stonehistory

ABOUT INDIAN STONES

Stones are classified based on their mineral content and the process of their formation. Good stones are characterized by durability, hardness, strength, compatibility to dressing, appearance, density, fineness of grain and compactness and porosity and property of absorption. For being suitable as building stone, a rock should have specific qualities such as capacity to stand the damage caused by time and weather, sufficient strength to take up strain and super-incumbent weight and attractive colours and general appearance for aesthetic acceptability. Its structure must also be such as to allow quarrying into sufficient sized blocks or sheets. It should possess inherent features such as joints and (in sedimentary rocks) bedding planes. India has extensive deposits of different kinds of building and monumental stones and is one of the few major countries known for the production and export of granites and marbles of various colours, along with slate and other building stones. The most important building stones in India are the granites/and allied rocks, the standstones, limestones and marbles, and the laterites.

IGNEOUS STONES

Igneous stone ranges from very soft rocks such as pumice and scoria to somewhat harder rocks such as tuff and hard rocks such as obsidian, granite and basalt. Igneous stones are formed from molten or partly molten material i.e. magma, through solidification of magma. Lava is a form of magma that cools and solidifies on the surface of the earth. Liquid magma cools and solidifies underneath the Earths surface, and then mineral gases and liquids penetrate the stone and create new crystalline formations and various colours. From all igneous rocks, Granite is the most common choice. It is a coarsely crystalline unstratified igneous rock composed of alkali feldspars, quartz, mica and hornblende. Since granite is highly speckled, it varies greatly in colour depending on the range of the minerals present and the location of the quarry. When the mica in granite is white, it is known as muscovite, when it is dark it is biolite. In certain types of granite, hornblende is an important ingredient and it may be even more abundant than the micas. Due to its chemical structure, granite is a much harder stone. It doesnt get scratched as easily and rarely needs heavy-duty grinding.

SEDIMANTORY STONES

There are two main types of sedimentary stone used in masonry work, limestones and sandstones. Sedimentary stones are layered rocks, formed through the accumulation and solidification of sediments, which may originally be made up of minerals, rock debris, or animal or vegetable matter. These stones come from such organic elements as glaciers, rivers, wind, oceans and plants. Tiny sedimentary pieces break off from these elements and accumulate to form rock beds. They are bonded through millions of years of heat and pressure and include limestone, sandstone and travertine. Some stone types are the result of some pre-existing rocks demise through the weathering process. Sandstone "Siliceous" The biogenic stones are 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 Limestones, Coralstones, Fossilstone, Shellstone "Calcareous". 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.

METAMORPHIC STONES

Metamorphic stones are formed in some fashion from a pre-existing rock, through heat, pressure, the effect of superheated fluids, or any combination of these forces. The change can be a development of crystalline formation, a texture change or even a color change. Metamorphic is derived from the Greek word "meta" denoting a change and "morph" meaning form or 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".

SLATE

As we have seen, India is the home of exquisite natural stones like granite, marble, slate and sandstone and has a tradition of usage of stones, since time immemorial. Slatestone is a very fine grained metamorphic rock derived from the sedimentary rock shale. Shale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by thin laminae or parallel layering or bedding less than one centimeter in thickness. Slate is composed mostly of mica, chlorite and quartz. Shale is subject to heat and pressure and the fine particles are "melted" together to form slate. Slate is characterized by a sub parallel orientation which imparts a strong parallel cleavage to the rock which allows the slate rock to be split into thin but tough sheets of slate. Slate has a fine to medium grained surface texture and has an excellent shining property. Chemically they are Multi-Mineral rocks made of an aggregate of minerals and colloidal substances. Essentially mineral composition includes members of Mica group commonly sericite & muscovite, Clay group commonly paragonite, chlorite & kaonilite. The accessory minerals are oxides, calcites & minor amounts of quartz and feldspar along with subordinate amounts of ferromagnesian minerals. Free silica is normally poor. It is resistant to acids and staining.

PHYSICAL PROPERTIES OF SLATES

  Composition:

    Muscovite, chlorite, kaolinite, micas, quartz and other accessory minerals (wide variation in composition).

  Hardness:

    Soft - easily scratched. They can also be as hard and dense as granite

  Absorption:

    Low - medium oil, water other liquids Weathering characteristics: Due to the wide ranges of slate, this stone has wide ranges of weathering. Cleft planes provides permeability for water penetration resulting in spalling and flaking. Slates that have a high kaolinite content (a clay) display accelerated rate of deterioration in wet environments.

  Tendencies:

    Absorbs oils and liquids...Softer/medium varieties scratches easily.

  Colors:

    Colors vary widely

  Finish:

    Different finishes such as acrylic, wax, natural cleft, honed etc. Some types may be flamed.

  Colors vary widely

CHEMICAL PROPERTIES OF SLATES

 

Slate stone is chemically a multi-mineral rock made of an aggregate of minerals and colloidal substances. Essentially mineral composition includes members of Mica group commonly sericite & muscovite, Clay group commonly paragonite, chlorite & kaonilite. The accessory minerals are oxides, calcites & minor amounts of quartz and feldspar along with subordinate amounts of ferro-magnesium minerals. Free silica is normally poor. It is resistant to acids and staining.