Properties of marble, slate, amphibolite

Marble

• Origin: metamorphism (contact or regional) of limestone or dolomite.
• Physical properties: crystalline, granular ("sugary") texture; non-foliated; white, pink, grey or green with veins; hardness 3-4; specific gravity 2.6-2.8; effervesces with dilute HCl; takes a fine polish.
• Engineering properties: compressive strength 60-140 MPa; low porosity (under 1%); soluble in acidic water, reducing durability in polluted air; massive.
• Uses: flooring, wall cladding, steps, statues, monuments, decorative and ornamental stone, lime.

Slate

• Origin: low-grade regional metamorphism of shale or clay.
• Physical properties: very fine-grained, dense, dark grey, black, green or red; slaty cleavage, splits into thin, flat slabs; dull to slightly silky lustre; hardness 3-4; specific gravity 2.7-2.8.
• Engineering properties: compressive strength 50-100 MPa across the cleavage plane, but strongly anisotropic and weak along it; low water absorption; durable; fairly impermeable.
• Uses: roofing slates, flooring, damp-proof courses, paving, blackboards; it is poor for aggregate and weak for slopes and tunnels.

Amphibolite

• Origin: regional metamorphism of basic igneous rocks (basalt, gabbro) or impure carbonate rocks.
• Physical properties: medium- to coarse-grained, dark green to black; mainly hornblende and plagioclase; weakly foliated or massive; hardness 5-6; specific gravity 2.9-3.2.
• Engineering properties: hard, dense and tough; compressive strength 100-250 MPa; low porosity; good durability and resistance to abrasion, but weathers along foliation and joints.
• Uses: good foundation rock, aggregate, road metal, railway ballast, building and crushed stone.