It is important to have at least a basic understanding concerning the geology of the dimensional stones being used commercially.
All stone is not the same…there are Three Major Groups from which the stones that are used for decorative and finishing purposes come from. The difference between and within these stone groups has a dramatic effect on the performance of a stone. These differences are attributed to mineral components and the processes by which the stone was formed.
GEOLOGICAL CLASSIFICATIONS
MINERALS-STONE COMPONENTS Stones are made up of a mosaic of interlocking mineral grains that have definite chemical compositions. Minerals have been divided into groups. The groups we are primarily concerned with are the Common Rock-Forming Mineral Groups which can be subdivided into two mineral categories Silicates and the Carbonates.
Silica is the most common chemical component of the earth. Silicates which contain silica are generally the harder minerals and the most common rock-forming minerals. These stones can be referred to as Siliceous based stones.
Carbonates are the next most important minerals in the rock-forming group. This mineral group generally contains the softer minerals. These stones can be referred to as Calcareous based stones.
THE THREE MAJOR GROUPS
Rocks have been divided into major and minor groups according to the process by which they are formed and each of these groups are subdivided into further stone categories. Only the Three Major Groups from which dimensional stones being used commercially will be covered.
Igneous : This group has been formed from molten or partly molten material i.e. from magma. Lava is a form of magma that has cooled and solidified on the surface of the earth. Granite is a igneous rock that has cooled and crystallized below the earth’s surfaces, having a fine to coarse-grained mineral crystal structure. It is a “Siliceous” based stone. Composed primarily of quartz, feldspar, and other accessory minerals.
Sedimentary : There are four main types of sedimentary rocks…
1. First there are the types that are the result of some preexisting rock’s demise through the weathering process. Sandstone “Siliceous”
2. The second kind is biogenic, stone produced by living organisms such as marine creatures or vegetation which eventually died and the remains becamepart of the sediment on the bottom of ancient sea beds. Certain Limestone’s, Coralstones, Fossilstone, Shellstone “Calcareous”
3. The third is produced by inorganic chemical material being deposited on the floor of ancient sea, lake beds and springs through the process of precipitation or evaporation. The white fur depoe sits in ktles in which hard water has been boiled is a good example of this type of stone. Certain Limestone’s and all
Travertine’s. “Calcareous”
4. The last sedimentary category are produced as a result of volcanic activity. This stone is composed of volcanic ash with a variety of secondary older rockmaterials from the surrounding area, volcanic and igneous in nature. This category is considered a tuff sedimentary stone. It is also classified as a igneous stone due to its volcanic origin. Cantera and Adoquin are of this variety. “Siliceous”
Metamorphic : Metamorphic is derived from the Greek meta denoting a change and morph meaning form…a change in form. This changing of One rock type into another new type takes place by an increase in temperature or pressure or a combination of both. The original minerals of the stone recrystallize, small crystals merge to form larger crystals with no changes in the mineral chemistry, fine-grained calcite in limestone recrystallizes to a coarse-grained calcite crystal structure in marble; or there may by a transformation into a different set of metamorphic minerals. This recrystallization transforms the…Shale into a colorful Slate “Siliceous”. Limestone becomes a multi colored Marble “Calcareous” and the grainy Sandstone alters into a highly crystallized sparkling Quartzite “Siliceous”. The caterpillar transforms into a beautiful butterfly.
NOTE : The names applied to stones within these three major groups are general in their geologic usage; and within the stone industry, commercially, these names become trade terms, a generalization classification, which is used to describe a wide range of stones. They have been applied to…marble, limestone, travertine, granite, slate and quartzite. Other stones may be referred to as a commercially accepted term due to similarities in color or other physical characteristics.
Marble…Commercially describes a wide range of stones that are capable of taking a polish. True geological Marbles, Limestone’s that can take a polish, Dolomite stones, Travertine’s, Onyx and Serpentines (green marbles).
Granite…The commercial classification has taken on a broader meaning, generally referring to granite-type stones with similar characteristics.
See STONE TYPES for more information pertaining to a particular stone.
The difference between the geological name of the stone and its commercial classification can be important in relation to its true geological characteristics, which governs the stones performance characteristics in its environmental setting and the conditions the stone will be subjected too.