Single-crystal HT X-ray diffraction study of anthophyllite.

The crystal chemistry of amphiboles is now well understood. However, very little is known about their physical properties, such as compressibility and thermal expansivity [1], both of which are fundamental to mineral stability. Of the few published expansivity data, all are for monoclinic amphiboles, including cummingtonite, tremolite and richterite and recently also pargasite [2]. There are no data for orthorhombic amphiboles. Furthermore, rapid disordering of divalent cations between M(1,2,3,4) sites in monoclinic amphiboles on heating has been observed in situ to occur within 2–4 hours at T ~ 300 – 700°C [3,4]. Beyond 600°C, dehydrogenation has been also observed [2]. The absence of thermal expansivity data for orthorhombic amphiboles and the possibility of in situ disordering of cations led us to undertake a high-temperature diffraction experiment on anthophyllite. A gem-quality Mg-rich crystal of anthophyllite coming from American Museum of Natural History (AMNH 34856) with composition ANa0.05 B(Mg1.34Mn0.56Ca0.09) C(Mg4.98Fe0.02) T(Si7.97 Al0.03) O22 (OH1.99F0.01) was studied by single-crystal XRD. Data were collected in situ using a micro furnace. Lattice parameters were refined at intervals of 25 K in the T range 298-1073 K; intensity data were collected at 298, 473, 673, 873, 973 and 1073 K. Above 973 K a homogeneous contraction in volume is observed which is related to dehydrogenation accompanied by a increase in mosaicity indicating the decomposition of the crystal. Thermal expansivities for V, a, b, c were obtained by fitting the equation V(T) = VTe !" #T-Tr)]: V = 3.668(3) !10-5 K-1, a = 1.49(1) !10-5 K-1, b = 0.969(2) !10-5 K-1, c = 1.206(9) !10-5 K-1. These values are the highest observed so far for amphibole. Thermal expansion is thus anisotropic with a: b: c = 1.53:1:1.24, with a larger expansion in the (010) plane, essentially along [100]. This behaviour contrasts with the chemically similar but monoclinic manganoan cummingtonite [2] which shows a lower c, although b values are almost the same in both monoclinic and orthorhombic structures. Structure refinements show that, after correcting for thermal motion, while tetrahedra behave as rigid units up to 1073 K, C-group octahedra expand at the same rate within experimental error ( M1,2,3 = 5(2) !10-5 K-1), whereas M(4) expands more ( M4 = 7.8(3.1) !10-5 K-1). A change in rate is observed in the c parameter which starts around 700 K and coincides with disordering of Mn from the B to the C sites in agreement to previous observations in manganocummingtonite [3]. However, on disordering at high-T in anthophyllite, the Mn2+ site preference is M(1,2) > M(3), in contrast with M(2) > M(1,3) reported by [3] in manganoan cumingtonite and also with M(1,3) $" M(2) found in richterite [4]. This result indicates that the composition of M(4) is more constraining than topology in determining the thermal behaviour of amphibole.