Page 26 - Curriculum Visions Dynamic Book

Sheet minerals
In many cases, silica molecules form extensive sheets. Stacks of these sheets are connected together by metal ions; potassium and magnesium are among the most common. The way the sheets are connected together
is very important, because it gives the minerals many of their properties.
Sheet silicates all break up into thin flakes. This can be important. Several of the sheet silicates are used as lubricants; for example, talcum powder is made from the
mineral talc.
Water molecules can
be absorbed between
the sheets of many clay minerals. This is why pottery clays swell when they are wet and shrink when they are fired, and also why soils shrink and swell.
Kaolinite
There are many different clay minerals, but all are composed of sheets containing aluminium silicates. Kaolinite, a soft mineral that
can absorb water, is the most common clay mineral in the world’s soils. It is mined from concentrated deposits as china clay, and is important as the raw material for pottery and ceramics.
Kaolinite crystals are
too small to be seen with an ordinary microscope.
 Hexagonal crystals of vermiculite.
Mica
Mica is the third most common group of minerals in the earth’s crust, after quartz and the feldspars. It easily recognised
as a silicate because it breaks up into thin, almost transparent sheets. The main elements, together with silicon and oxygen, are aluminium and potassium.
Varieties of mica are black biotite, and brown muscovite.
 Biotite is the black form of mica.
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