Slate, are metassedimentares rocks with low-grade metamorphic, formed from clay and silty-clay sequences. The scientific definition is based in the presence of preferential plans on parallel partitions, that provide the "slate fragmentation."
The plans of fragmentation are formed by the flow of glasses and placoid minerals, composing a schistose structure common to much of the metamorphic rocks. The distinction of the slates, among the other rocks with preferential cleavage planes, is determined by its very fine grain and a greater capability to partition into parallel surfaces.
Its main mineralogical constituents include fine white mica (sericite), quartz, chlorite and graphite . In general, variable quantities of carbonate, tourmaline, titanite, rutile, feldspar, oxides, iron pyrite, there may be additional.
Being essentially composed of stable minerals as quartz and phyllosilicates (mica and chlorite), the slates are resistant to weathering and therefore highly durable. Some impurities, especially the carbonates, contribute to the decrease of durability because they are attacked by acid solutions and corrode the slates.
In Minas Gerais, the areas of extraction and processing of slate covering an area of 7,000 km2, which represents 90% of Brazilian production. The varieties extracted, are commercially categorized by colour, recording slate gray, green, purple (wine), black and graphite.
The varieties of slate gray, black and graphite may give rise to the slate "rust" as a result of oxidation of thin slabs of pyrite with fibrorradial structure, interstratified. Where the split are more spaced by the separation of the planes defined by Slate Cleavage, formed the slate type “matacao”.
The main commercial product of slates in Minas Gerais, which refers to at least 70% of the 15 million m2 of flat sheets produced annually, are for slabs for flooring and walls. The remaining production is aimed at partitions, table tops, cabinets, pool tables and furniture, tiles, slabs ("black boxes), crafts.