NEW FE-BEARING MEMBERS OF THE OLIVENITE GROUP FROM THE BLACK PINE MINE, MONTANA

Members of the olivenite group are widespread, occurring as secondary minerals in many deposits rich in both copper and arsenic. Existing members are characterised by the formula M1M2TO₄OH, where M1 and M2 are medium-sized divalent cations (e.g. Cu²⁺, Zn²⁺, Mn²⁺) and T is P⁵⁺ or As⁵⁺. The M1 site exhibits 5-fold trigonal bipyramidal coordination and the M2 site exhibits 6-fold octahedral coordination. The M1 and M2 sites can both be occupied by the same cation (e.g. libethenite, Cu₂PO₄OH; olivenite, Cu₂AsO₄OH; adamite, Zn₂AsO₄OH; eveite, Mn₂AsO₄OH) or by different cations (e.g. zincolibethinite, ZnCuPO₄OH; zincolivenite, ZnCuAsO₄OH).

A new member of the group, IMA2009-037, has recently been approved from the Black Pine mine, located 14.5 km NW of Philipsburg, Granite Co., Montana, USA. This mine has been sporadically worked since the 1880s for Au, Ag and W. The ore zone is also enriched in Te, Sb, As and P, resulting in a large suite of secondary minerals. The Black Pine mine is also the type locality for philipsburgite and joëlbruggerite, the Sb⁵⁺-analogue of dugganite.

IMA2009-037 has the formula Fe³⁺Cu²⁺(As,Sb)O₄O and corresponds to the Fe³⁺-analogue of zincolivenite, with Fe in 5-fold trigonal bipyramidal coordination. A wide range of chemical analyses exists for the new mineral. The chemical analyses suggests that a complete solid solution exists between it and olivenite, following the coupled substitution (Cu²⁺OH)↔(Fe³⁺O). Likewise, a complete solid solution exists between the new mineral and an as yet unnamed Fe³⁺Fe³⁺-end-member with the formula Fe³⁺₂(As_0.8 _0.2)O₄O. Both of these solid solution series have an additional component based on Sb⁵⁺↔As⁵⁺ substitution. The majority of the analyses show As⁵⁺>Sb⁵⁺, however, two analyses show Sb⁵⁺ dominance. These analyses correspond to the Sb⁵⁺-analogue of the Fe³⁺Fe³⁺-end-member mentioned above.