Slate

Slates are dense and fine grained rock. The colour of slates are generally gray, red, brown and green ranging from dark to light. It is produced by the compression of clays, shale, and various other rocks developing a characteristic cleavage consisting sericite and quartz with biotite, chlorite, and hematite as principal accessories.

In other words slates are generally formed by low-grade regional metamorphism of pelithic sediments or fine-grained tuffs. It can be associated with other metamorphic sedimentary or volcanic rocks.

Slates are or argillaceous origin of metamorphic rocks and can also be defined as a layered stone. Fossils may be sometimes preserved in slate but are invariably distorted. Folds are often apparent in the field. Too fine-grained are tough to distinguished with the naked eye.

Physical Properties of Slate

Physically these are Metamorphic-Argillaceous rocks, which has developed a well-marked splittable cleavages with or without developed recrystallization & spots/knots of minerals like garnet, pyrite, andulasite, etc. They are normally fine to medium grained, moderately hard & compact, less homogeneous with varying textures & shades. It has an ability to split into thin layers along the plane of cleavages due to flattened & elongated minerals along the plane of cleavage.

Hardness	3.0 to 4.0 on Moh's scale
Density	2.6 to 2.8 Kg/cm3
Modulus of Rupture	22 to 34 N/mm2
Compressive Strength	24 to 36 N/mm2
Specific Gravity	2.65 to 2.80
Porosity	Low to very low
Water Absorption	1.5 to 2%
Weather Impact	Resistant
Corrodibility	0.4 to 0.7%

Chemical Properties of Slate

Chemically they are Multi-Mineral, metamorphic argilaceous rocks made of an aggregate of minerals and colloidal substances. Essentially mineral composition includes quartz, mica, chlorite, sericite & oxides of iron with occasional spots/knots of minerals like garnet, pyrite, andulasite, etc.

SiO2	55-65%
Iron (Fe2O3)	4%-8%
Alumina (Al2O3)	15 to 20%
Soda (Na2O) & Potash (K2O)	2-4%
Lime (CaO)	0.5-1%
Magnesia (MgO)	0.5-3%
TiO2	0.4-0.5%
Loss On Ignition (LOI)	3-4%

It is fairly resistant to acids and staining.

Uses of Slate

• Paving
• Stepping stone in garden
• Combined with other materials gives quality and attractiveness
• Square blocks for a break band in section of granite paving
• Decorative clocks and thermameters
• Picture frame, mirrors, coasters, trivets, switch plates, candle holders.
• Engraved and painted in jewellery like earrings and pins.

Ocurrances of Slate

Wales	Corris, Gwynedd Llangollen, Clwyd Neath, Mid Glamorgan Blaenau Ffestiniog, Gwynedd Clynderfen, Dyfed
India	Khanyara slate quarries are situated 11 to 15 Km. North East of Dharamshala, Distt. Kangra (Himachal Pradesh) Chamba district Himachal Pradesh