Predicted model for hydrous modified olivine (HyM-α)

A possible structure for hydrous modified olivine (HyM-α) has been obtained by the subtraction of Mg₃SiO₅ from forsterite by crystallographic shear along a direction parallel to the 010] direction of olivine. The subtraction of Mg₃SiO₅ results in the subtraction of MgO from bulk chemistry (?Mg₃SiO₅=?Mg₂SiO₄?MgO). A possible structure for HyM-α thus obtained has the chemical formula Mg₉Si₅H₂O₂₀ (= 5 × Mg_1.8SiH_0.4O₄) with monoclinic unit cell a=4.754 Å, b=10.19 Å, c=29.90 Å, ρ=3.126 g cm^?3, and space group=Ac2m (no. 39). Since the X-ray powder diffraction pattern of HyM-α proposed in this study is very close to that of clinohumite, there is the possibility of this phase having been undiscovered. The humite group minerals and HyM-α proposed in this study make a homologous series as recombination structures: Mg_{(2 m + n )}Si _m H_{2 n} O_{2(2 m + n )} for the humite group and Mg_{(2 m + n )}Si_{( m + n )}H_{2 n} O_{4( m + n )} for HyM-α A characteristic feature is that Mg/Si > 2 for the humite group and Mg/Si < 2 for HyM-α. Forsterite specimens containing around 100 ppm H₂O reported in mantle xenoliths might be the disordered case with n=1 and m=1200 of the humite group or HyM-α.