The structure of a strained intermediate microcline in cryptoperthitic association with twinned plagioclase Available to Purchase

The crystal structure of a strained intermediate microcline (a = 8.643, b = 12.929, с = 7.190A; α = 90.1°, β = 116.2°, γ = 89.6°; V = 720.6A³; C1), intergrown as untwinned lamellae in a cryptoperthitic ternary feldspar of bulk composition Or₃₃Ab₅₈Аn₈Сn₁, has been refined to R = 0.049 using 893 reflections. The cryptoperthite is specimen K-235 from the Kûngnât syenites of SW Greenland. Unit-cell volume gives the best estimate of the composition of the microcline phase, Or₉₁Ab₆Cn₃, which constitutes ∼36 percent of the bulk feldspar. The plagioclase composition, determined by mass balance, is approximately Ab₈₇An₁₃.

The plagioclase is twinned at a scale of 100-500A on both albite- and pericline-twin laws, and special caution was required to exclude from the refinement diffracted peaks from the microcline lattice which overlapped with diffracted peaks from these four plagioclase lattices. In spite of the non-continuous, lamellar nature of the microcline phase in the cryptoperthite, the mosaic texture resulted in only O.4°2θ diffraction peak widths.

Due to its intergrowth with twinned plagioclase, the microcline is strained (Δа = 0.30A) in the manner described by Stewart and Wright (1974). This apparently does not prohibit the use of b-c and α*-β* plots to estimate Al/Si distribution in the tetrahedral sites (t₁o ∼ 0.51, t₁m ∼ 0.35, t₂o = t₂m ∼ 0.07 Al); mean T–O distances (T₁O = 1.671, T₁m = 1.651, T₂O = 1.622, T₂m = 1.627A) give similar values (0.47, 0.32, and 0.10 Al, respectively). Without a meaningful basis of comparison, the effects of strain on individual bond lengths and angles could not be evaluated quantitatively.