Abstract
The crystal structures and compositions of the phases in a natural cryptoperthite from the Rabb Canyon pegmatite, Grant County, New Mexico, have been determined using singlecrystal X-ray diffraction, transmission electron microscopy, and electron microprobe analysis. The cryptoperthite consists of an untwinned, monoclinic sanidine [cell dimensions: a = 8.558(1), b = 12.997(1), c = 7.179(l)A, β = 116.07°(1)] and a pericline-twinned, triclinic high albite [cell dimensions: a = 8.144(2), b = 12.989(3), c = 7.160(2)A, α = 92.10°(2), β = 116.56(2)° and γ = 90.21(2)°]. The similarity of the b and c cell dimensions and precession X-ray photographs indicate that the phases are partially, but not completely, coherent in the intergrowth plane, ∽(601). The bulk composition of the crystal determined by microprobe analysis is Or0.51Ab0.48An0.01, and the mole fraction of the sanidine phase in the crystal determined by X-ray scale factor refinement is 0.68(1). TEM examination showed that the high-albite lamellae are ∽500A wide with ∽50A-wide pericline twin lamellae. The sanidine lamellae are ∽1000A wide. The composition of the sanidine phase, as determined by direct crystallographic site refinement, is Or0.65(1)Ab0.35. The composition of the high-albite phase, as determined by mass-balance, is Or0.22(2)Ab0.78.
Both phases in this cryptoperthite are strained, although the nature and amount of strain differ from predictions of models of elastic strain in perfectly coherent feldspars. No previously-reported method of predicting the composition of the phases in a cryptoperthite from the cell dimensions yields correct results for this specimen. The observed compositions lie on an experimentally-determined coherent solvus at a temperature of 465 ± 20°.