End-members compositions of augite and its varieties
Bulakh, Andrei G.; Zolotarev, Anatoly A.
published: Aug 6, 2004
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(Si2O6) 64 %, FeFe(Si2O6) 19 %, with Al [IV] 10 %, others 7 %. Some ferroaugite have two dominant end-members CaMgSi2O6 (39 mol. %) and CaFeSi2O6 (29 mol. %). An average subcalcic augite consists of FeMg(Si2O6) (47mol. %) and CaMgSi2O6 (29 mol. %). Titanium has a charge of 4+ in all augites. A maximum CaFe3+ AlSiO6 content (average 22 mol. %) is typical for ferrian augite, but the major end-member is CaMgSi2O6 (63 mol. %). During chemical evolution of augite in the Skaergaard intrusion Fe2+ initially fills both the M(1) and M(2) positions, and later Fe2+ preferentially fills the M(1) position. For pigeonite, Fe2+ tends to occupy progressively the M(2)site.