Precipitate rock is sedimentary rock formed not by the settling of transported grains, but by dissolved substances coming out of saturated water. It is the counterpart of clastic sedimentary rock, in which sand, clay and gravel particles are carried by water or wind and deposited.
Two routes of formation
- Physico-chemical — water saturated with calcium carbonate, gypsum or salt loses its capacity to hold the substance in solution through evaporation, pressure change or a rise in temperature, and the mineral precipitates. Travertine and tufa form this way at springs, and rock salt and gypsum in evaporating lagoons
- Biogenic — the precipitation is mediated by organisms. Shells, coral, calcareous algae and the skeletons of single-celled creatures build up calcium carbonate; much limestone and all chalk formed in this manner
Familiar building examples
- Travertine — a porous, banded limestone from hot springs, recognisable by its characteristic voids. Widely used as flooring and cladding, and a classic of Roman architecture
- Tufa — travertine’s lighter, spongier relative
- Limestone and chalk — biogenic in origin, and the raw material for lime and cement
- Gypsum and anhydrite — the feedstock for plasterboard, gypsum blocks and plaster
What it means in practice
Precipitate rocks are nearly always calcareous or sulphate-bearing, with direct consequences:
- Acid sensitive. Acidic cleaners, descalers and acid rain attack the surface
- Porous. Travertine for flooring is usually filled with a paste; outdoors, water in the pores can freeze and cause damage
- Soft. Scratch resistance is well below that of granite; choose the location deliberately
- Soluble. Gypsum-bearing rock is not frost resistant and is unsuitable outdoors
Points to watch
- Ask specifically about frost resistance if the material is going outside
- Impregnation limits staining and dirt pick-up but does not make the stone acid-proof
- In conservation the rule is to make good precipitate rock with comparable material, never with something harder
Related terms
- Sedimentary rock
- Limestone
- Lime
- Marble
- Natural stone finishes
- Hydrophobizing
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