Abstract
Cabrerite (IMA2023-123), NiMg2(AsO4)2·8H2O, is a newly approved mineral species from the Nickel mine, Cottonwood Canyon, Table Mountain district, Churchill County, Nevada, U.S.A. that was originally described in 1863 from Sierra Cabrera, Almería, Andalusia, Spain. At the Nickel mine, cabrerite occurs in divergent groups of green blades up to 1 mm long. Blades are elongated and striated parallel to [001], flattened on {010}, and exhibit the forms {010}, {110}, and {20-1}. The mineral is transparent with vitreous luster and white streak. The Mohs hardness is ~2½. The mineral has moderately sectile tenacity, irregular and stepped fracture, and three cleavages: perfect on {010}, fair on {100} and poor on {-102}. The measured density is 2.93(2) g·cm-3. The mineral dissolves slowly in RT H2O. The mineral is optically biaxial (+), α = 1.609(2), β = 1.633(2), γ= 1.667(2) (white light); 2Vmeas = 82(2)°; slight r < v dispersion; orientation X = b, Z ^ c = 37° in obtuse β; nonpleochroic. Electron microprobe analysis provided the empirical formula (Mg1.46Ni1.55)∑3.01(As1.00O4)2·8H2O. Cabrerite is monoclinic, C2/m, a = 10.2054(11), b = 13.3772(13), c = 4.7382(4) Å, β = 105.057(7)°, V = 624.66(11) Å3, and Z = 2. The mineral has a vivianite-type structure (R1 = 0.0353 for 668 I > 2σI reflections) in which MlO2(H2O)4 octahedra and M22O6(H2O)4 edge-sharing octahedral dimers are linked together via TO4 tetrahedra (where T = P or As) and hydrogen bonds to form layers parallel to {010}; successive layers are linked by hydrogen bonds only. Cabrerite is the ordered intermediate between hörnesite and annabergite, with Ni dominant at M1, Mg dominant at M2, and T = As.
