Geology Reference
In-Depth Information
Evaporites
Another group of minerals, called
evaporites,
are classified according to how they form
rather than by their composition. Evaporite minerals form when water they are dis-
solved into evaporates, leaving the mineral crystals behind. Common evaporite minerals
include gypsum and anhydrite, which are listed in Table 6-3 under their compositional
category: sulfates.
Other common evaporite minerals include halite (sodium chloride, or table salt) and
sylvite (potassium chloride).
Gemstones
Some minerals are considered especially valuable and beautiful and are called
gem-
stones.
Some of these minerals are valuable because they are rare, such as diamonds or
emeralds. But others are very common, such as amethyst or opal. Minerals that are con-
sidered gemstones form under conditions that allow the individual crystals to grow
fairly large. These conditions occur when molten rock cools very slowly underground,
or when rocks are metamorphosed by heat and pressure (see Chapter 7 on metamorph-
ism).
Many of the gemstones you may be familiar with are quartz minerals that have small
amounts of other elements, or
impurities,
in their crystals. These impurities change the
color of the crystal. For example, amethyst is a quartz crystal with lavender or purple
color due to small amounts of manganese in it. Citrine is also a quartz crystal, with small
amounts of iron giving it a yellow to orange color.
Diamonds
In the 1983 movie
SupermanIII
(what, you haven't seen it?), the hero crushes a lump of coal in his fist
until it becomes a diamond. How likely is this scenario? Both coal and diamonds are made of the ele-
ment carbon, but diamonds are not naturally created from coal. The carbon in coal is the remains of
decayed plant material from millions of years ago. Diamonds result from intense pressure deep in the
earth, creating large, pure carbon crystals.
Diamonds form under conditions of high pressure found in the earth's mantle, about 100 miles below the
crust. They are brought to the surface by explosive eruptions of molten materials up through the crust.
These explosions of molten material cool and form a structure called a
kimberlitepipe
(first discovered
in Kimberley, South Africa). Most of these kimberlite pipes seem to have formed during the breaking
