Electron microprobe and electron diffraction study of a new ferric iron sanidine from the leucite hills, wyoming

The group of minerals classified as feldspars are among the most common rock-forming minerals in the earth’s crust. The majority of feldspar compositions fall within the ternary system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8, though substitution of other elements, usually in minor amounts, is common. Described here is a naturally occurring ferric iron sanidine feldspar composition in which up to ∼70 mole% of the KAlSi3O8 molecule is replaced by KFe3+ Si3O8. It thus constitutes a new end-member feldspar composition and the data have been submitted to the Commission on New Mineral Names for evaluation.The ferric iron sanidine is found in the 1.1ma volcanic rocks of the Leucite Hills, Wyo. The composition of these lavas and plugs is unusually rich in K and Fe3+, and depleted in Al compared to typical basaltic rocks. Sanidine grains are among the last phase to crystallize in the groundmass of the rocks classified as orendites, and rarely exceed 150um in length.

Subsolidus crystallization behaviour in the system KAlSi3O8–NaAlSi3O8–H2O

SummaryWhen gels and glasses of alkali feldspar composition are crystallized hydrothermally below their solidus the crystals present when crystallization is incomplete do not have the same Na:K ratio as the starting material but bear a relation to the starting material analogous to that between crystals and liquids coexisting at the liquidus. This effect has been investigated in the temperature range 700–850 °C at water-vapour pressures between 5000 lb/in.2 and 30000 lb/in.2 Fractionation and normal zoning of crystals may, in principle, occur within fixed bulk compositions held under isothermal, isobaric conditions.