Winchite and the actinolite–glaucophane miscibility gap
Leake et al.1 have recently emphasized the apparent scarcity of winchite s.l., NaCa(Mg, Fe2+)4(Al, Fe3+) [Si8O22/(OH)2] (ref. 2), an amphibole intermediate in composition between the glaucophane–riebeckite series of high-pressure blueschists and the tremolite–actinolite series of lower-pressure greenschists. The rarity of winchite is considered consistent with the existence of a miscibility gap3,4 between these two amphibole series at the metamorphic P–T conditions normally attained in nature. Here we present optical and X-ray properties of Mg- and Al-rich winchite from Venezuela and draw attention to several other recent winchite analyses. In fact, as Schliestedt's5 new data on co-existing sodic and calcic amphiboles in blueschists show, only the present Mg–Al-rich winchite definitely lies within the above miscibility gap.
Exsolution of cummingtonite, actinolite and sodic amphibole in hornblende in high-pressure metamorphism
Exsolution phenomena in amphiboles indicate the existence of miscibility gaps1,2. The phase relationships of hornblende, the commonest amphibole in igneous and metamorphic rocks, to the other monoclinic amphiboles such as cummingtonite, actinolite and sodic amphibole has particular petrological importance. In these systems a solvus has been directly confirmed by exsolution in the hornblende–cummingtonite system3–5. Chemical discontinuities suggesting miscibility gaps have been shown for other systems6–18. In the hornblende–actinolite system, which was recently confirmed to have a solvus by experimental synthesis19, fine lamellar texture that is too fine for microprobe resolution is expected to result from exsolution9,10. However, there has been no report of amphibole exsolution from high-pressure metamorphic rocks. We have found exsolution lamellae of three kinds of clinoamphibole, cummingtonite, actinolite and sodic amphibole, in a zoned hornblende from high-pressure metamorphic rock, and present here direct evidence to confirm the miscibility gaps between hornblende and these three clinoamphiboles in high-pressure metamorphic conditions. The genesis of exsolution lamellae discussed below is based on back-scattered electron-scanning images and microprobe analyses.
Regional strategies for the minerals industries of the states of the SADCC
The SADCC has been in existence for eleven years but little progress has been made in regional coordination of the minerals sector even though this sector is the most important exporter of the region. There is however scope for minerals development at the regional level.