Brochantite, Cu (sub 4) So (sub 4) (OH) (sub 6) ; OD character, polytypism and crystal structures
Brochantite, Cu (sub 4) So (sub 4) (OH) (sub 6) ; OD character, polytypism and crystal structures (in Special issue dedicated to the memory of Luciano Ungaretti, Maria Chiara Domeneghetti (prefacer))
European Journal of Mineralogy (April 2003) 15 (2): 267-275
Single-crystal XRD studies on brochantite demonstrate its order-disorder character; the structure is built of equivalent order-disorder layers with symmetry Pn2 (sub 1) m. Two maximum degrees of order (MDO) polytypes have been identified in brochantite from various localities. The MDO (sub 1) polytype corresponds with 'normal' brochantite P12 (sub 1) /a1, with a 13.140, b 9.863, c 6.024 Aa, beta 103.16 degrees , whereas the MDO (sub 2) polytype is monoclinic, P2 (sub 1) /n11 with a 12.776, b 9.869, c 6.026 Aa, beta 90.15 degrees . Given their polytypic relationships, the two MDO (sub 1) and MDO (sub 2) polytypes can be designated as brochantite-2M (sub 1) and brochantite-2M (sub 2) , respectively. The refined structure of brochantite-2M (sub 1) is in substantial agreement with earlier work; a structural model for the newly discovered 2M (sub 2) polytype is derived and has been refined to R 0.062.