• journal_title：American Mineralogist
• Contributor：Hexiong Yang ; Robert T. Downs ; Stanley H. Evans ; William W. Pinch
• Publisher：Mineralogical Society of America
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4326
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：2-3
• firstpage：478
• section：Article

A new mineral species, scottyite, ideally BaCu<sub>2sub>Si<sub>2sub>O<sub>7sub>, 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 <strong>cstrong> 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<sup>3sup>. Optically, scottyite is biaxial (-), with α = 1.750(1), <strong>ßstrong> = 1.761(1), and γ = 1.765(1), 2V<sub>meassub> = 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<sub>2sub> 27.01(61), SrO 0.28(5), and Na<sub>2sub>O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba<sub>1.00sub>Sr<sub>0.01sub>Na<sub>0.01sub>Cu<sub>2.04sub>Si<sub>1.97sub>O7.

Scottyite is the natural analog of synthetic BaCu<sub>2sub>(Si,Ge)<sub>2sub>O<sub>7sub>, 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) Å, and V = 622.21(6) Å<sup>3sup>. The structure of scottyite is characterized by flattened CuO<sub>4sub> tetrahedra sharing corners with one another to form chains parallel to the <strong>cstrong> axis. These chains are interlinked by Si<sub>2sub>O<sub>7sub> tetrahedral dimers and Ba<sup>2+sup>. The Ba<sup>2+sup> 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 BaM<sup>2+sup><sub>2sub>Si<sub>2sub>O<sub>7sub>, where M = Be (barylite and clinobarylite), Fe (andrémeyerite), Mg, Mn, Co, and Zn.