The physical properties of transparent calcic labradorite phenocrysts, USNM 115900, found in basalt in Lake County, Oregon, are described in detail. The phenocrysts occur in a groundmass of labradorite, augite, olivine, magnetite, and the alteration products of these minerals. The cores of the phenocrysts are not zoned and comprise at least 90% of the volume; the rims are complexly zoned toward more sodic compositions. The high-intermediate structural state of the feldspar as deduced by optical and X-ray measurements seems to be typical of the plagioclase in rapidly quenched hypabyssal and volcanic basaltic rocks.

Chemical analysis showed the composition to be (mol per cent) An 67.2 Ab 31.5 Or 1.3. Observed S.G. 2.720, calculated density 2.710 g/cm3. Space group I1. Unit-cell parameters: a = 8.176 Å, b = 12.865, c = 14.204, α = 93° 27.1′, β = 116° 2.9′, γ = 90° 30.6′, molar volume = 100.84 cm3/mol. A powder diffraction pattern indexed on the C1 sub-cell is given. Optically (+), 2V = 85°, α = 1.5625, β = 1.5668, γ = 1.5718.

The cell-edge parameters and linear and volume expansion were determined for 10 temperatures up to 1141° C. An equation is given for the variation of molar volume with absolute temperature; differentiation of the equation yields the thermal expansion, but the values may not be useful at high temperatures because of complications introduced by a possible step on the volume curve that may be caused by a reversible high-order transformation between 650° and 1050° C. It is suggested that this transformation may be the change from a body-centered lattice to a C-centered lattice. It may be possible to establish an equilibrium phase diagram for calcic plagioclases by high temperature X-ray measurements, and to determine the kind and order of the transformations that occur.

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