The effects of composition upon the high-pressure behaviour of amphiboles: compression of gedrite to 7 GPa and a comparison with anthophyllite and proto-amphibole

摘要

A single-crystal X-ray diffraction study of a sample of natural gedrite from North Carolina, USA, with the crystal-chemical formula ~ANa_(0.47)~B(Na_(0.03)Mg_(0.97)Fe_(0.94)~(2+)Mn_(0.02)Ca_(0.04))~C(Mg_(3.52)Fe_(0.28)~(2+)Al_(1.15)Ti_(0.05)~(4+)) ~T(Si_(6.31)Al_(1.69))O_22)~W(OH)_2, up to a maximum pressure of 7 GPa, revealed the following bulk and axial moduli and their pressure derivatives: K0T = 91.2(6) GPa [K0T' = 6.3(2)]; K01{a) = 60.5(6) GPa [K_(0T)(a)' = 6.1(2)]; K_(0T)(b) = 122.8(2.6) GPa [K_(0T)(b)' = 5.7(8)]; K_(0T)(c) = 119.7(1.5) GPa [K_(0T)(c)' = 5.1(5)]. Gedrite has a much higher bulk modulus than anthophyllite (66 GPa) and proto-amphibole (64 GPa). All of the three axial moduli of gedrite are higher than those of these two other ortho-amphiboles. The greater stiffness of gedrite along [100] is due to its high ~ANa content, which is almost zero in anthophyllite and proto-amphibole. The much greater stiffness parallel to the (100) plane of gedrite compared with the two other amphiboles is probably due to its high ~CAl content. A comparison is made with published data available for orthorhombic ~B(Mg,Mn,Fe) and monoclinic ~BCa amphiboles to identify correlations between crystal-chemistry and compressibility in amphiboles.