Brushite in the bat guano deposit from the "dry" Cioclovina Cave (Sureanu Mountains, Romania)
Dumitraş, Delia-Georgeta; Marincea, Ştefan; Fransolet, André -Mathieu
published: Apr 15, 2004
ArtNo. ESP154018001003, Price: 29.00 €
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 10μm in length. They are roughly prismatic, platy on (010) and generally associates in sheaf- or cluster-like aggregates. The indices of refraction are α = 1.540(5), β = 1.546(2) and γ (calculate for 2V = 86) = 1.551. The measured density [Dm = 2.32(1) g/cm3] agrees well with the calculated value [Dx = 2.321(5) g/cm3]. Brushite from Cioclovina closely matches the CaHPO4 · 2H2O end-member. S-for-P substitutions in the tetrahedral site however account for up to 8.8 % from these sites. The average unit cell parameters refined from 30 sets of X-ray powder data are a = 5.811(4)Å, b = 15.175(12)Å, c = 6.241(7)Å and β = 116.37(8). 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 C. Dehydroxylation of the protonated phosphate groups occurs at ≈433 C. The fully dehydrated material, which is initially X-ray amorphous, recrystallizes at ≈490 C as β-Ca2P2O7 [a = 6.686(3)Å, c = 24.089(18)Å]. The band multiplicity on the IR-absorption spectrum suggests that the protonated phosphate groups have Cs 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.