Slate And Quartzites Of India

www.stonexp.com  2010-07-22 10:45:05  Popularity Index:0  Source:Internet

Slate is a fine grained metamorphic rock, defined commercially as layered stone that can be split into slices of thickness ranging from 8 mm to 10 mm, where as quartzite is a hard metamorphic rock which was originally sandstone.

SLATE
Slate has been formed by the metamorphism of pre-existing clay rocks (shales). The most remarkable feature of this rock is that it has very marked cleavage planes, which enable it to be either manually or mechanically split to obtain these slabs. Certain planes of easy split called "schistosity" are well developed, which makes these rocks suitable for decorative use. Slate schists having good and even cleavage are valued much.

Occurrences
The Aravali mountains in the state of Rajasthan and Haryana, Cuddaph in the state of Andhra Pradesh, Tamil Nadu and Himalayas in the   northern states have undergone metamorphism and given rise to deposits of slate. The availability of slate has been reported from the states of Haryana, Himachal Pradesh, Andhra Pradesh, Tamil Nadu, Rajasthan and Madhya Pradesh. The total reserves of slate in India are around 500 million tonnes. Out of this 50 million tonnes are available in Rajasthan. The Haryana Minerals Ltd. has estimated a reserve of 10.5 million cu m slate in its lease hold area in Haryana.

The Directorate of Geology and Mining of different states has carried out exploration for slate in association with the other building stones.

In the last decade slate of Rajasthan has come up in a big way, both in production as well as in export. The important mining and processing centres in Rajasthan are Alwar, Ajmer, Bharatpur, Tonk, Sawai Madhopur, Pali, Udaipur, Churu and Chittorgarh districts. The slate produced in these centres is used for cladding and marketed under the commercial names such as Golden Copper, Green Mica, Speckle, Deoli Green, Mahu, Silver Grey etc.

In Andhra Pradesh the slate is quarried in Markapur slate belt of Remedicherla area of Prakasam and Guntur districts.

In Haryana the slate is quarried at Rewari and other nearby areas in the state.

In Himachal Pradesh, which has been producing slate for the last many years, the important mining centres are Chamba, Kangra, Mandi, Shimla and Kulu districts. The slabs produced in the state is mainly used as a roofing tile.

Properties
Slate is mainly composed of inert mineral constituents like sericite mica, chlorite, quartz, hematite, rutile etc. Based on mineralogical composition, varieties of slate are as under:
•    Bended Slate : The original beds from which these are formed, were of different colours, say black, greyish black, greenish multicoloured, and the cleavage planes cutting across the bedding planes gave the bended appearance.
•    Ferruginous Slate: It contains iron. The colour of such slate is generally reddish, which is also called multicoloured.
•    Carbonaceous Slate : It is usually black and contains carbon finely dispersed reddish, which is also throughout the mass of the rock.

Chemically slate is composed of SiO2, TiO2, Fe2O3, Al2O3, CaO, MgO, Lol, K2O and Na2O. The percentage of these radicals differ in slate extracted from different locations.

Different variety of slates are suitable for different kind of environment. However slate of Methawas, Ghilot, Jakharana and Basai in Alwar district of Rajasthan has been found suitable in both, acidic and alkline environment.

Use of slate is also determined by its physical properties, such as its specific gravity, percentage of water absorption, modules of rupture and compressive strength.

Mining and Processing
Mining of slate is done by open cast method. The slate bands are exposed by removing the overburden by drilling and blasting process. The mining in most of the places is done manually, but in some mines, semi-mechanised method is adopted deploying jack-hammers for drilling, jib cranes for pulling, lifting and loading slate blocks into trucks and dumpers at the pit bottom or sites.

After removing the thick slab of slate it is split using hammer and specially made chisel and cutting knives. Thus saleable slab or tile is obtained in      6 mm to 10 mm thickness suitable for cladding, and in thickness of 20 mm to 35 mm for flooring or for panels. Sometime thickness varies from 40 mm to 60 mm with sizes varying up 100 cm x 50 cm in slate slabs quarried in Rajasthan. To provide smooth and regular edges, diamond wheel cutting machine is used manually to cut edges. The machine is also used where two diamond disc cutters traverse in same direction so as to cut two sides at a time, saving time and money. Other two sides are cut by turning the slab at right angle.

Since slate is fragile its recovery is quite low, not more than 25 per cent, during mining and processing. There is huge accumulation of broken pieces in and around the quarrying and processing centre.

Applications and Specifications
There are two main uses or applications of slate as a natural stone in building work: first is for roofing in the form of roofig tiles, and second for flooring and cladding in the form of tiles.

For roofing tiles, the slate should be exfoliated and it should be free from minerals like iron sulphides and carbonates, which in time could cause corrosion.

For flooring and cladding purposes, the slate should be able to bear the process of cutting in required sizes and polishing process by machines, and should not peel off during the process of laying and fixing.

Bureau of Indian Standards has adopted Standard IS: 6250: 1981 and reaffirmed in October 1998 for roofing slate tiles.

Market Potential
Indian slate is regarded as one of the best in the world. Apart from its large consumption in domestic market, Indian slate is exported to Italy, Germany, Spain, Belgium, UK, Canada, USA, Australia, France and China.

In the international market Indian slate has direct competition with Spanish Slate, the world leader in the sector, but its demand in European and other countries speaks of its high rating. A proper mining and processing technique with modern equipment can definitely improve its demand.

Availability
Depending upon application and demand, the commonly used varieties of slate tiles are:
•    Hammer cut in sizes of 63x73, 43x43, 33x33 and 33x23 cm
•    Hand-cut pieces shaped and sized by hand cutters are available in sizes of 40.5x28, 38x18, 30x30, 30.5x20.5, 60x30, 50x25, 40x20 and 40x30 cm in thickness varying from 7 mm to 12 mm.

Steps in slate are available in all common sizes in thickness of 25 mm to 35 mm.

Packing
Being fragile slate slabs and tiles require utmost care in packing. It is therefore packed in wooden crates or wooden stripes on all sides with thick wooden planks fixed at the top and bottom. The products are packed in standing position.

QUARTZITE
Quartzite usually is metamorphic rock composed of firmly cemented quartz grains. Quartzite is originally sandstone which got converted into quartzite through heating and pressure usually related to tectonic compression. Most often it is white, light grey, yellowish and light brown, though sometimes blue, purple and black shades are also available due to its mineral mix.

Quartzite is most easily distinguished from sandstone by the fact that it fractures across its constituent grains of sand, while sandstone fractures along the line of the cementing material between the grains of sand. Although most quartzites are metamorphic some are sedimentary in origin, resulting from cementation of quartz sandstone by groundwater solution containing pure quartz.

When sandstone is metamorphosed to quartzite, the individual quartz grains recrystalise along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of sandstone are erased by the metamorphism. Minor amounts of former cementing materials, iron oxide, carbonate and clay often migrate during recystallisation and metamorphism. This causes streaks and lenses to form within the quartzite.

The very pure form of quartzite is Orthoquartzite, composed of usually well rounded quartz grains cemented by silica. It is often 99 per cent SiO2 with only very minor amounts of iron oxide and traces of zircon, rutile and magnetite.

Quartzite is resistant to chemical weathering and often forms ridges and resistant hilltops.

Quartzite is a decorative stone and may be used for wall cladding, roofing tiles, flooring and stair steps. Crushed quartzite is sometimes used in road construction and for rail tacks. Its purest form is used to produce ferrosilicon, industrial silica sand, silicon metal and silicon carbide.

The quartzite varieties available in India are : Badnor Shine, Bhilwara Pink, Copper, Deoli, Golden, Heritage Pink, Himachal Black, Himachal Gold, Himachal Green, Himachal White, Jeera Green, Simla White, Ocean Black, Ocean Green, Silver Grey and Silver Shine.