Scottyite, the natural analog of synthetic BaCu2Si2O7, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

Abstract

A new mineral species, scottyite, ideally BaCu 2 Si 2 O 7 , has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. The mineral appears to have formed as a result of a hydrothermal event and is associated with wesselsite, pectolite, richterite, sugilite, and lavinskyite. Scottyite forms blocky grains with striations parallel to the c axis. Crystals are found up to 0.4 x 0.3 x 0.3 mm. No twinning is observed. The mineral is dark-blue in transmitted and under incident lights, transparent with pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of 4~5; cleavage is perfect on {100} and {010} and no parting was observed. The calculated density is 4.654 g/cm 3 . Optically, scottyite is biaxial (-), with α = 1.750(1), s = 1.761(1), and γ = 1.765(1), 2 V meas = 66(2)°. It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis produced an average composition (wt%) (8 points) of CuO 36.98(31), BaO 35.12(16), SiO 2 27.01(61), SrO 0.28(5), and Na 2 O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba 1.00 Sr 0.01 Na 0.01 Cu 2.04 Si 1.97 O7. Scottyite is the natural analog of synthetic BaCu 2 (Si,Ge) 2 O 7 , which exhibits novel one-dimensional quantum spin-1/2 antiferromagnetic properties with tunable super-exchange interactions. It is ortho-rhombic, with space group Pnma and unit-cell parameters a = 6.8556(2 ), b = 13.1725(2), c = 6.8901(1) A, and V = 622.21(6) A 3 . The structure of scottyite is characterized by flattened CuO 4 tetrahedra sharing corners with one another to form chains parallel to the c axis. These chains are interlinked by Si 2 O 7 tetrahedral dimers and Ba 2+ . The Ba 2+ cations are bonded to seven O atoms in an irregular coordination. The average Si-O, Cu-O, and Ba-O bond lengths are 1.630, 1.941, and 2.825 A, respectively. Scottyite is topologically related to a group of compounds with the general formula BaM 2+ 2 Si 2 O 7 , where M = Be (barylite and clinobarylite), Fe (andremeyerite), Mg, Mn, Co, and Zn.