Sanidine: Predicted and Observed Monoclinic-to-Triclinic Reversible Transformations at High Pressure

+ See all authors and affiliations

Science  01 Oct 1976:
Vol. 194, Issue 4260, pp. 105-107
DOI: 10.1126/science.194.4260.105


High sanidine, (K,Na)AlSi3O8, transforms reversibly to a triclinic phase at high pressure. This is analogous to the high-temperature monalbite transformation. Disordered alkali feldspars of various compositions have unit-cell dimensions which are very similar at the transition (a = 8.30 Å, b = 12.97 Å, c = 7.14 Å, and β = 116.2°), indicating that the transformation is structurally controlled. Changes in temperature, pressure, and the sodium/potassium ratio cause similar structural variations: angles between adjacent, rigid tetrahedra vary to accommodate changing effective alkali cation sizes.