Rocks

Chemical Classification of Rocks

Chemical Classification of Rocks

The Chemical Classification of Rocks are,

(a) Siliceous rocks : These rocks have silica (and, quartz and flint) as their principal constituent and are very hard and durable, unaffected by weathering. Chief types of siliceous rocks are Granites, Traps, Quartzite and Sandstones.

(b) Calcareous rocks : Calcium carbonate or lime is the main constituent of these rocks. Crystalline and compact types are hard and durable. Clay is very often found mixed in such rocks. Marbles and limestone are calcareous rocks.

(c) Argillaceous rocks : Rocks of the clayey types which are more or less composed of alumina mixed with small quantities of other minerals. Slates and laterites belong to this group.

Chemical Classification of Rocks – Also see geological classification of rocks

Test for Stones

Test for Stones

Some of the simple test for stones are listed below,

(a) Crushing test for stones: The crushing strength of a stone greatly depends upon its texture and specific gravity. A stone of even texture arid of specific gravity greater than 2.7 can take heavy loads. Safe compressive loads on stones should be taken not more than one-tenth of the crushing loads determined by cube test. Stones generally begin to crack or split under about half of their crushing loads.

(b) Porosity or absorption test : Porous stones such as coarse grained sandstones should not be used. A good building stone should not absorb more than 5 per cent of its weight of water after 24 hours immersion. Any stone absorbing more than 10 per cent or having specific gravity less than 25 should be rejected.

(c) Structure test : Small pieces of the stone are kept for about an hour in a glass of water and then shaken vigorously. 1f the water gets dirty it shows the stone particles are not properly cemented together.

(d) Acid test : A small sample is immersed into 1 per cent solution of hydrochloric acid and kept for about seven days. During this time the solution is frequently agitated. If the sample has still maintained its edges and corners as sharp as before, the stone will weather well. If a drop of weak sulphuric or hydrochloric acid on a piece of stone causes effervescence, the stone contains chalk and is poor in weathering qualities unless it is marble.

(e) Hardness test for stones: may be tested by scratching with a penknife, which should not make an impression on a hard stone.

(f) Toughness : may be tested by breaking the stone under a hammer. A hard and tough stone is required for road mental.

These are the simple tests.

Geological Classification of Rocks

Geological Classification of Rocks

It is estimated that three-fourth of the land area of the globe is underlain by sedimentary rocks and the other fourth by igneous and metamorphic rocks. Main three geological classification of rocks are, :

Igneous rocks:

Are of volcanic origin, formed as a result of consolidation of molten materials either in the interior of the earth’s crust or upon its surface. They represent a crystalline glassy or fused texture. Generally, igneous rocks are hard, tough, dense, impervious, strong and durable.

Granite, Dolerite, Basalt, Trap, are examples. Form excellent concrete aggregate.

Acid rocks:

Igneous rocks containing over 65 per cent of silica. Compared with basic rocks, acid rocks are of lighter colour and in coarsely crystalline varieties ; free silica or quartz can be seen without the use of a lens. Granite is an acid rock. Basic rocks: Igneous rocks containing less than 52 per cent of silica, Compared with acid rocks they are of darker colour, and only rarely show free silica or quartz. Basalt and dolerite are basic rocks,

Sedimentary or Aqueous rocks .:

Are formed by the sediments deposited chiefly by water and to some extent by wind and ice, (sand, gravel, clay, cemented together by silica, lime, etc.) They represent a bedded or stratified structure in general, the individual beds lying one above another, often being distinguishable by differences in colour, texture or composition, May be close grained, compact or open textured. Sandstones, Limestones and Shale are examples, Gravel, sand, silt, clay and peat are considered as uncemented and unconsolidated sedimentary rocks.

Metamorphic rocks:

Are either igneous or sedimentary in their origin but subsequently changed due to movements of the crust as a result of metamorphic action of heat and pressure. Metamorphic rocks have a foliated structure in general and also show layers of stratification which are not always uniform. These rocks are hard and durable, Slates, Schists Gneisses, quartzite, some hard Shales and Marbles, etc., are formed in this way.

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Geological Classification of Rocks