Abstract
Amphiboles have been grown across the join NaCa2Mg4AlSi6Al2O22(OH)2–NaCa2Fe4 AlSi6Al2O22(0H)2 on QFM and CCH4 oxygen buffers to gain insight into the Fe−Mg substitution in an amphibole without the local charge imbalance caused by Na in the M(4) site. Oxygen fugacity was found to have no effect upon the unit-cell dimensions of amphibole. Unit-cell parameters (C2/m) for amphibole grown across the series are [a, b, c(Å), β, V(Å3)]:
| Mg4 | 9.892(1),1 | 17.941(2), | 5.277(1), | 105°33′(1), | 902.2(3) |
| Mg3Fe | 9.904(1), | 17.989(5), | 5.291(2), | 105°27′(1), | 908.6(5) |
| Mg2Fe2 | 9.915(3), | 18.031(7), | 5.301(3), | 105°24′(1), | 913.6(1.0) |
| MgFe3 | 9.930(5), | 18.104(6), | 5.320(2), | 105°16′(1), | 922.6(9) |
| Fe4 | 9.953(5), | 18.152(3), | 5.330(2), | 105°16′(2), | 928.8(4) |
| Mg4 | 9.892(1),1 | 17.941(2), | 5.277(1), | 105°33′(1), | 902.2(3) |
| Mg3Fe | 9.904(1), | 17.989(5), | 5.291(2), | 105°27′(1), | 908.6(5) |
| Mg2Fe2 | 9.915(3), | 18.031(7), | 5.301(3), | 105°24′(1), | 913.6(1.0) |
| MgFe3 | 9.930(5), | 18.104(6), | 5.320(2), | 105°16′(1), | 922.6(9) |
| Fe4 | 9.953(5), | 18.152(3), | 5.330(2), | 105°16′(2), | 928.8(4) |
No change in cell parameters is observed with temperature on a given buffer. The essentially linear trend indicates disorder of Mg and Fe in M(1), M(2), and M(3) sites. Plagioclase and pyroxene were present in all charges. The amount ranged from a few percent to greater than 30 percent, without affecting the unit-cell parameters of coexisting amphibole.
