Mica-chlorite intermixing and altered chlorite from the Nevado-Filabride micaschists, Southern Spain
Mellini, Marcello; Nieto, Fernando; Alvarez, Fernando; Gomez-Pugnaire, Maria Teresa
published: Feb 21, 1991
manuscript accepted: Jul 26, 1990
manuscript received: Jul 11, 1989
ArtNo. ESP147050301011, Price: 29.00 €
Abstract Minerals with biotite-like optical properties occur in the Nevado-Filabride micaschists. The grains are highly pleochroic, variable in colour from light yellow to deep red, and strongly recall the descriptions given for "oxychlorite". Based on X-ray powder diffraction, microprobe analysis and transmission electron microscopy, two main types are recognized in optically similar grains : 1) Those consisting of a chlorite matrix interleaved with approximately 5 % by volume of very thin biotite lamellae. 2) Those consisting of a chlorite-like, hydrated material that produces irrational 16-16.5 Å spacings in the X-ray diffraction pattern. These values progressively decrease upon heating. This material gives microprobe analyses close to coexisting chlorite, but shows systematically lower oxide sums, of the order of 79-85 wt.%. Wavy rather than straight lattice fringes are obtained on these grains. Lamellae of haematite, or of a precursor of haematite, a few hundred Å in thickness and with granular appearance, may occur interleaved with the 16 Å material. The 16 Å hydrated chlorite is a retrograde alteration product which is supposed to have been derived from the grains consisting of chlorite-mica association. Low temperature reactions occurred during the very late evolution stage of the Veleta unit, producing the hydration of chlorite and alteration of plagioclase, chloritoid and garnet. These reactions occurred during the recent, relatively rapid uplift of the Veleta rocks.