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The Effects of Composition Upon the High-Pressure Behaviour of Amphiboles: Compression of Gedrite to 7 Gpa and a Comparison with Anthophyllite and Proto-Amphibole

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The Effects of Composition Upon the High-Pressure Behaviour of Amphiboles: Compression of Gedrite to 7 Gpa and a Comparison with Anthophyllite and Proto-Amphibole Mineralogical Magazine, August 2012, Vol. 76(4), pp. 987–995 The effects of composition upon the high-pressure behaviour of amphiboles: compression of gedrite to 7 GPa and a comparison with anthophyllite and proto-amphibole 1,2, 1 3 4 F. NESTOLA *, D. PASQUAL ,M.D.WELCH AND R. OBERTI 1 Dipartimento di Geoscienze, Universita`di Padova, via Gradenigo 6, 35131 Padova, Italy 2 CNR Istituto di Geoscienze e Georisorse, UOS Padova, via Gradenigo 6, 35131 Padova, Italy 3 Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK 4 CNR Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, 27100 Pavia, Italy [Received 10 February 2012; Accepted 23 March 2012; Associate Editor: G. Diego Gatta] ABSTRACT A single-crystal X-ray diffraction study of a sample of natural gedrite from North Carolina, USA, with A B 2+ C 2+ 4+ the crystal-chemical formula Na0.47 (Na0.03Mg0.97Fe0.94Mn0.02Ca0.04) (Mg3.52Fe0.28Al1.15Ti0.05) T W (Si6.31Al1.69)O22 (OH)2, up to a maximum pressure of 7 GPa, revealed the following bulk and axial moduli and their pressure derivatives: K0T = 91.2(6) GPa [K0T’ = 6.3(2)]; K0T(a) = 60.5(6) GPa [K0T(a)’ = 6.1(2)]; K0T(b) = 122.8(2.6) GPa [K0T(b)’ = 5.7(8)]; K0T(c) = 119.7(1.5) GPa [K0T(c)’ = 5.1(5)]. Gedrite has a much higher bulk modulus than anthophyllite (66 GPa) and proto-amphibole (64 GPa). All of the three axial moduli of gedrite are higher than those of these two other ortho- amphiboles. The greater stiffness of gedrite along [100] is due to its high ANa content, which is almost zero in anthophyllite and proto-amphibole. The much greater stiffness parallel to the (100) plane of gedrite compared with the two other amphiboles is probably due to its high CAl content. A comparison is made with published data available for orthorhombic B(Mg,Mn,Fe) and monoclinic BCa amphiboles to identify correlations between crystal-chemistry and compressibility in amphiboles. KEYWORDS: high pressure, bulk modulus, single-crystal XRD, gedrite, amphibole group. Introduction constants. Diffraction studies which measure the COMPRESSIBILITY and thermal expansivity are variation of unit-cell parameters with pressure fundamental thermodynamic parameters that allow the derivation of bulk and axial moduli K0T, relate directly to the stability of a mineral K0T(a), K0T(b)andK0T(c), where the final assemblage. Despite their importance, few brackets refer to values in different crystal- compressibilities and expansivities have been lographic directions. Superior datasets also allow determined for amphiboles (Welch et al., 2007). the pressure derivatives (K0T’, K0T’’, K0T’(a), Although the primary goal of compression studies K0T’’(a)...) of these elastic moduli to be calculated. is to determine the bulk modulus (inverse In highly topologically anisotropic phases such as compressibility, bÀ1) of a phase, other valuable amphiboles and sheet silicates, the principal information is contained within the set of elastic crystallographic axes relate to key structural directions defined by silicate chains or sheets. In these cases the axial moduli can give insights into compression mechanisms, even if full structural refinements at high P cannot be obtained due to * E-mail: [email protected] structural complexity and instrumental limitations DOI: 10.1180/minmag.2012.076.4.14 in the angular range for data collection. # 2012 The Mineralogical Society F. NESTOLA ET AL. The effect of composition on thermodynamic (prefixed ferro-) are geologically relevant. properties is of particular interest in the earth Gedrites commonly contain significant ANa sciences as it provides information which is (usually up to 0.5 atoms per formula unit, a.p.f.u.). valuable for modelling the Earth’s interior. The The crystal-chemistry of gedrite has been thermodynamics of solid solutions is particularly revised by Schindler et al. (2008) and relevant to petrological studies. For example, the Hawthorne et al. (2008). Zema et al. (2012) question as to whether or not amphibole solid reported the high-T behaviour of a crystal from solutions are thermodynamically ideal at geolo- sample A(26) of Schindler et al. (2008), with a A B 2+ gical P and T is still open. Very few thermo- formula Na0.47 (Na0.03Mg1.05Fe0.86Mn0.02 C 2+ 4+ T dynamic studies of binary or near-binary Ca0.04) (Mg3.44Fe0.36Al1.15Ti0.05) (Si6.31Al1.69) W amphibole solid solutions have been reported: O22 (OH)2, the structure of which is shown in fluoro-[tremoliteÀedenite] was studied by Fig. 1. Orthorhombic Pnma amphiboles (e.g. Graham and Navrotsky (1986), and tremolite– anthophyllite and gedrite) contain two non- richterite by Pawley et al. (1990). These equivalent double-chains of tetrahedra, the A- measurements were made at ambient conditions, and B-chains. Proto-amphibole, a much rarer and their relevance to amphibole stability at high orthorhombic structure, has space group Pnmn P and T is unclear. Volume is a thermodynamic and two equivalent double-chains of tetrahedra. quantity that can be used to quantify solid- The Al contents of tetrahedra in the gedrite A(26) solution behaviour (ideality vs. non-ideality via crystal studied by Zema et al. (2012) are as DVmix). The determination of the bulk modulus, follows: T1A = 0.53 a.p.f.u., T2A = 0.03 a.p.f.u., K0T, from compression studies allows the volume T1B = 0.76 a.p.f.u., T2B = 0.36 a.p.f.u. The higher at high P (and T, if expansivity data are available) Al content of the B-chain results in larger to be calculated. If the bulk moduli across a solid tetrahedra and a more kinked chain. The ribbon solution can be measured, the pressure depen- of octahedrally coordinated edge-sharing M1, M2 dence of DVmix can be determined. However, and M3 sites is the least compressible component given the number of distinct samples that are of the amphibole structure, and Zema et al. (2012) required for this type of study (at least 7), the showed that in gedrite its thermal expansivity collection of such data is time consuming and is depends strongly on the oxidation state of Fe. In only possible on a realistic timescale using gedrite, CAl is completely ordered at the smallest synchrotron X-ray powder diffraction on well octahedrally coordinated site (M2) at the edge of characterized synthetic samples. It is, nonetheless, the ribbon. At higher T, the M4 polyhedron loses a worthwhile objective as little is known about the almost all of its Fe, which migrates to the thermodynamics of amphiboles at mantle pres- octahedrally coordinated sites and oxidizes to sures. The first step is the acquisition of good Fe3+, leading to dehydrogenation (Zema et al., compressibility and expansivity data for 2012). The M4 polyhedron also deforms in endmember amphiboles and other geologically response to rotation of the double-chains, which relevant compositions. in turn respond to the deformation of the ribbon of In addition to providing quantitative data on octahedra. In general, larger M4 cations (e.g. Na amphibole solid solutions at high P and T, in situ and Ca in pargasite and richterite) resist high-P studies provide valuable insights into compression more than smaller ones (Mg, Fe2+). compression mechanisms. Using single crystals The A-cation is also expected to stiffen the of good quality it is possible to quantify the structure across the channel perpendicular to pressure dependence of bulk and axial moduli, K’ (100), and indeed the thermal expansion coeffi- (qK/qP) and thereby predict structural hiatuses cient a a is smaller in gedrite A(26) that may destabilize amphibole-group minerals at (1.11(1).610À5 KÀ1; Zema et al., 2012) than in high P, or alternatively, stabilize them. anthophyllite (1.49610À5 KÀ1; Welch et al., Solid solutions between the orthorhombic 2011a). A B C amphiboles gedrite, ideally & Mg2 (Mg3Al2) Welch et al. (2011b) determined the bulk and T W A B C (Si6Al2)O22 (OH)2,and Na Mg2 Mg5 axial moduli of a natural near-endmember T W A (Si7Al)O22 (OH)2 (in the new classification anthophyllite with a formula Na0.04 B C scheme prepared by the subcommittee on (Mg1.30Mn0.57Ca0.09Na0.04) (Mg4.96Fe0.02 T W amphiboles chaired by F.C. Hawthorne and R. Al0.02) (Si7.99Al0.01)O22 (OH)2 to 7 GPa, and Oberti and approved by IMA-CNMNC in April compared these moduli with those of a natural 2012), and their Fe(II)-substituted counterparts proto-amphibole with a formula 988 HIGH-PRESSURE BEHAVIOUR OF GEDRITE FIG. 1. The structure of gedrite (A26) projected onto (100), after Zema et al. (2012). A B C T & (Mn1.39Fe0.59) (Fe3.98Mg1.02) Si8O22 group of gedrite crystals used by Schindler et al. W (OH)2 reported by Zanazzi et al. (2010). It was (2008) and Zema et al. (2012) in their studies. concluded that the composition of the ribbon of Structure refinement of our gedrite A(26) prior to edge-sharing octahedra, which is the most rigid the high-P treatment showed substantial agree- component of the amphibole structure, exerts the ment with the sample studied by Zema et al. main control upon compressibility. No significant (2012), which has the crystal-chemical formula A B 2+ symmetry effects (Pnma vs. Pnmn) on compres- Na0.47 (Na0.03Mg1.05Fe0.86Mn0.02Ca0.04) C 2+ 4+ T sibility were identified. (Mg3.44Fe0.36Al1.15Ti0.05) (Si6.31Al1.69)O22 W As part of our ongoing research on the effects of (OH)2. The sample used in this study has a total composition on the physical behaviour of of 107.85 electrons per formula unit (e.p.f.u.) for amphiboles, we have undertaken a high-P study the M1À4 and A sites, compared to of a gedrite crystal taken from the same sample as 107.30 e.p.f.u.
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