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MINERAL COMPOSITION & PHYSICAL PROPERTIES

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
became part 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 deposits in kettles 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 rock materials 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”.


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.

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.