Magnesiostaurolite and zincostaurolite: mineral description with a petrogenetic and crystal-chemical update

Abstract

Magnesiostaurolite, ideally A □ 4 B Mg 4 C Al 16 D (Al 2 □ 2 ) T Si 8 O 40 X [(OH) 2 O 6 ], occurs together with talc, clinochlore and kyanite (or alone, armouring corundum) as inclusions in pyrope megablasts from the ultra-high-pressure metamorphic terrane of the Dora-Maira massif, Italian Western Alps. It is transparent colourless in thin section; non pleochroic, biaxial with a 2 V angle close to 90 0 ; the birefringence is low ( n = 1.709(2) at 592 nm. A few crystals show a ‘tweed’ texture under crossed nicols. The calculated density is 3.54 g/cm 3 . Zincostaurolite, ideally A □ 4 B Zn 4 C Al 16 D (Al 2 □ 2 ) T Si 8 O 40 X [(OH) 2 O 6 ], occurs with kyanite, muscovite, margarite, ± chloritoid, gahnite and either quartz or diaspore in a karst-filling meta-argillite of the Mesozoic Barrhorn series, Zermatt valley, Swiss Western Alps; it may have formed concurrently with kyanite from the breakdown of gahnite + pyrophyllite + diaspore, i.e. near 400 0 C. Zincostaurolite is non pleochroic and biaxial positive, with positive elongation (γ = c); α = 1.722(2) and γ = 1.734(2) at 592 nm. The calculated density is 3.78 g/cm 3 . Both magnesiostaurolite and zincostaurolite samples are monoclinic, C2/m , with the β angle equal or very close to 90 0 , which implies a very low degree of cation order; this feature is confirmed by the nearly equal site-scattering values and the similar mean bond lengths refined at the relevant pairs of sites; it is unrelated to metamorphic grade. Electron and ion microprobe analyses were combined to new single-crystal structure refinements to give the following crystal-chemical formulae: magnesiostaurolite, A (Fe 2+ 0.16 Mg 0.72 □ 3.12 ) B (Mg 1.86 Li 0.94 Zn 0.02 □ 1.18 ) C (Al 15.96 Ti 0.04 ) D (Al 1.58 Mg 0.45 □ 1.97 ) T (Si 7.96 Al 0.04 ) O 40 X [(OH) 3.98 O 402 ]; zincostaurolite, A(Fe 2+ 0.13 Mg 0.10 □ 3.77 ) B (Zn 2.45 Li 0.51 Fe 2+ 0.20 □ 0.84 ) C (Al 15.98 Ti 0.02 ) D (Al 1.95 Mg 0.09 □ 1.96 ) T Si 8 O 40 X [(OH) 3.67 O 4.33 ]. The magnesiostaurolite crystal, with several hundreds ppm BeO, probably shows the highest Li and Be contents ever measured in staurolite. The increased occupancy of the M4 octahedron at the expense of the T2 tetrahedron from Fe- or Zn-rich to Mg-rich staurolite may be the key to the Mg-staurolite paradox (a high-pressure phase with expectedly four-fold coordinated Mg) and to the complex thermodynamic behaviour of the staurolite series ( e.g. reversal in Fe-Mg partitioning with garnet).