Minerals of the Wooltana Cave, Flinders Ranges, South Australia

Abstract

Analysis of guano specimens from Wooltana cave, formerly occupied by ghost bats (Macroderma gigas), shows the principal mineral is whitlockite with seven other crystalline phases. These include dittmarite NH4MgPO4 ·H2O, biphosphammite (NH4K)H2PO4, syngenite K2Ca(SO4)2 ·H2O, görgeyite K2Ca5(SO4)6 ·H2O, brushite CaHPO4 ·2H2O, gypsum CaSO4 ·2H2O and whewellite CaC2O4 ·H2O. The mineralogy of the guano samples was determined by powder X-ray diffraction and field emission SEM and the phase analysis was quantified by Reitveld methods. Dittmarite is a very rare mineral and only reported from a small number of localities. It cannot be formed directly in caves as it is normally crystallised from boiling water. We propose that it is formed by decomposition of struvite, NH4MgPO4 ·6H2O, in the presence of ammonia arising from the fresh bat guano. The minerals dittmarite, struvite and newberyite, MgHPO4 ·3H2O, can transform between themselves in caves and in kidney stones (in vitro) according to the conditions. Two minerals, görgeyite, and whewellite, have not previously been reported from guano deposits. Gypsum and brushite are present in some samples and show up to 18% PO4 for SO4 in gypsum substitution and up to 18% SO4 for PO4 in brushite. Some pure end-member crystals were also identified. The whitlockite is also substituted by sulfate by up to 30% and averaging 10%. The cave walls are dolomite and a speleothem present shows calcite, magnesium calcite, gypsum and leonite, K2Mg(SO4)2 ·4H2O an unusual sulfate mineral. The extent and presence of the sulfates and of whewellite confirms that substantial ground water has entered the cave consistent with the known wetter past climate.