A scheme of cation distribution in the amphiboles By SUBRATA G~OSE, M.Sc., M.S., Ph.D.

Summary. The four principles governing the cation distribution i various octa-hedral metal sites in the amphibole structure are: Ionic size--the M 4 position prefers large cations like Ca 2+, Na+; charge balance--when the] I t position is occupied by a monovalent ion, like Na +, Li +, the neighbouring M S position is occupied by a trivalent ion like A1 a+, Fea+; role of the strongly polarizing cations--cations like Fe S+ prefer the M t site, since they tend to form covalent bonds with the Oa 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 A1-Si distribution i the two different tetrahedral sites are governed by three principles: relief of the Si-O-Si bond strain A1 a+ in the Si 1 site causes fewer Si ~§ cations to share more than two anions with other Si t+ cations; charge balance--with the introduction of alkali ions in the ' A ' position, A1 a+ is introduced in the Si 1 position, since the coordinating oxygens around the A position mostly belong to Si 1; and steric considerations--occupation of the Mt site by a large cation tends t