Tosudite in very low-grade metamorphic graywackes from the Malaga area (Betic Cordilleras, Spain)
Cruz, Maria Dolores Ruiz; Andreo, Bartolomé
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1391 - 1400
published: Jan 8, 1997
manuscript accepted: Jul 12, 1996
manuscript received: May 3, 1996
ArtNo. ESP147050806016, Price: 29.00 €
Abstract The regularly interstratified dioctahedral chlorite/smectite, tosudite, has been identified in Carboniferous graywackes from the Malaga region (southern Spain). It mainly occurs replacing large phyllosilicate grains, frequently intergrown with muscovite and dickite. X-ray diffraction data reveal a basal spacing of 29.2 Å that shifts at 31 Å after ethylene-glycol treatment and contracts at 23.2 Å after heating at 500°C. Transmission electron microscopy confirms the nature of a regular 1:1 interstratification and shows sequences resulting in 24 Å periodicity, due to chlorite (14 Å) and collapsed smectite (10 Å). Chemical data (AEM) indicate high Si content and about 0.60 atoms of Fe + Mg in the octahedral sheets. The Málaga tosudite can be included within the very low-grade metamorphic association determined in the Maláguide complex (Betic Cordilleras), and ascribed to the Alpine orogeny. Minimum P-T conditions of tosudite stability are estimated around 150-170°C and 2-4kbar on the basis of chlorite geothermometry and phengite geobarometry.