Brushite in the bat guano deposit from the 'dry' Cioclovina Cave (Sureanu Mountains, Romania)

摘要

This paper provides new mineralogical information on brushite from the famous bat guano deposit from the "dry" Cioclovina Cave (Sureanu Mountains, Romania). The mineral occurs as snow-white earthy masses generally associated with hydroxylapatite, but also with ardealite and crandallite, in the guano mass from the cave. Individual crystals do not exceed 10 mu m in length. They are roughly prismatic, platy on (010) and generally associates in sheaf- or cluster-like aggregates. The indices of refraction are alpha = 1.540(5), beta = 1.546(2) and gamma (calculate for 2V = 86 deg) = 1.551. The measured density [D_m = 2.32(1) g/cm~3] agrees well with the calculated value [D_x = 2.321(5) g/cm~3]. Brushite from Cioclovina closely matches the CaHPO_4 centre dot 2H_2O end-member. S-for-P substitutions in the tetrahedral site however account for up to 8.8 percent from these sites. The average unit cell parameters refined from 30 sets of X-ray powder data are a = 5.811(4) A, b = 15.175(12) A, c = 6.241(7)A and beta = 116.37(8) deg. The thermal analysis shows that water is lost in three steps; the loss of molecular water is a two-step process and is complete before 210 deg C. Dehydroxylation of the protonated phosphate groups occurs at approx 433 deg C. The fully dehydrated material, which is initially X-ray amorphous, recrystallizes at approx 490 deg C as beta-Ca_2P_2O_7 [a= 6.686(3) A, c = 24.089(18) A]. The band multiplicity on the IR-absorption spectrum suggests that the protonated phosphate groups have C_s punctual symmetry. The mineral derives from the reaction between calcium carbonate from the moonmilk flows or the cave floor and phosphoric solutions derived from guano, with or without hydroxylapatite as a precursor.