Abstract

The crystal structure of a homogeneous unstrained intermediate microcline has been refined to R = 0.043 using 2183 reflections (I > 2σI) collected with an automated singlecrystal X-ray diffractometer. Its composition (Or89Ab11Cn1 and structural state (t1o = 0.54, t1m = 0.32, o = t2m = 0.07) are similar to those of the strained intermediate microcline (coherently exsolved from a cryptoperthitic ternary feldspar) for which the refined structure was reported by Ribbe (1979). A comparison of the two structures has led to a preliminary assessment of the crystallographic effects of strain in terms of bond lengths and bond angles, including a crystallographic explanation of strained lattice parameters.

Relative to corresponding sites in the strained feldspar, 〈T1O–O〉 is larger in the unstrained feldspar by 0.010Å, whereas the T1m and T2O sites are the same size, and 〈T2m–O〉 in the unstrained feldspar is slightly smaller. As pointed out by many previous workers, coherent exsolution of the strained feldspar parallel to (601) requires compression of b and c in the K-rich phase, with a concomitant expansion of a predicted by elasticity theory. Our results support these predictions. The strained a cell dimension is longer by 0.019Å due to an extension of the double crankshaft chain by rotation of tetrahedra about c* and lengthening of the OCO–OBm and OBO–OCm tetrahedral edges. These tetrahedral rotations about c* and accompanying tilts about a result in the shortening of the b cell edge in the strained feldspar by 0.046Å. The c cell edge of the strained feldspar is shorter by 0.015Å due principally to the smaller size of the T1O site.

With the exception of compression along c, previous predictions that feldspars respond to stress by tilting of tetrahedra rather than by alteration of <T–O> distances, and that the potassium site is a “soft” site, are confirmed. Envisioning tetrahedral tilts in terms of changes in T–O–T angles does not lead to an accurate perception of the geometrical changes that accompany coherent strain.

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