Key optical features of Sillimanite in thin section: Colourless Needle-like shape, with diamond-shaped cross sections Moderate relief One cleavage parallel to length and typically fractured across length of needle Moderate order interference colours Straight extinction. Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rocks. Common minerals that might be confused with sillimanite and occur in similar rocks: The remainder of the … A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. The first layer type is composed of garnet (slightly lower relief and isotropic) and coarse-grained sillimanite (slightly lower relief and medium birefringence colours). In thin section the coarse-grained texture is dominated by two types of layer.
2020 sillimanite thin section