Holmquistites from seven different localities, which are representative of the known compositional variations, have been collected and characterised using single-crystal structure refinement and EMP and SIMS analyses. The agreement between the results is good, and reliable site populations could thus be obtained. All but one samples can be classified as holmquistite; the remaining one can be referred as "ferro-holmquistite" (IMA approval still pending). Lithium at the M4 sites is always nearly stoichiometric (> 1.85 apfu). Contrarily to clinoholmquistites, no Li occupancy has been detected at the octahedral M3 site, and the A site is always vacant. Also, there is an inverse correlation between the Fe<SUP>2+</SUP> and the Fe<SUP>3+</SUP> contents, which indicates a structural limit to the expansion of the strip of octahedra. In mineral families which undergo more or less complex solid-solution, as well as in clinoholmquistites, the changes in the unit-cell parameters are directly related to those of all the individual polyhedra, which in turn depend on their population. In holmquistites, both the a and b edges significantly lengthen as a function of the Fe<SUP>3+</SUP>Al<SUB>-1</SUB> exchange at the M2 site, but are rather invariant as a function of the Fe<SUP>2+</SUP>Mg<SUB>-1</SUB> exchange at the M1 and M3 sites. The geometries of the three octahedral sites are strongly non-ideal (octahedral angular variance = M1 59$IMM_COM_001575, M2 29$IMM_COM_001534, M3 74$IMM_COM_001597, the highest values in the Fe-richest samples). Of the two independent double-chains of tetrahedra, which are both O-rotated, one is significantly but invariantly kinked (O5B-O6B-O5B = 163.2?164.1°), and the other is more extended and more sensitive to changes in the M2 site population (O5A-O6A-O5A = 166.5$IMM_COM_0015168.1°). The coordination of Li at the M4 site is rather peculiar, with 4 short bonds (with the oxygen atoms belonging to the strip of octahedra) and 1 longer bond with the O5A atom. In clinoholmquistites, Li has a more regular [6]-coordination which involves the O6 and not the O5 oxygen atoms, and solid solution with Na is commonly observed. The crystal-chemical reasons for the orthorhombic symmetry are not yet completely clear. However, it stability is restricted to a narrow compositional field close to the ideal composition for the A, B and T sites, but with Fe<SUP>2+</SUP>/(Fe<SUP>2+</SUP>+Mg) between 0.15 and 0.60. Dimensional requirements are likely to play the major role in this regard.

Li-bearing amphiboles: anomalous solid-solution properties in holmquistites [Li2 (Al2Mg3) Si8 O22 (OH)2, Pnma]

Oberti R
2004

Abstract

Holmquistites from seven different localities, which are representative of the known compositional variations, have been collected and characterised using single-crystal structure refinement and EMP and SIMS analyses. The agreement between the results is good, and reliable site populations could thus be obtained. All but one samples can be classified as holmquistite; the remaining one can be referred as "ferro-holmquistite" (IMA approval still pending). Lithium at the M4 sites is always nearly stoichiometric (> 1.85 apfu). Contrarily to clinoholmquistites, no Li occupancy has been detected at the octahedral M3 site, and the A site is always vacant. Also, there is an inverse correlation between the Fe2+ and the Fe3+ contents, which indicates a structural limit to the expansion of the strip of octahedra. In mineral families which undergo more or less complex solid-solution, as well as in clinoholmquistites, the changes in the unit-cell parameters are directly related to those of all the individual polyhedra, which in turn depend on their population. In holmquistites, both the a and b edges significantly lengthen as a function of the Fe3+Al-1 exchange at the M2 site, but are rather invariant as a function of the Fe2+Mg-1 exchange at the M1 and M3 sites. The geometries of the three octahedral sites are strongly non-ideal (octahedral angular variance = M1 59$IMM_COM_001575, M2 29$IMM_COM_001534, M3 74$IMM_COM_001597, the highest values in the Fe-richest samples). Of the two independent double-chains of tetrahedra, which are both O-rotated, one is significantly but invariantly kinked (O5B-O6B-O5B = 163.2?164.1°), and the other is more extended and more sensitive to changes in the M2 site population (O5A-O6A-O5A = 166.5$IMM_COM_0015168.1°). The coordination of Li at the M4 site is rather peculiar, with 4 short bonds (with the oxygen atoms belonging to the strip of octahedra) and 1 longer bond with the O5A atom. In clinoholmquistites, Li has a more regular [6]-coordination which involves the O6 and not the O5 oxygen atoms, and solid solution with Na is commonly observed. The crystal-chemical reasons for the orthorhombic symmetry are not yet completely clear. However, it stability is restricted to a narrow compositional field close to the ideal composition for the A, B and T sites, but with Fe2+/(Fe2++Mg) between 0.15 and 0.60. Dimensional requirements are likely to play the major role in this regard.
2004
Istituto di Geoscienze e Georisorse - IGG - Sede Pisa
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.14243/212420
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