Magnesiochloritoid: Compressibility and high pressure structure refinement

The response of magnesiochloritoid to pressure has been studied by single crystal X-ray diffraction in a diamond anvil cell, using crystals with composition Mg_1.3Fe_0.7Al₄Si₂O₁₀(OH)₄. The unit cell parameters decrease from a = 9.434 (3), b = 5.452 (2), c = 18.136 (5) Å, β = 101.42° (2) (1 bar pressure) to a = 9.370 (7), b = 5.419 (5), c = 17.88 (1) Å, β = 101.5° (1) (42 kbar pressure), following a slightly anisotropic compression pattern (linear compressibilities parallel to unit cell edges: β _a = 1.85, β _b = 1.74, β_c = 3.05 × 10^?4 kbar^?1) with a bulk modulus of 1480 kbar. Perpendicular to c, the most compressible direction, the crystal structure (space group C2/c) consists of two kinds of alternating octahedral layers connected via isolated SiO₄ tetrahedra. With increasing pressure the slightly wavy layer [Mg_1.3Fe_0.7AlO₂(OH)₄] tends to flatten. Furthermore, the octahedra in this layer, with all cations underbonded, are more compressible than the octahedra in the (A1₃O₈) layer with slightly overbonded aluminum. Comparison between high-pressure and high-temperature data yields the following equations: $$begin{gathered} a_{P,T} = 9.434{text{ }}{AA} - 174 cdot 10^{ - 5} {text{ }}{AA}{text{kb}}^{{text{ - 1}}} cdot P hfill {text{ }} + 9 cdot 10^{ - 5} {text{ }}{AA}^circ C^{ - 1} cdot (T - 25^circ C) hfill b_{P,T} = 5.452{text{ }}{AA} - 95 cdot 10^{ - 5} {text{ }}{AA}{text{kb}}^{{text{ - 1}}} cdot P hfill {text{ }} + 5 cdot 65 cdot 10^{ - 5} {text{ }}{AA}^circ C^{ - 1} cdot (T - 25^circ C) hfill c_{P,T} = 18.136{text{ }}{AA} - 549 cdot 10^{ - 5} {text{ }}{AA}{text{kb}}^{{text{ - 1}}} cdot P hfill {text{ }} + 16 cdot 2^{ - 5} {text{ }}{AA}^circ C^{ - 1} cdot (T - 25^circ C) hfill end{gathered} $$ with P in kbar and T in °C. These equations indicate that the unit cell and bond geometry of magnesiochloritoid at formation conditions do not differ greatly from those at the outcrop conditions, e.g. the calculated unitcell volume is 917.3 ų at P = 16 kbar and T=500 °C, whereas the observed volume at room conditions is 914.4 ų. In addition, they show that the specific gravity increases from formation at depth to outcrop at surface conditions.