Nevadaite, (Cu2+, Al, V3+)6 [Al8 (PO4)8 F8] (OH 2 (H2O)22, a new phosphate mineral species from the Gold Quarry mine, Carlin, Eureka County, Nevada: Description and crystal structure

Nevadaite,  (Cu²⁺, □, Al, V³⁺)₆ (PO₄)₈ F₈ (OH)₂ (H₂O)₂₂, is a new supergene mineral species from the Gold Quarry mine, near Carlin, Eureka County, Nevada, U.S.A. Nevadaite forms radiating clusters to 1 mm of prismatic crystals, locally covering surfaces more that 2 cm across; individual crystals are elongate on [001] with a length:width ratio of > 10:1 and a maximum diameter of ~30 μm. It also occurs as spherules and druses associated with colorless to purple-black fluellite, colorless wavellite, strengitevariscite, acicular maroon-to-red hewettite, and rare anatase, kazakhstanite, tinticite, leucophosphite, torbernite and tyuyamunite. Nevadaite is pale green to turquoise blue with a pale powder-blue streak and a vitreous luster; it does not fluoresce under ultra-violet light. It has no cleavage, a Mohs hardness of ~3, is brittle with a conchoidal fracture, and has measured and calculated densities of 2.54 and 2.55 g/cm³, respectively. Nevadaite is biaxial negative, with 1.540, β 1.548, γ 1.553, 2V(obs.) = 76°, 2V(calc.) = 76°, pleochroic with X pale greenish blue, Y very pale greenish blue, Z blue, and with absorption Z ≫ X > Y and orientation X = c, Y = a, Z = b. Nevadaite is orthorhombic, space group P2₁mn, a 12.123(2), b 18.999(2), c 4.961(1) Å, V 1142.8(2) Å ³, Z = 1, a:b:c = 0.6391:1:0.2611. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 6.077(10)(200), 5.618(9)(130), 9.535(8)(020), 2.983(6)(241), 3.430(4)(041), 2.661(4)(061), and 1.844(4)(352). A chemical analysis with an electron microprobe gave  P₂O₅ 32.54, Al₂O₃ 27.07, V₂O₃ 4.24, Fe₂O₃ 0.07, CuO 9.24, ZnO 0.11, F 9.22, H₂O (calc.) 23.48, OH ≡ F–3.88, sum 102.09 wt.%; the valence states of V and Fe, and the amount of H₂O, were determined by crystal-structure analysis. The resulting empirical formula on the basis of 63.65 anions (including 21.65 H₂O pfu) is ((Cu_2.00 ²⁺ Zn_0.02 V_0.98 ³⁺ Fe_0.01 ³⁺ Al_1.15)_{∑ 4.16} Al₈ P_7.90 O₃₂ [F_8.37 (OH)_1.63]_∑10 (H₂O)_21.65. The crystal structure of nevadaite was solved by direct methods and refined to an R index of 4.0% based on 1307 observed reflections collected on a four-circle diffractometer with MoKα X-radiation. The structure consists of ordered layers of vertex-sharing octahedra and tetrahedra alternating with layers of disordered vertex-sharing and face-sharing octahedra in the b direction. [Alϕ₅] chains of octahedra are decorated by (PO₄) tetrahedra that share vertices with octahedra adjacent in the chain. These chains link in the c direction by sharing vertices between octahedra and tetrahedra to form an ordered layer of the form [Al₈(PO₄)₈F₈(H₂O)₈]. In the disordered layer, octahedra containing positionally disordered Cu²⁺, V³⁺, Al and □ (vacancy) share trans faces to form columns that link by sharing octahedron vertices to form ribbons extending in the c direction; the resulting layer has the form {(Cu₂ ²⁺□₂V³⁺,Al)_∑6 (H₂O)₁₂ (OH)₂ (H₂O)_x}, x ≈ 2. The layers link in the b direction by sharing vertices between octahedra and tetrahedra. Although decorated chains topologically equivalent to that in nevadaite are common in many oxysalt minerals, its chain is geometrically distinct from those topologically equivalent chains. The M–M linkage along the [Mϕ₅] chains in most minerals take place through trans vertices of the octahedra, with one example of linkage through cis vertices; in nevadaite, the M–M linkage involves both trans and cis vertices, as does the chain in slavíkite. In most of these decorated chains, alternate tetrahedra along the chain occur either in a trans or a cis arrangement. In nevadaite and slavíkite, the tetrahedra are arranged in both trans and cis arrangements; the arrangements in these two minerals are geometrically distinct, however.