High-T behaviour of gedrite: thermoelasticity,cation ordering and dehydrogenation

The thermoelastic behaviour of a natural gedrite having the crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.05 Fe_0.86²⁺ Mn_0.02 Ca_0.04) ^C(Mg_3.44 Fe_0.36²⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69)O₂₂ ^W(OH)₂ has been studied by single-crystal X-ray diffraction to 973 K (Stage 1). After data collection at 973 K, the crystal was heated to 1,173 K to induce dehydrogenation, which was registered by significant changes in unit-cell parameters, M1–O3 and M3–O3 bond lengths and refined site-scattering values of M1 and M4 sites. These changes and the crystal-chemical formula calculated from structure refinement show that all Fe²⁺ originally at M4 migrates into the ribbon of octahedrally coordinated sites, where most of it oxidises to Fe³⁺, and there is a corresponding exchange of Mg from the ribbon into M4. The resulting composition is that of an oxo-gedrite with an inferred crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.93 Ca_0.04) ^C(Mg_2.56 Mn_0.02²⁺ Fe_0.10²⁺ Fe_1.22³⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69) O₂₂ ^W[O_1.12²⁻ (OH)_0.88]. This marked redistribution of Mg and Fe is interpreted as being driven by rapid dehydrogenation at the H3A and H3B sites, such that all available Fe in the structure orders at M1 and M3 sites and is oxidised to Fe³⁺. Thermoelastic data are reported for gedrite and oxo-gedrite; the latter was measured during cooling from 1,173 to 298 K (Stage 2) and checked after further heating to 1,273 K (Stage 3). The thermoelastic properties of gedrite and oxo-gedrite are compared with each other and those of anthophyllite.