Tinsleyite in the ''dry'' Cioclovina Cave (Sureanu Mountains, Romania): the second occurrence

Abstract

This paper documents the occurrence of tinsleyite in the dry Cioclovina Cave, Sureanu Mountains, Romania. This is the first mention of tinsleyite as a cave mineral and the second find of this mineral species in the world. The mineral occurs in small quantities, as a rare authigenic, early diagenetic mineral, in the bat guano deposits in this cave. Low (alpha) quartz is the only closely associated mineral. This assemblage corresponds to a possible reaction in the natural guano - clay system upon decomposition of illite. Chemical analyses show variable leucophosphite contents (up to 31.68 mol.%). The unit-cell parameters calculated for an iron-poor sample are a = 9.638(5) A , b = 9.522(4) A , c = 9.540(4) A and b = 103.10(3)° , whereas those deter- mined for an iron-rich sample are a = 9.639(6) A , b = 9.538(8) A , c = 9.553(7) A and b = 103.04(4)° . Thermal analyses show that water is lost in three steps and that the loss of molecular water is complete before 250° C. The fully dehydrated material is com- pletely X-ray amorphous. Ammonium is expelled at about 490° C. The infrared spectrum of tinsleyite affords reasonable evidence for the presence of PO 4, NH4 and OH groups, together with H 2O molecules. The punctual symmetry of the PO 4 groups is C s.