AbstractNestolaite (IMA 2013-074), CaSeO3·H2O, is a new
mineral species from 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 μm thick consisting of tightly
intergrown oblique-angled, flattened to acicular crystals up to 30 μm long
and up to 7 μm (very rarely up to 15 μm) 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 2½. Nestolaite is brittle, has uneven fracture and perfect
cleavage on {100}. The measured and calculated densities are
Dmeas. = 3.18(2) g/cm3 and Dcalc. = 3.163 g/cm3. Optically, nestolaite is biaxial
positive. The refractive indices are α = 1.642(3), β = 1.656(3), γ =
1.722(6). The measured 2V is 55(5)° and the calculated
2V is 51°. In transmitted light nestolaite is
colourless. It does not show pleochroism but has strong pseudoabsorption
caused by high birefringence. The chemical composition of nestolaite (wt.%,
electronmicroprobe data) is: CaO 28.97, SeO2 61.14, H2O (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
Ca0.96Se1.02O3·H2O. The Raman
spectrum is dominated by the Se–O stretching and O–Se–O bending vibrations
of the pyramidal SeO3 groups and O–H stretching modes of the
H2O molecules. The mineral is monoclinic, space group
P21/c, with
a = 7.6502(9), b = 6.7473(10),
c = 7.9358(13) Å, β = 108.542 (12)°, V
= 388.37(10) Å3 and Z = 4. The eight strongest
powder X-ray diffraction lines are [dobs in Å(hkl) (Irel)]: 7.277 (100)(100), 4.949 (110)(37), 3.767 (002)(29), 3.630
(200)(58), 3.371 (020)(24), 3.163 (02)(74),
2.9783 (21)(74) and
2.7231 (112)(31). The crystal structure of nestolaite was determined by
means of the Rietveld refinement from the powder data to Rwp = 0.019. Nestolaite possesses a layered structure consisting
of CaΦ–SeO3 sheets, composed of edge-sharing polyhedra. Adjacent
sheets are held by H bonds emanating from the single (H2O) group
within the sheets. The nestolaite structure is topologically unique.