Scottyite, the natural analog of synthetic BaCu₂Si ₂O₇, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

A new mineral species, scottyite, ideally BaCu₂Si ₂O₇, 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 × 0.3 × 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/cm³. Optically, scottyite is biaxial (-), with α = 1.750(1), β = 1.761(1), and γ = 1.765(1), 2Vmeas = 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), SiO₂ 27.01(61), SrO 0.28(5), and Na₂O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba_1.00Sr_0.01Na_0.01Cu _2.04Si_1.97O₇. Scottyite is the natural analog of synthetic BaCu₂(Si,Ge)₂O₇, which exhibits novel one-dimensional quantum spin-1/2 antiferromagnetic properties with tunable super-exchange interactions. It is orthorhombic, with space group Pnma and unit-cell parameters a = 6.8556(2), b = 13.1725(2), c = 6.8901(1) Å, and V = 622.21(6) Å3. The structure of scottyite is characterized by flattened CuO₄ tetrahedra sharing corners with one another to form chains parallel to the c axis. These chains are interlinked by Si₂O ₇ tetrahedral dimers and Ba²⁺. The Ba²⁺ 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 Å, respectively. Scottyite is topologically related to a group of compounds with the general formula BaM₂²⁺Si₂O₇, where M = Be (barylite and clinobarylite), Fe (andrémeyerite), Mg, Mn, Co, and Zn.