Summary: Unit-cell dimensions and refractive indices of lithium-iron micas decrease with decreasing iron and increasing lithium. Indices β and γ as well as parameters a and b can be used to estimate the composition of lithium-iron micas but basal spacing and 2V_α are poor indicators of composition.

The chemical composition of natural lithium-iron micas from the Krušné hory and the Erzgebirge along the Czechoslovak—German border was discussed in Part A of this study (Rieder et al., 1970). It was concluded that the composition and crystallography of these micas fit best the series siderophyllite-polylithionite. The compositions were expressed by the ratio A′ = LR/(LR+‘Fe’). In this expression, LR is the subscript value of Li or octahedral R³⁺ (whichever is the smaller) in the crystallochemical formula, ‘Fe’ is the sum of the values in the formula of Fe²⁺ and Mn²⁺. A′ therefore defines the position of a particular mica on the siderophyllite-polylithionite join. This paper deals with the correlation between composition, cell dimensions, and refractive indices.