Hydroxylellestadite from Cioclovina Cave (Romania): Microanalytical, structural, and vibrational spectroscopy data

Abstract

Electron-microprobe analyses of hydroxylellestadite from the Cioclovina Cave (Romania) gave the composition Ca 10.27 [(SiO 4 ) 2.53 (SO 4 ) 2.17 (PO 4 ) 1.27 ] Σ=5.97 [(OH) 1.66 F 0.21 Cl 0.16 ] Σ=2.03 . The mineral is translucent to transparent, light orange, slightly fl uorescent, has a vitreous luster and <1.5 mm in length. A singlecrystal X-ray structure investigation gave the average space-group symmetry P6 3 /m [R1(F) = 0.038 for 783 refl ections up to 2θ MoKα = 70° and 42 variables, a = 9.496 (2), c = 6.920(2) Å, V = 540.4 Å 3 , and Z = 2]. Some atoms exhibit large anisotropic displacements. Ordering of atoms along with a symmetry reduction is not verifi ed. Fourier-transformed infrared (FT-IR) and micro-Raman spectra exhibit a distinct contribution from (PO 4 ) 3-modes along with the characteristic (SO 4 ) 2-and (SiO 4 ) 4-modes. The occurrence is quite unusual and suggests that an intense thermal process affected a restricted area within the cave. Hydroxylellestadite is associated with berlinite, another high-temperature mineral. It is likely to have formed within highly phosphatized, silicate-rich, carbonate-mudstone sediments heavily compacted and thermally transformed due to in situ bat guano combustion.

Chlorellestadite, Ca5(SiO4)1.5(SO4)1.5Cl, a new ellestadite- group mineral from the Shadil-Khokh volcano, South Ossetia

“…5) is analogous to other spectra of the ellestadite group minerals (Harada et al 1971;Livingstone et al 1987;Onac et al 2006). The main bands correspond to the Si-O and S-O vibrations in tetrahedral sites.…”

“…The first region (a) is characterized by the appearance of many overlapping bands marked by low-intensity. Relatively intense bands at 649 and 630 cm − 1 are in this range; they correspond to the ν 4 bending vibrations connected with the SO 4 2− group (Onac et al 2006). Two lower intense bands at 536 and 572 cm −1 are related to the ν 4 vibrations caused by SiO 4 4− groups (Onac et al 2006).…”

“…Both spectra of F-Cl-bearing hydroxylellestadite and OHCl-bearing fluorellestadite from the xenolith from the ShadilKhokh volcano have bands in the 3500-3600 cm −1 range related to the presence of a hydroxyl group in the channel sites (Onac et al 2006;Comodi et al 2016). In this range, the fact that the chlorellestadite spectrum does not show bands reflects the low water content in the chemical composition of the studied fragment of the mineral.…”

“…There exists the following structural data for the ellestadite group minerals: hydroxylellestadite from Cioclovina Cave, Romania (P 6 3 / m, a = 9.496(2), c = 6.920(2) Å, V = 540.4 Å 3 ; Onac et al 2006); Crestmore, California, USA (P2 1 /m, a = 9.5224, b = 9.5268, c = 6.9087 Å, β = 119.9483°, V = 531.48 Å 3 ; Hughes and Drexler 1991) and Chichibu, Japan (P2 1 /m, a = 9.476(2), b = 9.508(2), c = 6.919(1) Å, β = 119.53°, V = 542.41 Å 3 ; Sudarsanan 1980). The symmetry reduction of hydroxylellestadite relates to F, Cl and (OH) ordering in channel sites as well as cation ordering in tetrahedral sites (Sudarsanan 1980;Hughes and Drexler 1991).…”

Revisiting three minerals from Cioclovina Cave (Romania)

“…1b) is of great interest for the occurrence of unusual authigenic cave minerals. To date, over 25 mineral species attributed to six mineral classes (carbonates, oxides and hydroxides, phosphates, sulfates, silicates, and halides) have been documented, some rather unusual for a cave environment (Marincea et al, 2002;Onac et al, 2002Onac et al, , 2005aOnac et al, , 2005bOnac et al, , 2007Onac & Effenberger, 2007;Onac & Tămaş, 2010). Phosphates are by far the best represented class of minerals in Cioclovina Cave.…”