Composition, orderdisorder and lattice parameters of olivines: determinative methods for Mg-Mn and Mg-Ca silicate olivines

A method has been derived to estimate both the bulk composition and the distribution of Mg and Mn and of Mg and Ca atoms between the Ml and M2 octahedral sites in olivines of the forsterite-tephroite and forsterite-monticellite series, making use of the regression equations, a = 0.93216 + 0.236ry2 + 3.918, D:0.505rr,,rr + 3.Zllrvt2+7.535, determined by Lumpkin and Ribbe (1983). The strong dependence of a on ry1, the radius of the Ml cation, and of D on ry2 is exploited on an a-b plot contoured for composition, 100 Mg/[Mg + (Mn or Ca)1, and for the Mn (or Ca) content of the M2 site. Predictions of bulk composition from a-b are within-+l.sVo Fo (on the average), and of site occupancies are usually better than +0.05 Mn or Ca. Natural olivines of the forsterite-tephroite series and zincian varieties from Franklin, New Jersey are found to be highly ordered with nVo or more of the total Mn in M2 for samples with less than one Mn atom per formula unit and with>0.94 Mn in M2 for those specimens with more than one Mn atom per formula unit. Synthetic Mg-Mn olivines are more highly disordered; synthetic Mg-Zn olivines are completely disordered. Single-phase Mg-Ca olivines synthesized by Warner and Luth (1973) are highly ordered: in Fo55>EIVo of the Ca present is in M2, in Mo55>95Vo of the Ca present is in M2. The d-spacings for the 130 and l3l peaks in X-ray powder patterns of olivines are very sensitive to Mg-Mn and Mg-Ca order-disorder, but d112 was found to be insensitive (to within one part in 2500) to cation distribution in Ml and M2. Regression equations to estimate compositions are 100 Me/[Me + Mn + minor (Fe,Zn,Ca)] : 2484.87- 970.42dtn, and 100 Fo/(Fo + Mo) = 19tt6.25- 752.l4drn