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

Abstract

The crystal structures and site populations of 8 calcic amphiboles from a leucogabbro from Punta Falcone, Sardinia, have been refined to R indices of similar to 2 % using single-crystal MoK alpha X-ray diffraction data. These amphiboles are unusual in that they show a significant degree of disorder of [4]-coordinated Al over the T(1) 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(1) and T(2) tetrahedra. It is also shown that the [T(2)-O] distance is significantly affected by variation in occupancy of other sites in the amphibole structure.
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\nThe ordering behaviour of Al-[4] in amphiboles is examined in terms of local bond-valence requirements. In most amphiboles, Al-[4] occurs completely at T(1) 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 Al-[4] similar to 2 atoms per formula unit observed in amphiboles occurs so as to avoid Al-O(7)-Al linkages that are very unfavourable from a bond-valence viewpoint, and to avoid the occurrence of Al at T(2). When Al-[4] > 2 apfu, the additional Al-[4] normally occurs at the T(2) site. Medium-temperature (high-pressure) amphiboles from the Liset eclogite pod show all Al-[4] to occur at T(1) up to a limit of 2.0 apfu, with all additional Al-[4] occurring at T(2). High-temperature amphiboles from Punta Falcone have Al-[4] less than 2.0 apfu and yet have Al at both T(1) and T(2). Intermediate-temperature amphiboles from potassic volcanics at Monte Vulture, Italy, have Al-[4] greater than 2.0 apfu with ([4])A1 occurring at T(1) and T(2), It is apparent that long-range Al-[4] disorder over T(1) and T(2) in amphiboles is induced by high temperatures of crystallization.