Guanacoite, Cu2Mg2(Mg0.5Cu0.5)(OH)4(H2O)4(AsO4)2, a new arsenate mineral species from the El Guanaco Mine, near Taltal, Chile: Description and crystal structure

Abstract

Guanacoite, ideally Cu 2 Mg 2 (Mg 0.5 Cu 0.5 )(OH) 4 (H 2 O) 4 (AsO 4 ) 2 , is a new arsenate mineral from the El Guanaco Mine, around 93 km east of Taltal and 230 km southeast of Antofagasta, 2 nd Region, Northern Chile. The mineral occurs as blue to pale blue prismatic to acicular crystals up to 0.7 mm in length, in association with arhbarite, conichalcite, olivenite, chrysocolla, brochantite, quartz and enargite. The crystals are brittle, have a vitreous luster and are transparent. The streak is white to pale blue, the Mohs hardness is 3. Electron microprobe analysis gave (wt. %) CuO 29.67, MgO 17.12, As 2 O 5 35.67, H 2 O 18 (from thermal analysis), total 100.46. The empirical formula is Cu 2.32 Mg 2.64 (OH) 4.13 (H 2 O) 4.15 (AsO 4 ) 1.93 , simplified to Cu 2 Mg 2 (Mg 0.5 Cu 0.5 ) (OH) 4 (H 2 O) 4 (AsO 4 ) 2 . Guanacoite is monoclinic, space group P 2 1 / c , with a = 5.482(4), b = 16.84(1), c = 6.911(5) A, β = 99.98(7)°, V = 628.5(5) A 3 from X-ray powder data, and a = 5.475(1), b = 16.865(3), c = 6.915(1) A, β = 99.80(3)°, V = 629.2(2) A 3 from single-crystal data. The measured density is 3.31 g/cm 3 , the calculated density ( Z = 2) is 3.36 g/cm 3 for the ideal composition and 3.30 g/cm 3 for the average Cu/Mg ratio measured by microprobe. Guanacoite is optically biaxial negative with α 1.664(1), β 1.691(1), γ 1.695(1), 2V(meas.) 31(1)°, 2V(calc.) 42° at 589 nm; pleochroism was not observed, dispersion is r > v weak, and the orientation is X ~ c , Y ⁁ a 10° (in obtuse β), Z = b. The mineral is isotypic with akrochordite, (Mn,Mg) 5 (OH) 4 (H 2 O) 4 (AsO 4 ) 2 . Dominating elements of the crystal structure are edge-sharing, distorted (Cu,Mg)-, Cu- and MgO 6 octahedra which form ribbons extending along the a -axis, topologically very similar to those in amphiboles. These ribbons are linked to adjacent, parallel ribbons via AsO 4 tetrahedra, thus forming heteropolyhedral slabs parallel to (010). The slabs are linked to each other only by very strong hydrogen bonds crossing the (010) plane. Guanacoite has also been found at Taghouni (Tarouni), Bou Azzer district, Morocco, in association with quartz, dolomite, chalcopyrite, chromite, cuprite, malachite and agardite-(Ce). Cation contents (normalized to 5): Cu 2.33, Mg 2.57, Co 0.10; unit-cell parameters: a = 5.477(1), b = 16.873(3), c = 6.898(1), A, β = 99.86(3)°, V = 628.1(2) A 3 ; measured density: 3.29(2) g/cm 3 ; optical properties: α 1.663(1), β 1.691(1), γ 1.693(1) (589 nm), 2 V (meas.) = 31(1)°, dispersion r > v (strong), pleochroism Z greenish blue >> X pale greenish blue > Y nearly colorless, orientation is Z = b and Y ⁁ a = 17° (in obtuse β).