Siliceous - Argillaceous rocks, Slates

Slates are meta-sedimentary rocks, of low metamorphic degree, formed from argillaceous and silicic-argillaceous sequences. The scientific definition of ‘slate’ is yet based on the presence of preferential plans of parallel partition which provide the "slate cleavage".

The cleavage plans are formed by isorientation of placoid and prismatic minerals, setting a schistous structure, common to an important part of metamorphic rocks. Their very fine granulation and a more important capacity of partition into parallel surfaces determine the distinction of the slates among the other rocks with preferential cleavage plans.

Their main mineral components include white fine mica (sericite), quartz, chlorite and graphite. Variable and usually accessory amounts of carbonate, tourmaline, titanite, rutile, feldspar, iron oxides and pyrite may occur.

Being essentially constituted of steady minerals such as quartz and phylosilicates (mica and chlorite), slates are resistant to subliming and therefore highly durable. Some impurities, specially the carbonate ones, contribute to the reduction of durability, because they are attacked by acid solutions that corrode the slates.

In the State of Minas Gerais, the areas of extraction and processing of slates enclose a 7.000 sq./km region, which responds for 90% of Brazilian production. The extracted varieties are commercially distinguished by the color, being noticed the gray, green, purple (wine), black and graphitic colored slates.

The gray, black and graphitic colored varieties can originate the “rust” slate, as a result of the oxidation of fine pyrite lamellas, with fibroid-radial interstratified structure. Where the displacement plans defined by the slate cleavage are more spaced, the slates of the type "matacão" (solid rock) are formed.

The main commercial production of slates from Minas Gerais, responsible for at least 70% of the 15 million sq./m of the annually elaborated plates, is of tiles for floor covering. The remaining production is used for partitions (interior walls), table boards, sinks and billiards, furniture, roofing tiles, blackboards, handcraft etc.