Data for the 23 K-feldspars whose structures have now been refined serve to construct curves used to derive the Al contents of the four tetrahedral sites from unit-cell parameters; the data are utilized according to principles different than those used in the Stewart-Wright-Ribbe, Kroll and similar curves. The new curves. applicable to K-feldspars with Or 2 >= similar to 85 mol. %, are designed to yield (l) the mean tetrahedral cation-oxygen distance (1)/T-3-O (A) for each T-j site and (2) the corresponding Al content t(j) (atomic %), urtilizing in effect either of two commonly used t versus EO linear relationships, a "modified Jones-Ribbe-Gibbs".(J-R-G) relationship (Si-O 1.604, Al-O 1.759 angstrom) and a "modified Smith-Bailey" (S-B) relationship (1.609, 1.745 angstrom). The curves consist in linear regression lines through plots of observed distances : (T-1-O) over bar and (T-2-O) over bar (A) against cell parameters b, c (A) and c*/b*(=6d(060)/2d(002)) for monoclinic K-feldspars (sanidines, orthoclases, adularias), and through plots of observed distances (T1O-O) over bar (T(1)m-O) over bar, and the mean of (T-2 O-O) over bar and (T(2)m-O) over bar (A) against interaxial angle gamma (degrees) for triclinic K-feldspars (microclines). Scales for t assumming each of the J-R-G and S-B t versus (T-O) over bar relationships are added in diagrams for both. the monoclinic and the triclinic cases. and scales for gamma* (degrees) and triclinicity triangle (Goldsmith & Laves 1954) are added parallel to the T scale in the the diagram. The errors in derived results are estimated to be in most cases +/- 0.003 angstrom in (T-j -O) over bar and +/- 3 Al (in atomic %) in t(j) for a given pair of t and (T-O) over bar values. The diagrams show that a completely Si/Al disordered (high) sanidine would have b = 13.047. c = 7.171 angstrom, and that the triclinic microcline Series can be interpreted as having a monoclinic end-member with (T-1-O) over bar and (T-2-O) over bar equal to 1.660 and t. 624 angstrom,, and, t(1) and t(2) equal to similar to 36 and 14% Al, which correspond to the values for orthoclase.
