Leucophosphite and Associated Minerals in the Fossil Bat Guano Deposit in Gaura cu Muscă Cave, Locvei Mountains, Romania

This paper presents a new account of the mineralogy of the bat guano deposit in Gaura cu Muscă Cave, Locvei Mountains, Romania. The cave, which, in its main proportion, is a wet, “live” cave, has a dry portion hosting guano. Biogenic leucophosphite is one of the main compounds of the fossil bat guano association in the cave, where it occurs together with hydroxylapatite, taranakite, ardealite, calcite, quartz and illite (the 2M1 polytype). The mineral species from the cave were characterized by optical methods, scanning electron microscopy, wet-chemical analysis, X-ray powder diffraction, Fourier-transform infrared and inductively coupled plasma – atomic emission spectrometry. The crystal-chemical formula of leucophosphite from Gaura cu Muscă is [K_0.978Na_0.003(NH₄)_0.014](Al_0.085Fe_1.903Mg_0.001Mn_0.006)(PO₄)₂(OH)_0.973·2H₂O. The cell parameters calculated for the same sample are a = 9.813(6) Å, b = 9.749(6) Å, c = 9.631(9) Å and β = 102.30(2)°. The infrared spectrum affords the presence of (PO₄)³⁻, (HPO₄)²⁻, (NH₄)⁺ and (OH)⁻ ions, together with H₂O molecules. The band multiplicity on the IR absorption spectrum suggests that the phosphate groups in the structure have C_s punctual symmetry. The host deposit was formed under extremely “dry” conditions that favored a sharp decrease in the pH of solutions derived from the guano mass.