Black Granite

What is Granite?

Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly black, white, pink, red, green or gray in color, depending on their mineralogy. As an igneous rock it has between 20% and 60% of quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term “granite” is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equiangular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals

Granite is nearly always massive (lacking any internal structures), hard and tough, and therefore it has gained widespread use throughout human history as a construction stone. The average density of granite is between 2.65 and 2.75 g/cm3 , its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6 • 1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F); it is strongly reduced in the presence of water, down to 650 °C at a few kBar pressure. Granite is the perfect natural stone to weather natural conditions courtesy the intensity by which it was formed. It can be used in flooring, cladding, sculptures & as a protective surface both in private and public spaces. Granite has poor primary permeability, but strong secondary permeability.

Black Granite- timeless, enduring & the classiest!

Black Granite is a dark colored ingenious rock containing gabbro, basalt, diabase, anorthosite & diorite. These are used for buildings, facades, monuments, pavements and even used in crushed forms. These have an interlocked crystalline texture with little or no quartz or alkali feldspar but are composed of calcic plagioclase with minerals such as biotite & pyroxenes. They have a high ferrous & magnesium content.

When blocks are cut into slabs and are being honed or polished to bring out the color & texture they could have specks (tiny white or colored spots) in various sizes. When the slab is ready it could range in color from dark black to greyish or greenish black.  The biggest challenge during the quarrying process is the consistency of a desired color as transitions can happen even as often as fractures. Quarrying is an art as much as a science especially in getting consistency of the color hence the demand for black granite.

Typology of Black Granite 

1.    GABBROS

2.    MONZODIORITE

3.    DIORITE

4.    DOLORITE

 

Grading

Black Granite is graded based on the depth of its color, grain size & density. The texture createsthe difference through specks, grains, veins and spots. As these are natural formations which are elevated through polishing sometimes the desired effect may not be achieved due to its aesthetic properties and at the same time it could also end up as an exotic find none like the others.

Possible defects

The most frequent defect in black granite is the change in color or variations. The finer the grain the more visible are the smallest of defects. Variations in colour and texture in the varieties marketed as “uniform” are rarely acceptable. A close petrographic examination by a buyer can also lead to conclusions of decay or degradation of the stone over time as now its being exposed to the forces of weathering. Dyke unlike Sill deposits require special attention as quality varies laterally, especially colourwise and in most cases it gets darker in depth.