Rochas Ornamentais do Brasil, Seu modo de ocorrência geológica, variedade tipológica, explotação comercial e utilidades como materiais nobres de construção
This present paper reports a general review of Brazilian ornamental rocks, in special respect of "granites", in their mode of field occurrence, type variety, quarry explotation and utilities as construction materials. Brazilian market classifies rocks in four commercial categories: "granite", "marble", "slate" and "basalt".
The "granites" correspond to holocrystalline cause-grained igneous and metamorphic rocks made up of silicate minerals, such as granite, granodiorite, quartz diorite, nepheline syenite, augen gneiss and charnockite. They have very high lustre on polished surface and, according to the colour, are classified in red, brown, yellow, blue, green, black and grey groups. The blue granite of the State of Bahia, in fact sodalite syenite, is of the highest commercial value. The granitic rocks characterised by abundant red alkaline feldspar are also highly evaluated and they commonly take place as small rock bodies intruding in late Precambrian continental collision zones. Their quarry operation is limited in boulders. Because of high surface lustre, good chemical resistance, and excellent physical durability, the "granites" are classified to be the best ornamental rocks and used mainly for floor, wall, column and table counter.
The "marbles" involve general carbonate rocks, either metamorphic or non-metamorphic ones, such as marble and limestone, and characterised by have high polished surface lustre next to "granites". Because of relative low hardness, they are easy to be cut and polished. Most of the Brazilian marble is Precambrian metamorphic rocks. The rock composed purely of carbonates is white in colour. When the certain type of secondary mineral is included, the rocks appear to be yellow, pink, green, brown and black. The light yellow-coloured limestone of the State of Bahia is widely used in Brazil due to its brecciated texture. The "marbles" are mined in outcrops and applied mainly to floor, wall, column and table counter.
The "slates" and "basalts" are classified to be semi-ornamental rocks and used without surface polish. Brazilian "slates" are extracted from outcrops of Permian sedimentary rocks and used mainly for interior floor. Grey-coloured slate is of high commercial value. Exceptionally, some types of Precambrian quartzite, muscovite schist and paragneiss are mined and sold as "slate".
The "basalts" are peculiar semi-ornamental rocks that occur exclusively in southern end of Brazilian territory. In fact, they are rhyoritic pyroclastic flow deposits of extremely high-grade welding with well-developed secondary flow texture. Their normal colour is light grey, but according to secondary minerals and weathering, they appear to be yellow, brown, red, dark grey and black. The quarry operation is in outcrops and the main use is for floor and wall.
Palavra chave: Rochas ornamentais, granito, mármore, ardósia, basalto