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Pores
Pores
The cavities between soil particles that are occupied by air and water. Large pores (as found in sandy soils) may allow too much water to be drained away, leaving the soils dry and plants subject to drought. Small pores (as found in clay soils) may hold the water so well that the soil becomes waterlogged and air cannot get to the roots of the plants.
Loam soils are thought to have the best size of pores for allowing good drainage, yet retaining enough water for plants to use through a dry spell.
Porous
A rock or soil containing small cavities, or pores. Porous is not the same as permeable. To be permeable, the pores in a rock must connect to allow a liquid to pass through.
Porphyry, porphyritic rock
An igneous rock in which larger crystals (phenocrysts) are enclosed in a fine-grained matrix.
Prairie soil
Another word for chernozem.
Prismatic
A crystal that has formed with one axis very much longer than the others (see: Axis of symmetry).
Pyrite
Iron sulphide. Also known as ‘fool’s gold’ because of its superficial resemblance to gold.
It is common in sedimentary rocks that were poor in oxygen during their formation. It sometimes fills the spaces left
by ancient animals and can show the animal’s shape in beautiful detail. Iron pyrite also forms yellow cubic crystals.
Q
Quartz
A glassy mineral composed of silicon dioxide. Quartz is colourless when pure, but impurities can give it a variety of colours, especially brown. Quartz almost always forms irregular
 Pyrite – Pyrite
often forms cubic crystals.
grains, rather than crystals, with well-defined faces because it has a low melting point temperature and so is the last mineral to crystallise in an igneous rock. It fills all
the remaining gaps between the crystals that have already formed. Quartz is quite hard (7 on Mohs’ scale of hardness), and its compact structure makes it extremely resistant to weathering. As a result, it is found in igneous, metamorphic and sedimentary rocks. Quartz is the most common mineral in sandstone, where it makes up most of the sand grains.
R
Regional metamorphism
Changes in rocks resulting from both heat and pressure. It is
usually connected with mountain building and occurs over a large area. The main rocks formed by regional metamorphism are schists and gneisses. (See also: Contact metamorphism and Metamorphic rock.)
Rendzina
A thin soil that develops directly on limestone or chalk rock. The soil consists of a thin, dark topsoil in which there is a large percentage
 Quartz – Quartz crystals often have a glassy appearance, as here in the case of rose quartz.
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