Published online by Cambridge University Press: 05 July 2018
A single-crystal X-ray diffraction study of a sample of natural gedrite from North Carolina, USA, with the crystal-chemical formula ANa0.47 B(Na0.03Mg0.97Fe0.94 2+Mn0.02Ca0.04)C(Mg3.52Fe0.28 2+Al1.15Ti0.05 4+)T(Si6.31Al1.69)O22 W(OH)2,up to a maximum pressure of 7 GPa, revealed the following bulk and axial moduli and their pressure derivatives: K 0T = 91.2(6) GPa [K 0T ' = 6.3(2)]; K 0T (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 muchhigher 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 orthoamphiboles. The greater stiffness of gedrite along [100] is due to its high ANa content, which is almost zero in anthophylliteand 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 BCaamphiboles to identify correlations between crystal-chemistry and compressibility in amphiboles.