Amphiboles are a supergroup of rock-forming minerals with a really intricate crystal-chemistry, which however is able to record a number of markers of petrogenetic relevance. Hence, HT studies of amphiboles both allow experimental control on our present knowledge of cation ordering and provide precious information on phase stability, molar volumes and on the condition of water release and thus on the water budget in the Earth mantle. Diffraction data measured up to 750 °C on holotype ferro holmquistite A(K0.01Na0.01) B(Li1.88Mg0.08Na0.03Fe2+0.01) C(Al1.89Fe2+1.70Mg1.39Mn2+0.02) TSi8.00 W(O22OH1.97F0.03)[1] (FeHOL) and on its partially dehydrogenated (about 1 H pfu) counterpart (OXO), where part of the Fe2+ content oxidized to Fe3+ thus allowing for H loss, are presented and compared with those previously obtained for the other end-members of orthorhombic amphiboles, anthophyllite[1] and gedrite.[2] In this way, the effects of compositional variations such as A?-0.5 ANa0.5, B(Mg,Fe)2+-2 BLi2, C(Mg,Fe)2+-2 CAl2 and TSi-2 TAl2 are taken into account. Contrary to the almost linear behaviour observed for changes in unit-cell parameters in anthophyllite and gedrite, ferro-holmquistite shows a non-linear thermal expansion of the c edge, and a significant discontinuity around 500 °C for both the b and c edges of the partially dehydrogenated phase (measured on reversal). Best fits of our data in the range RT-650 °C for FeHOL and RT-500 °C for OXO give: ?aFeHOL: 1.36(2)o10-5, ?aOXO: 1.324(9)o10-5, ?bFeHOL: 0.56(1)o10-5, ?bOXO: 0.60(1)o10-5, ?cFeHOL: 1.27(6)o10-5-8.9(7)o10-9, ?cOXO: 0.68(2)o10-5, ?VFeHOL: 3.5(1)o10-5-1.3(2)o10-8 or 2.57(6)o10-5 (linear fit), ?VOXO: 2.59(2)o10-5. Structure refinements done at different T values during heating and at RT on the partially dehydrogenated phase provide information on the mechanism and the structural consequences of the differential thermal expansion of the structural units, as well as on cation exchange and structural changes related to dehydrogenation. Differences in thermoelastic behaviour and in the starting T of the dehydrogenation processes as a function of the overall composition and cation order in the distinct orthorhombic end-members will be discussed. These data allow construction of a reliable model for orthorhombic amphiboles, that will be compared to that still in preparation for monoclinic amphiboles, the terms of the amphibole supergroup that are more relevant to petrogenetic studies but that have a far more complex compositional variability. References 1.Cámara, F., Oberti, R. (2005) Am. Mineral. 90, 1167-1176. 2.Welch, M. D., Cámara, F. & Oberti, R. (2011) Phys. Chem. Minerals 38, 321-334. 3.Zema, M., Welch, M. D. & Oberti, R. (2012) Contrib. Mineral. Petrol. 163, 923-937.
High temperature behaviour of ferroholmquistite: thermal expansion and dehydrogenation
2012
Abstract
Amphiboles are a supergroup of rock-forming minerals with a really intricate crystal-chemistry, which however is able to record a number of markers of petrogenetic relevance. Hence, HT studies of amphiboles both allow experimental control on our present knowledge of cation ordering and provide precious information on phase stability, molar volumes and on the condition of water release and thus on the water budget in the Earth mantle. Diffraction data measured up to 750 °C on holotype ferro holmquistite A(K0.01Na0.01) B(Li1.88Mg0.08Na0.03Fe2+0.01) C(Al1.89Fe2+1.70Mg1.39Mn2+0.02) TSi8.00 W(O22OH1.97F0.03)[1] (FeHOL) and on its partially dehydrogenated (about 1 H pfu) counterpart (OXO), where part of the Fe2+ content oxidized to Fe3+ thus allowing for H loss, are presented and compared with those previously obtained for the other end-members of orthorhombic amphiboles, anthophyllite[1] and gedrite.[2] In this way, the effects of compositional variations such as A?-0.5 ANa0.5, B(Mg,Fe)2+-2 BLi2, C(Mg,Fe)2+-2 CAl2 and TSi-2 TAl2 are taken into account. Contrary to the almost linear behaviour observed for changes in unit-cell parameters in anthophyllite and gedrite, ferro-holmquistite shows a non-linear thermal expansion of the c edge, and a significant discontinuity around 500 °C for both the b and c edges of the partially dehydrogenated phase (measured on reversal). Best fits of our data in the range RT-650 °C for FeHOL and RT-500 °C for OXO give: ?aFeHOL: 1.36(2)o10-5, ?aOXO: 1.324(9)o10-5, ?bFeHOL: 0.56(1)o10-5, ?bOXO: 0.60(1)o10-5, ?cFeHOL: 1.27(6)o10-5-8.9(7)o10-9, ?cOXO: 0.68(2)o10-5, ?VFeHOL: 3.5(1)o10-5-1.3(2)o10-8 or 2.57(6)o10-5 (linear fit), ?VOXO: 2.59(2)o10-5. Structure refinements done at different T values during heating and at RT on the partially dehydrogenated phase provide information on the mechanism and the structural consequences of the differential thermal expansion of the structural units, as well as on cation exchange and structural changes related to dehydrogenation. Differences in thermoelastic behaviour and in the starting T of the dehydrogenation processes as a function of the overall composition and cation order in the distinct orthorhombic end-members will be discussed. These data allow construction of a reliable model for orthorhombic amphiboles, that will be compared to that still in preparation for monoclinic amphiboles, the terms of the amphibole supergroup that are more relevant to petrogenetic studies but that have a far more complex compositional variability. References 1.Cámara, F., Oberti, R. (2005) Am. Mineral. 90, 1167-1176. 2.Welch, M. D., Cámara, F. & Oberti, R. (2011) Phys. Chem. Minerals 38, 321-334. 3.Zema, M., Welch, M. D. & Oberti, R. (2012) Contrib. Mineral. Petrol. 163, 923-937.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
