The crystal structures of two oligoclases : A comparison with low and high albite

Abstract

The crystal structures of two pegmatitic oligoclases, Anl6 and An28, have been refined using intensities from the type a reflections which give average atomic parameters, bond lengths and anglcs and thus avcrage Al/Si distributions. These are: TIO0.74,0.63 AI; TIm 0.11, 0.21 AI; T20 0.16,0.22 AI and T2m 0.16, 0.22 AI, respectively. Structural and site-occupancy par-ameters are compared with low and high albite, using data from RIBBE, ~IEGAWand TAYLORand WAINWRIGHT and STARKEY. The Na/Ca anisotropy and (Na/Ca)-O coordination polyhedra of the oligoclascs arc remarkably similar to those of high albite. The (Na/Ca)-O bond lengths for each of the first five nearest oxygen atoms decrease linearly with the mean of the two T-O bond lengths to each oxygen atom. Those oxygen atoms in the oligoclases bonded to TIO show remarkable similarities to those in low albite, whereas 0,\(2) (bonded to 1'20 and T2m) is like that in high albite. Aluminum occupancy is, of course, the major factor governing individual 1'-0 bond lengths in feldspars, although the importance of linkage (i.e., Si-OSi or Si-O-Al) is clearly evidenced in plots of mean T-O distances 1)8. individual T-O bond lengths. The small differences in T-O-T angles for a given oxygen atorn amongst the four sodic plagioclases are inversely related to mean Al content of the two T sites. Although low albite (AI: Si = 1: 3) is ordered and high albite disordered with respect to Al/Si and the two low oligoclases have different AI: Si ratios (1.16: 2.84 and 1.28: 2.72) with partly disordered AljSi distributions, it appears that the lattice parameters b, c, iX, iX*, y, y* and 20131 20131 vary linearly with the diiierence in the Al contents of the '1\0 and TIm sites (where Ahlin = Ah,o = Ah,m).