Compressibility mechanisms of alkali feldspars: New data from reedmergnerite

Citation
Rt. Downs et al., Compressibility mechanisms of alkali feldspars: New data from reedmergnerite, AM MINERAL, 84(3), 1999, pp. 333-340
Citations number
26
Categorie Soggetti
Earth Sciences
Journal title
AMERICAN MINERALOGIST
ISSN journal
0003004X → ACNP
Volume
84
Issue
3
Year of publication
1999
Pages
333 - 340
Database
ISI
SICI code
0003-004X(199903)84:3<333:CMOAFN>2.0.ZU;2-9
Abstract
Structural and volume compressibility data for reedmergnerite, NaBSi3O8, we re obtained by single-crystal X-ray diffraction at pressures up to 4.7 GPa. The bulk modulus was determined to be 69.8(5) GPa with the pressure deriva tive constrained to 4. Unit-cell compression is anisotropic, as indicated b y unit strain tensors. Tetrahedral bond lengths and angles remained relativ ely constant over the pressure interval, whereas Na-O bonds decreased syste matically. T-O-T angles underwent a variety of behaviors, remaining constan t or decreasing with pressure. The compression for reedmergnerite is similar to that of low albite, wherei n bending of the (Al,B)-Oco-Si angle compresses the Na-bearing zigzag chann els. In contrast, microcline compresses by shearing the four-membered rings , which in turn compresses the K-bearing channels. At about 4 GPa, a new bo nd between K and Obm appears that alters the compression mechanism and expl ains the discontinuity in the pressure variation of crystallographic parame ters observed by Allan and Angel (1997). Thus, the compression mechanism of the alkali feldspars is dominated by the compression of alkali containing channels. However, because of low symmetry, this can be accomplished in sev eral ways. The observed variety of compression pathways resulted from T-O-T angle bending energetics that were coupled with the effects of alkali cati on bonding.