The new mineral species grandaite, ideally
Sr2Al(AsO4)2(OH), has been discovered on
the dump of Valletta mine, Maira Valley, Cuneo province, Piedmont, Italy.
Its origin is related to the reaction between the ore minerals and
hydrothermal solutions. It occurs in thin masses of bright orange to salmon
to brown coloured crystals, or infrequently as fan-like aggregates of small
(<1 mm) crystals, with reddish-brown streak and waxy to vitreous lustre.
Grandaite is associated with aegirine, baryte, braunite, hematite, tilasite,
quartz, unidentified Mn oxides and Mn silicates under study.
Grandaite is biaxial (+) with refractive indices α = 1.726(1), β = 1.731(1),
γ = 1.752(1). Its calculated density is 4.378 g/cm3. Grandaite is
monoclinic, space group P21/m,
with a = 7.5764(5), b = 5.9507(4),
c = 8.8050(6) Å, β = 112.551(2)°, V =
366.62(4) Å3 and Z = 2. The eight strongest
diffraction lines of the observed X-ray powder diffraction pattern are
[d in Å, (I), (hkl)]:
3.194 (100)(
11), 2.981
(50.9)(020), 2.922 (40.2)(
03), 2.743
(31.4)(120), 2.705 (65.2)(112), 2.087 (51.8) (
23), 1.685 (24.5)(321), 1.663 (27.7)(132). Chemical
analyses by electron microprobe gave (wt.%) SrO 29.81, CaO 7.28, BaO 1.56,
Al2O3 7.07, Fe2O3 2.34,
Mn2O3 1.88, MgO 1.04, PbO 0.43,
As2O5 44.95, V2O5 0.50,
P2O5 0.09, sum 96.95; H2O 1.83 wt.% was
calculated by stoichiometry from the results of the crystal-structure
analysis. Raman and infrared spectroscopies confirmed the presence of
(AsO4)3− and OH groups. The empirical formula
calculated on the basis of 9 O a.p.f.u., in agreement with the structural
results, is
(Sr1.41Ca0.64Ba0.05Pb0.01)∑=2.11(Al0.68Fe0.14
3+Mn0.12
3+Mg0.13)∑=1.07
[(As0.96V0.01)∑=0.97O4]2(OH),
the simplified formula is
(Sr,Ca)2(Al,Fe3+)(AsO4)2(OH)
and the ideal formula is
Sr2Al(AsO4)2(OH).
The crystal structure was solved by direct methods and found to be
topologically identical to that of arsenbrackebuschite. The structure model
was refined on the basis of 1442 observed reflections to R
1 = 2.78%. In the structure of grandaite, chains of edge-sharing
M
3+ octahedra run along [010] and share vertices with
T5+ tetrahedra, building up [M
3+(T
5+O4)2(OH, H2O)] units, which
are connected through interstitial divalent cations. Grandaite is named
after the informal appellation of the province where the type locality is
located. The new mineral was approved by the International Mineralogical
Association Commission on New Minerals, Nomenclature and Classification
(IMA2013-059). The discovery of grandaite and of other members of the group
(description still in progress) opens up the possibility of exploring the
crystal chemistry of the brackebuschite supergroup.