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.