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Obsidian
O
Obsidian
Glassy rocks often associated with the rapid cooling of very sticky lava. Obsidian has a low water content and a high silica content. It normally forms on the surface of a rhyolite type of magma.
This kind of magma, although also very acidic and with a high silica content, contains about 10% water, which keeps it mobile. When the magma reaches the surface,
the water boils off. As a result,
the magma increases rapidly in stickiness and, at the same time, cools. These are the conditions that produce volcanic glass. Obsidian is, therefore, the material that normally forms on top of rhyolite lava flows. You cannot see through the glass because it contains too many emerging crystals. As a result, there is no clear path for light to pass through.
Olivine
The name of a group of magnesium iron silicate minerals that have
an olive colour. They mostly form small crystals in dark-coloured igneous and metamorphic rocks. (See also: Garnet.)
 Obsidian – One of the few forms of natural glass.
Ore
A rock containing enough useful metal or fuel to be worth mining. The amount of waste rock (gangue) that can be tolerated depends on the price that can be obtained for the metal or fuel.
In the case of gold, uranium and molybdenum, for example, just a few grams per tonne of rock can make the ore worth mining. In the case of iron ore, the iron content normally has to be above one-third to make it worth mining. (See also: Lode; Ore mineral; Pegmatite.)
Ore mineral
A mineral that occurs in sufficient quantity to be mined for its metal. The compound must also be easy to process. (See also: Sulphides.)
Organic matter
Any material in a soil that comes from a living thing. Organic matter is often found on the surface as leaf litter or moder, mor and mull. Inside the soil it is found
as humus. (See also: Brown soil, brown earth.)
Orthoclase
The form of feldspar that is often pink. It contains more potassium than plagioclase feldspar.
Orthorhombic
A crystal system in which crystals have three axes all at right angles to one another but of unequal length (see: Axis of symmetry).
Outcrop
An exposure of a rock at the surface of the Earth. Outcrops are most noticeable at the coast, when they make cliffs, or in mountains where no soil forms.
 Outcrop – This is a place where a hard band of sandstone occurs, or crops out, at the surface. A soft band of shale
outcrops below it, but shale, being a soft rock, does not produce such dramatic outcrops.
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