Discreditation of “orthobrochantite” (IMA 78–64) as the MDO₁ polytype of brochantite

“Orthobrochantite,” IMA 78–64, was originally approved by the Commission on New Minerals, Nomenclature and Classification (IMA) as the orthorhombic polymorph of brochantite. Described by Wilson W. Crook III and Stanley G. Oswald from the Douglas Hill mine, Yerington, Nevada, USA, the mineral description was never formally published; however, the name and some data have been widely available since the late 1970s. Investigation of material from the Douglas Hill mine shows “orthobrochantite” to consist mostly of the MDO₁ polytype of brochantite, but probably also to contain small domains of the MDO₂ polytype. The crystal structure of the MDO₁ polytype [a = 13.1117(4), b = 9.8654(4), c = 6.0307(9) Å, β = 103.255(7)° and V = 759.31(12) ų] has been refined to R₁ = 6.37 % for 1245 unique reflections [F_o > 4σF] and 8.83 % for all 1724 reflections. The incorrect unit cell for “orthobrochantite” is either due to the choice of the B-centered pseudo-orthorhombic cell, which is virtually identical to the MDO₁ cell, or indexing based on {100} twinning of the MDO₁ polytype. New optical determinations for “orthobrochantite” suggest that the indices of refraction reported by Crook and Oswald were significantly in error.