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°.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.