allomerism Chem.
(æˈlɒmərɪz(ə)m)
[mod. f. Gr. ἄλλος other, different + µέρ-ος part + -ism.]
A term applied by Cooke to the variation in chemical constitution without change of form, incident to those minerals that contain isomorphous constituents which may wholly or partially replace one other.
Allomerism is well shown in Amphibole; its typical composition is RO . SiO2 in which RO represents any two or more of the bases magnesia (MgO), lime (CaO), oxide of iron (FeO), oxide of manganese (MnO), soda (N2O), potash (K2O) or water (H2O); while the silica (SiO2) is usually partly replaced by alumina (Al2O3) and often by the isomorphous sesquioxide of iron (Fe2O3).