Brochantite, Cu4SO4(OH)(6): OD character; polytypism and crystal structures

Single-crystal X-ray diffraction studies of brochantite demonstrate its Order-Disorder (OD) character. The OD structures of the brochantite family can be described as built up by equivalent OD layers with symmetry Pn2(1)m. Two MDO polytypes are possible in this family, and both have been identified in specimens of brochantite from various localities. The MDO1 polytype corresponds to "normal" brochantite P12(1)/a1, a = 13.140(2) Angstrom, b = 9.863(2), c = 6.024(1), beta = 103.16(3)degrees, whereas the newly discovered MDO2 polytype is monoclinic, P2(1)/n11, a = 12.776(2) Angstrom, b = 9.869(2), c = 6.026(1), a = 90.15(3)degrees. Given their polytypic relationships, the two MDO1 and MDO2 polytypes can be designated as brochantite-2M(1) and brochantite-2M(2), respectively. The refined structure (R-1 = 0.049) of brochantite-2M(1) substantially agrees with the model of Cocco & Mazzi (1959) and confirms the assignment of the OH bands due to Schmidt & Lutz (1993). On the basis of the OD relationships between the two polytypes, a structural model for the newly discovered 2M(2) polytype was derived and subsequently refined up to R-1 = 0.062.