Experimental Study of Clay Mineral, Greenschist, and Low-Pressure Amphibole Facies in the System Na2O-Al2O3-MgO-SiO2-H2O

Claude Triboulet
Laboratoire de Pétrographie, Université Pierre et Marie Curie, Paris1
1Equipe de Recherche associée au C.N.R.S.: ‘Pétrogénèse des roches éruptives et métamorphiques’.

Summary: Compositions concerning sodic amphiboles (glaucophane) in the system Na2O-Al2O3-MgO-SiO2-H2O have been studied at PH1O ⩽ 3 kb, 380 °C < T < 700 °C. The study attempts to establish the mineral parageneses under these physical conditions as they define the clay mineral (saponite-hectorite smectite) and greenschist (chlorite-albite) facies. The occurence of sodic amphiboles is defined by compositional parameters. Glaucophane does not appear in the present investigation.

The method of ‘reaction reversal’, i.e. using precrystallized mineral assemblages to form new ones, has been used to establish the three reactions: albite + chlorite → paragonite + saponite, paragonite + chlorite → saponite + cordierite, and saponite + paragonite → albite + cordierite. In general, the clay mineral → greenschist transformation is favoured by an increase in pressure in the system studied. The amphibole present is a sodic richterite with the type substitution NaA + MgX + 2Mg + 2MgY = NaX + 2AlY

Mineralogical Magazine; December 1976 v. 40; no. 316; p. 875-886; DOI: 10.1180/minmag.1976.040.316.09
© 1976, The Mineralogical Society
Mineralogical Society (www.minersoc.org)