1
Mineral
Sciences Department, Natural History Museum of Los Angeles County, 900
Exposition Boulevard, Los Angeles, CA 90007, USA
2
Division
of
Geological and Planetary Sciences, California Institute of Technology,
Pasadena,
CA 91125, USA
Pararaisaite,
Cu2+Mg[Te6+O4(OH)2]·6H2O,
is a new mineral from the North Star mine, Tintic district, Juab
County, Utah,
U.S.A. It is an
oxidation-zone mineral occuring in
vugs in a matrix of massive quartz with imbedded crystals of barite and
goldfieldite.
Crystals are deep blue, striated, prisms up
to 0.4 mm in length, elongated on [010] and exhibiting the forms {100},
{001},
{102}, {10-2}, and {114}. The mineral is transparent with vitreous
luster,
white streak, Mohs
hardness 2½, brittle
tenacity, splintery fracture, and
two cleavages: perfect on {001} and good
on {100}. The measured density is 2.85(2) g/cm3.
Pararaisaite is biaxial
(+) with α = 1.600(2), β =
1.616(2), γ = 1.713(3) (white light); 2V
= 47(1)°; slight r
> v
dispersion; orientation Z = b, X
≈ a, Y
≈ c; and pleochroism X very pale purple, Y purple, Z
blue green (X << Z
< Y). The Raman spectrum is
consistent with the presence of tellurate, OH, and H2O.
Electron-microprobe
analyses gave the empirical formula (Mg1.10Cu0.93Te0.96Sb0.01)Σ3O12H14.12.
The
mineral is monoclinic, space
group P21/c,
with a
= 9.6838(5), b =
5.75175(19), c = 17.6339(12) Å, β
= 90.553(6)°, V = 982.14(9) Å3,
and Z = 4. The five
strongest X-ray powder diffraction lines are [dobs
Å(I)(hkl)]:
8.77(100)(002), 4.824(71)(200,111),
4.248(85)(-202,202), 2.419(50)(400,024), and
1.8929(48)(-226,226). Pararaisaite
is dimorphous with raisaite. The
structure contains straight edge-sharing chains of alternating Cu2+O4(OH)2
and Te6+O4(OH)2
octahedra. The chains link to
one another via shared octahedral
corners to form charge neutral [Cu2+Te6+O4(OH)2]
sheets. Interlayer Mg(H2O)6
octahedra link the sheets via
hydrogen bonds.