Homogeneous and heterogeneous polymorphic transformations in alkaline earth feldspars

The displacive transformations in quartz, hexacelsian and the strontium equivalent of the latter, are homogeneous incipient transformations. These transformations are carried out mostly as a second (or higher) order reaction, and are finally completed abruptly as a first-order reaction which results from a sudden release of strain accumulated due to bond stretching. In such transformations the first-order mode of reaction seems to be eliminated to various extents by the introduction of impurities as solid solution to the structure. Such impurities promote heterogeneous transformations. The extent of sluggishness of displacive transformations and of stability relationship in alkaline earth feldspars is correlated with the various ionic sizes of barium, strontium and calcium. Displacive and diffusive transformations in alkali feldspars are also homogeneous incipient transformations, but due to the high mobility of the alkali ions, building up of strain is prevented and no abrupt change in symmetry occurs at the transformation. The hexacelsian ⇌ celsian and monotropic transformations in SrAl₂Si₂O₈ and CaAl₂Si₂O₈ are heterogeneous incipient reconstructive transformations. Differences in melting temperatures reported in the past for BaAl₂Si₂O₈ may probably be correlated with the grain size, crystalline modification and extent of Al/Si order in the sample investigated.