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Crumb structure
Crystal system
A group of crystals with the same arrangement of axes (see: Axis of symmetry). There are six crystal systems, as shown below.
(See also: Cubic; Hexagonal; Orthorhombic; Triclinic.)
Crystal system – Cubic.
Cubic crystals of iron pyrites
Crystal system – Tetragonal.
Tetragonal crystals of zircon
Crystal system – Hexagonal.
Hexagonal crystals of beryl
Crumb structure – The rounded lumps in this soil show it has a crumb-structured topsoil.
Crumb structure
A soil in which the clay particles clump together to form irregularly shaped balls. The balls of soil hold together well and resist erosion, while allowing good drainage.
It is a sign of a stable and fertile topsoil. (See also: Friable and Soil structure.)
Crust
The outermost layer of the Earth, typically 5km thick under the oceans and 50 to 100km thick under continents. It makes up less than 1% of the Earth’s volume. (See also: Tectonic plate.)
Crystal
A mineral that has a regular geometric shape and is bounded by smooth, flat faces. Crystals form in six crystal systems.
(See also: Cleavage; Fracture; Gem, gemstones; Geode; Phenocryst.)
Crystal system – Orthorhombic.
Orthorhombic crystals of barite
Crystal system – Monoclinic.
Monoclinic crystals of gypsum
Crystal system – Triclinic.
Triclinic crystals of kyanite
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