Vermiculite-like minerals in low-grade metasediments from the Coastal Range of central Chile M. D. RUIZ CRUZ 1, *, E. PUGA 2 , L. AGUIRRE 3 , M. VERGARA 3 AND D. MORATA 3 1 Departamento de Quı ´mica Inorga´nica, Cristalografı ´a y Mineralogı ´a, Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain, 2 Instituto Andaluz de Ciencias de la Tierra C.S.I.C., Avda. Fuentenueva, 18002 Granada, Spain, and 3 Departamento de Geologı ´a, Universidad de Chile, Plaza Ercilla, 803, Santiago, Chile (Received 10 January 2001; revised 17 August 2001) A B S T R A C T : Mixed-layer minerals with optical properties similar to metamorphic vermiculite were identified in rocks belonging to a Palaeozoic and a Triassic formation separated by an angular unconformity and exposed in the Coastal Range of central Chile. Both formations are affected by low-grade metamorphism. The mixed-layer minerals were studied by optical microscopy, X-ray diffraction (XRD), electron microprobe (EMPA), and transmission/analytical electron microscopy (TEM/AEM). Two types of phyllosilicates were identified: chlorite-vermiculite and mica-chlorite, which are present in the Palaeozoic and Triassic rocks respectively. Chlorite-vermiculite mixed layers form packets with well-defined boundaries and mainly show 1:1 ordered sequences. On the contrary, mica-chlorite mixed layers show, in most cases, random sequences evolving laterally toward chlorite. The AEM data indicate compositions close to that of chlorite in the ternary Si-Al- (Fe+Mg+Mn) diagrams for both types of mixed-layer phyllosilicates. Relative to the coexisting chlorite, they have lower (Fe+Mg) contents, and a higher Si/Al ratio. They are interpreted as products of the transformation of chlorite, developed during prograde metamorphism, and probably represent intermediate, metastable phases, in the chlorite to biotite transformation. KEYWORDS: Coastal Range, Chile, chlorite, mixed-layer minerals, TEM, vermiculite. High-temperature or metamorphic vermiculite was defined by Velde (1978) on the basis of optical properties, microprobe data and thermal stability, as a phyllosilicate with high birefringence (like biotite or stilpnomelane), a composition close to that of chlorite but containing K 2 O and CaO in small amounts, and with low-temperature stability. This mineral has been reported occasionally in low-grade metamorphic rocks, and interpreted either as a retrograde metamorphism (or weathering) mineral (Brown, 1967; Black, 1975) or a stable meta- morphic phase (Kerrik & Cotton, 1971) for which Velde (1978) deduced a stability field in P-T space, just below the biotite stability field. Because of the scarcity of this mineral and the common presence of chlorite or biotite, which prevent its accurate X-ray characterization, most of the descriptions of this mineral were based on its optical properties and microprobe data. Nevertheless, in recent years, the investigation by scanning or transmission electron microscopy (SEM, TEM) of metamorphic minerals showing optical and chemical properties similar to meta- morphic vermiculite has allowed the identification of fine-scale muscovite-chlorite intergrowths (Francescelli et al., 1986; Mellini et al., 1991) and a variety of mixed-layer minerals, including chlorite-smectite (Beaufort, 1987), mica-chlorite, * E-mail: mdruiz@uma.es DOI: 10.1180/0009855023720029 Clay Minerals (2002) 37, 221–234 # 2002 The Mineralogical Society