Mixed-layer mica-chlorite in very low-grade metaclastites from the Malaguide Complex (Betic Cordilleras, Spain)

Mixed-layered phyllosilicates with composition intermediate between mica an d chlorite were identified in very low-grade metaclastites from the Mal gui de Complex Betic Cordilleras, Spain), and studied by X-ray diffraction, and transmission and analytical electron microscopy. They occur both as small grains in the rock matrix, and associated with muscovite-chlorite stacks. T ransmission electron microscope observations revealed a transition from chl orite to ordered 1:1 interstratifications through complex 1:2 and 1:3 inter stratifications. Analytical electron microscopy data indicate a composition slightly different from the sum of discrete trioctahedral chlorite and dio ctahedral mica. The types of layer transitions suggest that mixed-layer for mation included two main processes: (1) the replacement of a brueite sheet by a cation sheet in the chlorite structure; and (2) the precipitation of m ica-like layers between the chlorite layers. The strongest diffraction line s in oriented X-ray patterns are: 12.60 Angstrom (002), 7.98 Angstrom (003) , 4.82 Angstrom (005) and 3.48 Angstrom (007).