Properties of schist, sandstone, phyllite

Schist

Medium to coarse grained metamorphic rock of medium to high grade.

• Texture: crystalloblastic, lepidoblastic (flaky minerals visible to naked eye).
• Structure: well developed foliation called schistosity.
• Minerals: mica (muscovite, biotite), chlorite, talc, quartz, garnet, hornblende.
• Engineering properties: strength is low and highly anisotropic. It splits along schistosity, so it is weak for foundations, slopes (sliding along foliation) and tunnels (overbreak, squeezing). It is not good as a building stone or aggregate.

Sandstone

Clastic sedimentary rock made of sand-size grains (0.06 - 2 mm) cemented together.

• Texture: arenaceous, clastic, grains of quartz held by cement.
• Structure: bedded, with cross bedding and ripple marks; jointed.
• Minerals: quartz mainly, some feldspar and mica; cement of silica, calcite, iron oxide or clay.
• Engineering properties: compressive strength 20 - 170 MPa depending on cement (siliceous strongest, argillaceous weakest), specific gravity 2.2 - 2.7, porosity 5 - 25 %, water absorption 1 - 10 %. Used as building stone, paving and aggregate. Weak cement type weathers and softens in water.

Phyllite

Fine grained low-grade metamorphic rock between slate and schist.

• Texture: very fine, lepidoblastic, with silky sheen.
• Structure: foliated (phyllitic cleavage), often wavy or crinkled.
• Minerals: sericite mica, chlorite, quartz.
• Engineering properties: weak and easily split along foliation, weathers rapidly to clayey soil, shows creep and slope failure. Poor foundation and tunnelling rock; widespread in the Lesser Himalaya of Nepal, where it causes landslides.