End-members compositions of augite and its varieties

Abstract

One hundred and five chemical analyses of augite were used to calculate an average end-members composition (mol. %). The end-members composition consists of: CaMg(Si 2 O 6 ) - 64%, FeFe(Si 2 O 6 ) - 19 %, with Al [ I V ] - 10%, others -7%. Some ferroaugite have two dominant end-members CaMgSi 2 O 6 (39 mol. %) and CaFeSi 2 O 6 (29 mol. %). An average subcalcic augite consists of FeMg(Si 2 O 6 ) (47 mol. %) and CaMgSi 2 O 6 (29 mol. %). Titanium has a charge of 4+ in all augites. A maximum CaFe 3 + AlSiO 6 content (average 22 mol. %) is typical for ferrian augite, but the major end-member is CaMgSi 2 O 6 (63 mol. %). During chemical evolution of augite in the Skaergaard intrusion Fe 2 + initially fills both the M(1) and M(2) positions, and later Fe 2 + preferentially fills the M (1) position. For pigeonite, Fe 2 + tends to occupy progressively the M(2) site.