The occurrence of taranakite in the 'dry' Cioclovina Cave (Sureanu Mountains, Romania)

This paper documents the chemistry, physical properties, thermal and infrared behavior and unit-cell parameters of taranakite in the "dry" Cioclovina Cave, which is the second documented occurrence of this mineral in Romania. The mineral occurs as earthy white nodules scattered throughout the terra rossa mass. Associated minerals are low (alpha) quartz and scarce illite. Individual crystals do not exceed 7μm in diameter, are roughly hexagonal and tabular on (0001). The indices of refraction are e = 1.508(1) and to = 1.502(1). The measured density [D m = 2.11(1) g/cm 3 agrees well with the calculated value [D x = 2.133 g/cm 3 ]. A wet-chemical analysis yielded (in wt.%): K 2 O = 9.06, Na 2 O = 0.11, (NH 4 ) 2 O = 0.33, MnO = 0.02, MgO = 0.07, CaO = 0.31, Al 2 O 3 = 18.78, Fe 2 O 3 = 0.34, P 2 O 5 = 42.32, H 2 O = 28.30, total 99.64, which leads to the chemical-structural formula: [K 2 . 5 8 1 Na 0 . 0 4 1 (NH 4 ) 0 . 1 6 9 Ca 0 . 0 7 4 Mg 0 . 0 2 3 Mn 0 . 0 0 4 ](Al 4 . 9 4 2 Fe 0 . 0 5 7 )(PO 4 ) 1 . 9 9 (HPO 4 ) 6 . 0 1 18.07H 2 O. The hexagonal cell parameters refined from three sets of X-ray powder data are a = 8.687(9)A, c = 95.69(5)A, V = 6254(16)A 3 , a: c = 0.091:1. Thermal analyses showed that the molecular water is lost in two steps, before 200°C; the dehydration is complete at 365°C. The band multiplicity on the IR-absorption spectrum suggests that the protonated and unprotonated phosphate groups have C 3 and C 3 v punctual symmetries, respectively, whereas the strong imprint of the bands assumed to P-O-H vibrations suggests the local presence of (H 2 PO 4 ) - groups. Taranakite from Cioclovina is authigenic and crystallized through the in situ fixation of the potassium and aluminum ions derived from a clay precursor into a phosphate structure.