Abstract
Submicroscopic exsolution in Mn-bearing alkali amphibole from Tirodi, Maharashtra, India, has been studied with transmission and analytical electron microscopy (TEM and AEM). The amphibole (crystals up to a few centimeters long) exhibits submicroscopic exsolution textures and has an average bulk composition , that is intermediate to magnesio-arfvedsonite, magnesio-riebeckite, manganese-cummingtonite, and richterite. The amphibole contains sparse inclusions (a few micrometers wide) of two secondary amphiboles, magnesio-arfvedsonite with a magnesio-riebeckite component, , and manganese-cummingtonite with a richterite component, , which do not exhibit exsolution textures.
Two sets of exsolution lamellae were observed in the coarse amphibole crystals: “101” lamellae oriented ∼11° from (101), and “l00” lamellae oriented ∼4.5° from (100). The orientations of these exsolution lamellae are consistent with predicted orientations, on the basis of optimal phase boundary theory. The (110) lattice fringes are continuous across interfaces of the “101” lamellae but change orientation by about 1.5° when viewed parallel to [112], implying coherent interfaces with minimum strain. The AEM analyses of both sets of lamellae and hosts give paired compositions that approach those of the two secondary amphiboles with no exsolution. The microstructures are consistent with an origin through spinodal decomposition, although homogeneous nucleation and growth is not completely excluded for the “101” and some of the “100” lamellae.
The exsolution texture in the primary Mn-bearing alkali amphibole and the presence of coexisting secondary amphiboles are consistent with a miscibility gap between alkali amphibole and ferromagnesian amphibole. Major differences in M4 site occupancies (Na vs. Mn) coupled with differences in M2 site occupancies (Fe3+ vs. Mg) imply that the exsolution relations are primarily controlled by occupancies of the M4 and M2 sites and can be ideally expressed by the substitution [M4]Na+ + [M2]Fe3+ ↔ [M4]Mn2+ + [M2]Mg2+. The two types of associated amphibole inclusions with compositions close to magnesio-arfvedsonite and manganese-cummingtonite with some richterite component, respectively, are inferred to have formed by dissolution-crystallization during retrograde metamorphism or subsequent hydrothermal alteration.
