ESO 1 Science 5

The Earth's Crust, the Rocks and the Minerals

Key Information

Rock Grains

Sedimentary rocks can be made up of clasts or detritus (rock grains) weathered from other rocks by river waters, wind, coastal sea waters, glaciers, living beings, etc. This kind of sedimentary rocks are called detrital rocks.

They're classified upon the grains that they are made up of. First up, grains may be all the same size, uniform, as in a sandstone, or different sizes, as in a conglomerate. Secondly, grains may be bigger or smaller. The following table shows roughly how rock grains are classified upon their size:

Clay< 0.004 mm
Silt< 0.06 mm
Sand< 2 mm
Gravel< 6 cm
Cobble< 25 cm
Boulders> 25 cm
Mineral Lustre

The term lustre refers to the appearance of a mineral surface in reflected light. The main types of mineral lustre are the following:

VitreousTranslucentQuartz, Halite, Olivine
ResinousTranslucentYellow and red sphalerite varieties
GreasyVariableMilky quartz
SilkyVariableFibrous gypsum
PearlyPoorly translucentOrthoclase, Mica aggregates
Dull metallicOpaqueCinnabar aggregates
MetallicOpaquePyrite, Chalcopyrite, Magnetite

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