Temperature-dependent Al order-disorder in the tetrahedral double chain of C2/m amphiboles

The crystal structures and site populations of 8 calcic amphiboles from a leucogabbro from Punta Falcone, Sardinia, have been refined to R indices of ~ 2 % using single-crystal MoKa X-ray diffraction data. These amphiboles are unusual in that they show a significant degree of disorder of [4]-coordinated Al over the T(l) and T(2) sites in the tetrahedral double-chain of the C2/m amphibole structure. New relationships are derived between the distances and the Al contents of the T(l) and T(2) tetrahedra. It is also shown that the distance is significantly affected by variation in occupancy of other sites in the amphibole structure. The ordering behaviour of [4]A1 in amphiboles is examined in terms of local bond-valence requirements. In most amphiboles, [41A1 occurs completely at T(l) in order to avoid reducing the incident bond-valence to O(4), the only [3]-coordinated non-bridging oxygen in the amphibole structure. The normal limit of [4]A1 ~ 2 atoms per formula unit observed in amphiboles occurs so as to avoid Al-0(7)-Al linkages that are very unfavourable from a bondvalence viewpoint, and to avoid the occurrence of Al at T(2). When t4]Al > 2 apfu, the additional Al normally occurs at the T(2) site. Medium-temperature (high-pressure) amphiboles from the Liset eclogite pod show all [41A1 to occur at T(l) up to a limit of 2.0 apfu, with all additional |41A1 occurring at T(2). High-temperature amphiboles from Punta Falcone have 14,A1 less than 2.0 apfu and yet have Al at both T(l) and T(2). Intermediate-temperature amphiboles from potassic volcanics at Monte Vulture, Italy, have [4]A1 greater than 2.0 apfu with I41A1 occurring at T(l) and T(2). It is apparent that long-range [4]A1 disorder over T(l) and T(2) in amphiboles is induced by high temperatures of crystallization