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 <T-O> distances and the Al contents of the T(l) and T(2) tetrahedra. It is also shown that the <T(2)-0> 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