End-member villyaellenite from Mapimi, Durango, Mexico; descriptive mineralogy, crystal structure, and implications for the ordering of Mn and Ca in type villyaellenite

The second occurrence of villyaellenite, at the Ojuela mine, Mapimi, Durango, Mexico, has yielded the nearly pure end member of the species. The mineral occurs as a compact spray of orange-pink prismatic crystals up to 4 cm in length, associated with ogdensburgite, arseniosiderite, adamite, and chalcophanite. Crystals are elongate parallel to [001] with forms {100}, {110}, and {101}. There is a good {100} cleavage, and the hardness is about 4. Optically, it is biaxial (−) with refractive indices α = 1.713(2), β = 1.723(2), and γ = 1.729(2); 2V_meas = 70(5)° and 2V_calc = 75°; X = b, Z ∧ c = 40°. Absorption isZ ≫ X > Y (X = very pale orange-pink, Y = exceedingly pale orange-pink, Z = pale orange-pink). The specific gravity is 3.69, and the density (calc.) is 3.72 g/cm³.

Chemical analysis by electron microprobe yielded CaO 0.5, MnO 36.2, FeO 0.1, ZnO 2.9, and As₂O₅ 50.6, and moisture evolution analysis gave 9.9 H₂O, total 100.2 wt%. The empirical formula based upon 20 oxygens is

(Z = 4), close to pure Mn₅(H₂O)₄(AsO₃OH)₂(AsO4)₂.

Single-crystal X-ray precession data indicate space groups Cc or C2/c; the latter is confirmed by structure analysis. Cell parameters determined during crystal-structure analysis are a = 18.015(5), b = 9.261(2), c = 9.770(3) Å, β = 96.238(7)°, V = 1620.3 ų. The strongest lines in the X-ray powder-diffraction pattern (d in Å, I, hkl) are 8.96(50)(200), 8.21(100)(110), 3.243(80)(511), 3.209(70)(222), 3.068(70)(222,511), 2.978(50)(600), and 1.678(60)(642).

The refractive indices, specific gravity, chemistry, and cell parameters differ markedly from those of type villyaellenite from Sainte-Marie aux Mines, France, which is near the midpoint of the villyaellenite-sainfeldite series. A third occurrence of villyaellenite at Sterling Hill, New Jersey, consists of material nearly identical chemically to the villyaellenite from Mapimi.

Villyaellenite is isostructural with sainfeldite and hureaulite. The crystal structure of the near end member from Mapimi has been refined by the method of least-squares on the basis of 1183 reflections measured on an automated single-crystal diffractometer to R = 0.076 (R_w = 0.089). The average Mn-O bond lengths for the three nonequivalent octahedra that make up its five-member edge-sharing chain fragments are Mn(l)-0 = 2.21, Mn(2>- O = 2.14, and Mn(3)-0 = 2.24 Å. Distance least-squares refinements of the type villyaellenite structure [Mn/(Mn + Ca) = 0.565] yield M(1)-0 = 2.27, M(2)-0 = 2.20, and M(3)- O = 2.36 Å, suggesting the ordering scheme M(1) = Mn + Ca, M(2) = Mn, and M(3) = Ca. The abnormally large a cell parameter in type villyaellenite relative to the Mn and Ca end members of the series and its structural and paragenetic relationships to fluckite further suggest that Mn and Ca are at least partially ordered in type villyaellenite.