The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations

The general formula of the amphiboles of this series may be written as Na x Mg 2 (Mg (5- y ) Al y ,)(Si (8- z ) Al z )O 22 (OH) 2 , where Mg = Mg + Fe 2+ + Mn 2+ and Al = Al + Fe 3+ + Ti. The individual < T –O> distances are linear functions of their [4] Al content, and the [4] Al content is strongly ordered in the following way: T 1 B > T 1A » T 2B » T 2A. The < M 1-O>, < M 2-O> and < M 3–O> distances are linear functions of the mean ionic radius of their constituent cations. End-member compositions may be written as follows: A ☐Mg 2 Mg 5 Si 8 O 22 (OH) 2 O 22 (OH) 2 ; A ☐Mg 2 (Mg 3 Al 2 )(Si 6 Al 2 )O 22 (OH) 2 ; A NaMg 2 Mg 5 (Si 7 Al) These compositions define a plane in xyz space across which the data of Schindler et al. (2008), measured on amphiboles from amphibolites, follow a tightly constrained trajectory. Anthophyllite–gedrite amphiboles equilibrated under significantly different P-T conditions (e.g. igneous rocks, contact-metamorphic rocks) follow trends that diverge from this trajectory, with greater Na and [4] Al contents and relatively smaller [6] Al contents. Detailed examination of the local bond topology involving the A and M 2 sites indicates that the maximum degree of bond-valence compensation will occur for incorporation of A Na and M 2 Al in the ratio 4:10, and hence 2.5 A Na = M 2 Al in these amphiboles. This relation closely fits the data of Schindler et al. (2008), suggesting that the variation in chemical composition in anthophyllite–gedrite amphiboles is strongly constrained by the anion bond-valence requirements of the Pnma amphibole structure. We further suggest that different compositional trends for ortho-amphiboles equilibrated under different P-T conditions are the result of the valence-sum rule operating with (different) bond-lengths characteristic of these P-T conditions.