CHALK

Chalk is a sedimentary rock of biochemical origin. It is soft, white and porous. It formed in the deep ocean far from land via the gradual accumulation of the calcite shells of coccolithophores. Layers of chalk are known that are hundreds of feet thick, indicating extremely long periods of accumulation.

Note that many products called chalk are not calcite. Gypsum is a common replacement (for example the "chalks" used on blackboards), as is talc. True chalk, however, is always composed of minute crystals of calcite.

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IGNEOUS: ANDESITE; ANORTHOSITE; BASALT; CARBONATITE; DACITE; DIORITE; DUNITE; GABBRO; GRANITE; KIMBERLITE; KOMATIITE; LAMPROPHYRES; MONZONITE; OBSIDIAN; PEGMATITE; PERIDOTITE; PUMICE; PYROXENITE; RHYOLITE; SCORIA; SYENITE
METAMORPHIC: GNEISS; MARBLE; QUARTZITE; PHYLLITE; SCHIST; SERPENTINITE; SLATE; SOAPSTONE
SEDIMENTARY: ANHYDRITE; BANDED IRON FORMATION; BRECCIA; CHALK; CHERT; CLAYSTONE; COAL; CONGLOMERATE; COQUINA; DOLOMITE; GEODES; GYPSUM; HALITE; LIMESTONE; MUDSTONE; PHOSPHORITE; SANDSTONE; SHALE; SILTSTONE; TILLITE
UNCONSOLIDATED SEDIMENTS: ALLUVIAL DEPOSITS; LAHARS; MORAINES; PEAT; SANDS; SOILS; TEPHRA; TILLS; ORES; MINING TALUS PILES
PRIMORDIAL: COMETS; ASTEROIDS; METEORITES; CHONDRITES; CHONDRULES; CAI's; PRESOLAR GRAINS; METEORIC MINERALS

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