Hydrothermal synthesis of manganese-richterite and the Mg/Mn-substitution of rithteritic amphiboles

Manganese-richterites have been synthesized hydrothermally from oxide mixes at a total fluid pressure of 1 kbar. Among the crystalline by-products, wollastonite, bustamite and serandite, but no phyllosilicates, were detected. Microanalyses of richteritic amphiboles formed at 700°C indicate that the substitution at the octahedral M(1)-M(2)-M(3) sites is restricted to 2.00±0.05 Mn per formula unit. Compared with pure richterite, the unit-cell is enlarged (up to 2.8% in volume) with increasing Mn-content, approximately linearly and mainly in the b-direction. The limit is inferred to depend on geometrical constraints on the structure, and it explains the absence of more Mn-rich varieties in natural assemblages