A Scheme of Cation Distribution in the Amphiboles

Subrata Ghose
Inst. Kristallogr. Petrogr., Eidgenössische Technische Hochschule, Sonneggstrasse 5, Zürich 6, Switzerland

Summary: The four principles governing the cation distribution in various octahedral metal sites in the amphibole structure are: Ionic size—the M4 position prefers large cations like Ca2+, Na+; charge balance—when the M4 position is occupied by a monovalent ion, like Na+, Li+, the neighbouring M2 position is occupied by a trivalent ion like Al3+, Fe3+; role of the strongly polarizing cations—cations like Fe2+ prefer the M4 site, since they tend to form covalent bonds with the O4 oxygens; and the role of the (OH) ions—these form part of the brucite layer and repel large or strongly polarizing cations, which tend to distort the regularity of the sheet structure.

The Al-Si distribution in the two different tetrahedral sites are governed by three principles: relief of the Si-O-Si bond strain—Al3+ in the Si1 site causes fewer Si4+ cations to share more than two anions with other Si4+ cations; charge balance—with the introduction of alkali ions in the ‘A’ position, Al3+ is introduced in the Si1 position, since the coordinating oxygens around the A position mostly belong to Si1; and steric considerations—occupation of the M4 site by a large cation tends to rotate the Si2 tetrahedron anticlockwise; Al3+ substitution in the Si1 site rotates the Si1 tetrahedron clockwise and thus the regularity and the constancy of the repeat distance of the silicate double chain is maintained.

Based on these principles, a scheme of cation distribution in the M1, M2, M3, M4, A, Si1, and Si2 positions in the amphibole structure has been derived for most of the common amphibole varieties, except richterite, whose position is anomalous from a crystallochemical point of view.

Mineralogical Magazine; March 1965 v. 35; no. 269; p. 46-54; DOI: 10.1180/minmag.1965.035.269.07
© 1965, The Mineralogical Society
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