, 2018)

Summary

Introduction

The sequential dehydration affecting guano masses in dry caves exposed to air circulation was described or supposed by a number of authors (e.g., Hill and Forti, 1997; Karkanas et al, 2000; Onac and Veres, 2003; Frost and Palmer, 2011; McFarlane and Lundberg, 2018; Audra et al, 2019, and references therein) in relation to the thermal or dehydration reactions in the guano mass. In tropical caves where complete sequences of fossil bat guano minerals were subjected to desiccation, the microbial activity seems responsible of the breakdown by dehydration of variscite in berlinite or gibbsite (e.g., McFarlane and Lundberg, 2018). The mineralogical approach proposed in this paper intends to clarify the early diagenesis of the fos-

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