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

• Main Author: Pnur H. Rlssr
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.494.7171; http://rruff.info/doclib/am/vol64/AM64_402.pdf

The crystal structure of a strained intermediate microcline (a: 8.643, b: 12.929, c:?.190,4; a = 90.1o, g:116.2 ",1 = 89.6o; V = 720.6As; CT). in tergrown as untwinned lamellae in a cryptoperthitic ternary feldspar of bulk composition OrrrAbrrAn.Cn,, has been refined to R: 0.049 using 893 reflections. The cryptoperthite is specimen K-235 from the K0ngnAt syenites of SW Greenland. Unit-cell volume gives the best estimate of the composi-tion of the microcline phase, OrrrAb.Cns, which constitutes-36 percent of the bulk feldspar. The plagioclase composition, determined by mass balance, is approximately AbrtAn". The plagioclase is twinned at a scale of 100-5004 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 0.4"N difrraction peak widths. Due to its intergrowth with twinned plagioclase, the microcline is strained (Ao: 0.30A) in the manner described by Stewart and Wright (1974). This apparently does not prohibit the use of b-c and a*-7 * plots to estimate Al/Si distribution in the tetrahedral sites (lro- 0.51, trm-