One measure of monoclinic geometry in alkali feldspars is the angle ϕ between b and b*. Monoclinic geometry is possessed if ϕ is zero, likewise if the function (1 − cos ϕ) is zero. This function has been found to behave linearly with composition for triclinic members of various alkali feldspar series and is a parameter which can be usèd to calculate triclinic–monoclinic transformation compositions, temperatures, and pressures. During the present investigation, linear least-squares fits of (1 − cos ϕ) as a function of mole fraction KAlSi3O8 were used to calculate transformation compositions for various ion-exchange and synthesis series based on data collected at room temperature and pressure. Transformation compositions using this method are significantly less potassic than those estimated previously by others from observations of peak broadening and splitting on X-ray powder diffraction charts. Furthermore, transformation composition becomes less potassic with increased Al–Si order of the feldspar series. This implies for an individual topochemically monoclinic alkali feldspar that transformation temperature decreases as Al–Si order increases. The critical parameter associated with the transformation seems to be unit-cell volume, as opposed to particular values of individual unit-cell dimensions. Structural changes related to the triclinic–monoclinic transformation in relatively disordered series are apparently paralleled in microcline-low albite series, even though the latter cannot attain monoclinic symmetry.

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