Nestolaite, CaSeO_3-H_2O, a new mineral from the Little Eva mine, Grand County, Utah, USA

摘要

Nestolaite (IMA 2013-074), CaSeO_3-H_2O, is a new mineral species frorri the Little Eva mine, Grand County, Utah, USA. It is named in honour of the prominent Italian mineralogist and crystallographer Fabrizio Nestola. The new mineral was found on sandstone matrix as rounded aggregates up to 2 mm across and up to 0.05 mm thick consisting of tightly intergrown oblique-angled, flattened to acicular crystals up to 30 urn long and up to 7 μm (very rarely up to 15μm) thick. Nestolaite associates with cobaltomenite, gypsum, metarossite, orschallite and rossite. The new mineral is light violet and transparent with a white streak and vitreous lustre. The Mohs hardness is 2 1/2. Nestolaite is brittle, has uneven fracture and perfect cleavage on {100}. The measured and calculated densities are D_(neas). = 3.18(2) g/cm~3 and D_(calc). = 3.163 g/cm~3. Optically, nestolaite is biaxial positive. The refractive indices are a = 1.642(3), (3 = 1.656(3), y = 1.722(6). The measured 2V is 55(5)° and the calculated 2 V is 51°. In transmitted light nestolaite is colourless. It does not show pleochroism but has strong pseudo-absorption caused by high birefringence. The chemical composition of nestolaite (wt.%, electron-microprobe data) is: CaO 28.97, SeO_2 61.14, H_2O (calc.) 9.75, total 99.86. The empirical formula calculated on the basis of 4 O a.p.f.u. (atoms per formula unit) is Ca_(0.96)Se_(1.02)O_3-H_2O. The Raman spectrum is dominated by the Se-O stretching and O-Se-O bending vibrations of the pyramidal SeO_3 groups and O-H stretching modes of the H_2O molecules. The mineral is monoclinic, space group P2_1/c, with a = 7.6502(9), b = 6.7473(10), c = 7.9358(13) A, β = 108.542 (12)°, V= 388.37(10) A~3 and Z= 4. The eight strongest powder X-ray diffraction lines are [d_(obs) in A (hkl) (I_(rel))]: 7.277 (100)(100), 4.949 (110)(37), 3.767 (002)(29), 3.630 (200)(58), 3.371 (020)(24), 3.163 (202)(74), 2.9783 (121)(74) and 2.7231 (112)(31). The crystal structure of nestolaite was determined by means of the Rietveld refinement from the powder data to R_(wp) = 0.019. Nestolaite possesses a layered structure consisting of Caφ-SeO_3 sheets, composed of edge-sharing polyhedra. Adjacent sheets are held by H bonds emanating from the single (H_2O) group within the sheets. The nestolaite structure is topologically unique.