INFLUENCE OF SANIDINE STRUCTURE ON TRACE-ELEMENT PARTITIONING

Trace-element partition coefficients for Rb, Sr, Ba, and REE are influenced by both composition and structure of sanidine. In particular, lattice strain created during substitution appears helpful in understanding why ions have different partitioning behavior. This study selected sanidines from peraluminous, metaluminous, and peralkalic volcanic systems to evaluate correlations between elemental ionic radii and partition coefficients. Partition coefficients for studied systems are plotted against ionic radii of eight-fold coordination -- Onuma diagram -- show systematized variations as functions of ionic radii. Monovalent cations show single peak curves with the highest partitioning near K; divalent cations show similar pattern and peak close to Ba; and trivalent cations show decreasing patterns from La to Yb and peak at La. For all elements in the study, Ba has the highest partition coefficients. For both +1 and +2 charged cations, the parabolas clearly define a single peak around 1.4 Å.

The peak position of the partition coefficient curve for alkali feldspar was formerly believed to be a function of Or content. Cations with radii best fitting the M site should cause the least lattice strain and should therefore have higher partition coefficients. Cations with radii greater or smaller than the site will cause increased lattice strain and have lower partition coefficients. The M site sizes for sanidines of this study are more similar to the radius of divalent Ba than monovalent K (VIII-fold ionic radii are 1.42Å for Ba2+ and 1.51Å for K+). Therefore, the occupation of alkali feldspar M position by Ba causes less strain and thus, barium has a larger partition coefficient than K.