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

Abstract

The crystal structures of two pegmatitic oligoclases, An16 and An28, have been refined using intensities from the type a reflections which give average atomic parameters, bond lengths and angles and thus average Al/Si distributions. These are: T10 0.74, 0.63 Al; T1m 0.11, 0.21 Al; T20 0.16, 0.22 Al and T2m 0.16, 0.22 Al, respectively. Structural and site-occupancy parameters are compared with low and high albite, using data from Ribbe, Megaw and Taylor and Wainwright and Starkey.The Na/Ca anisotropy and (Na/Ca)–O coordination polyhedra of the oligoclases are 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 T10 show remarkable similarities to those in low albite, whereas OA(2) (bonded to T20 and T2m) is like that in high albite.Aluminum occupancy is, of course, the major factor governing individual T–O bond lengths in feldspars, although the importance of linkage (i.e., Si–O– Si or Si–O–Al) is clearly evidenced in plots of mean T–O distances vs. individual T–O bond lengths. The small differences in T–O–T angles for a given oxygen atom amongst the four sodic plagioclases are inversely related to mean Al content of the two T sites.Although low albite (Al:Si = 1:3) is ordered and high albite disordered with respect to Al/Si and the two low oligoclases have different Al:Si ratios (1.16:2.84 and 1.28:2.72) with partly disordered Al/Si distributions, it appears that the lattice parameters b, c, α, α*, γ, γ* and 2 θ131–2 θ1[unk]1 vary linearly with the difference in the Al contents of the T10 and T1m sites (where AlT1m = AlT20 = AlT2m).