Texture and properties of quartzite, limestone, granite

Quartzite

Metamorphic rock formed from sandstone by recrystallisation.

• Texture: granoblastic (interlocking quartz grains), fine to medium grained; original grains fused with silica.
• Structure: massive, non-foliated; sometimes relict bedding.
• Minerals: more than 90 % quartz, with traces of mica, feldspar and iron oxides.
• Engineering properties: compressive strength 150 - 300 MPa, specific gravity 2.65, very hard (Mohs 7), very low porosity, highly resistant to weathering and chemicals. Used as road metal, railway ballast, concrete aggregate and foundation rock. Very hard to dress, and it may show jointing and slip planes.

Limestone

Sedimentary rock formed by chemical or organic deposition of calcium carbonate.

• Texture: crystalline, oolitic, fossiliferous or clastic (fine to coarse grains).
• Structure: bedded, often jointed; may contain fossils and cavities.
• Minerals: calcite (CaCO3), some dolomite, clay and silica as impurities.
• Engineering properties: compressive strength 50 - 150 MPa, specific gravity 2.4 - 2.7, hardness 3, soluble in acidic water (sinkholes, caves, karst), effervesces with dilute HCl. Used for cement, lime, building stone and aggregate. Dam sites on limestone need grouting for solution cavities.

Granite

Plutonic igneous rock.

• Texture: holocrystalline, phaneritic, medium to coarse granular (sometimes porphyritic).
• Structure: massive, often with sheeting and joints; no bedding.
• Minerals: quartz (20 - 40 %), orthoclase/microcline feldspar, plagioclase, biotite/muscovite or hornblende.
• Engineering properties: compressive strength 100 - 250 MPa, specific gravity 2.6 - 2.8, hardness 6 - 7, water absorption below 0.5 %, excellent durability and polish. Used for foundations, piers, dams, facing, aggregate. Weathering can produce deep clay-rich residual soil, which must be removed from foundations.