Cabrerite, NiMg2(AsO4)2<middle dot>8H2O, a new old mineral: The ordered intermediate between annabergite and hörnesite

Cabrerite (IMA2023-123), NiMg2(AsO4)(2)<middle dot>8H(2)O, 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, Almer & iacute;a, 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 {201<overline>}. The mineral is transparent with vitreous luster and white streak. The Mohs hardness is similar to 2 1/2. The mineral has moderately sectile tenacity, irregular and stepped fracture, and three cleavages: perfect on {010}, fair on {100}, and poor on {1<overline>02}. The measured density is 2.93(2) g<middle dot>cm(-3). The mineral dissolves slowly in RT dilute HCl. The mineral is optically biaxial (+), alpha = 1.609(2), beta = 1.633(2), gamma = 1.667(2) (white light); 2V(meas) = 82(2)degrees; slight r < v dispersion; orientation X = b, Z <^> c = 37 degrees in obtuse beta; nonpleochroic. Electron probe microanalysis provided the empirical formula (Mg1.46Ni1.55)(Sigma 3.01)(As1.00O4)(2)<middle dot>8H(2)O. Cabrerite is monoclinic, C2/m, a = 10.2054(11), b = 13.3772(13), c = 4.7382(4) & Aring;, beta = 105.057(7)degrees, V = 624.66(11) & Aring;(3), and Z = 2. The mineral has a vivianite-type structure (R-1 = 0.0353 for 668 I > 2 sigma(I) reflections) in which MlO(2)(H2O)(4) octahedra and M2(2)O(6)(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 hornesite and annabergite, with Ni dominant at M1, Mg dominant at M2, and T = As.