Geology Reference
In-Depth Information
Geo-Focus
Welcome to the Wonderful World of Micas
mica
probably comes from the Latin
term
micare
, “to shine,” a reference to
their shiny luster.
Even though there are more than
three dozen varieties of mica, only a
few are common in rocks. One of the
most common,
biotite
(black mica)
(Figure 3.13a), has no commercial uses,
although geologists use it in potassium-
argon dating (see Chapter 17).
Muscovite
(colorless, white, or pale
red or green) mica is also com-
mon (
What makes the paint on cars and
appliances so lustrous? Why are lip-
stick, eyeliner, and glitter so attractive?
Do you enjoy the amber glow seen
through the isinglass window of a
wood stove? Some of you may remem-
ber the line in the song “The Surrey
With the Fringe on Top” from the
musical
Oklahoma
: “isinglass curtains
that can roll right down in case there's
a change in the weather.” What is this
remarkable substance in paint, lipstick,
and isinglass? The answer—micas.
Mica is the name given to a group
of 37 sheet silicate minerals that
have similar physical properties, par-
ticularly the way they split or cleave.
Remember that mineral cleavage is
breakage along a plane or planes of
weakness determined by atomic struc-
ture. Some minerals have no cleavage
planes, others have two, three, four, or
six; but micas have only one. Never-
theless, their cleavage is perfect, mean-
ing that when cleaved they yield very
smooth planes. Indeed, the micas split
into thin, fl exible sheets. The name
Figure 1); it was named for
Moskva
(Moscow), where much of
Europe's mica was mined. Isinglass,
mentioned above, consists of thin,
transparent sheets of muscovite.
Muscovite and another mica called
phlogophite
(from Greek
phologopos
,
“fi ery”) have commercial value.
Micas used for products are called
scrap and fl ake micas, which either
occur naturally or are ground into
small pieces, and sheet mica, which is
cut into various shapes and sizes for
use in the electronics and electrical
industries. Scrap and fl ake micas
◗
◗
Figure 1
Muscovite mica is colorless,
white, pale red, or green. It is a sheet
silicate that has many industrial uses in
paint, wallboard compound, eyeliner, lipstick,
and nail polish.
are produced in many countries and
U.S. states, but about half the
U.S. production comes from North
Carolina. Micas are found in many
types of rocks, but our main concern
here is its uses.
When mica is ground up dry, it
loses much of its luster, but retains its
platy nature and is ideal for wallboard
A simple chemical test to identify the minerals calcite and
dolomite involves applying a drop of dilute hydrochloric acid
to the mineral specimen. If the mineral is calcite, it will react
vigorously with the acid and release carbon dioxide, which
causes the acid to bubble or effervesce. Dolomite, in contrast,
will not react with hydrochloric acid unless it is powdered.
(plagioclase feldspars and potassium feldspars) and quartz
make up more than 60% of Earth's crust. So, even though
there are hundreds of silicates, only a few are particularly
common in rocks.
The most common nonsilicate rock-forming minerals are
the carbonates calcite (CaCO
3
) and dolomite [CaMg(CO
3
)
2
],
the main constituents of the sedimentary rocks limestone and
dolostone, respectively (see Chapter 7). Among the sulfates
and halides, gypsum (CaSO
4
•
2H
2
O) in rock gypsum and ha-
lite (NaCl) in rock salt (see Chapter 7) are common enough to
qualify as rock-forming minerals. Even though these minerals
and their corresponding rocks might be common in some ar-
eas, however, their overall abundance is limited compared to
the silicate and carbonate rock-forming minerals.
Geologists use the term
rock
for a solid aggregate of one
or more minerals, but the term also refers to masses of
mineral-like matter, as in the natural glass obsidian (see
Chapter 4), and masses of solid organic matter, as in coal
(see Chapter 6). And even though some rocks may contain
many minerals, only a few, designated
rock-forming min-
erals
, are suffi ciently common for rock identifi cation and
classifi cation (Table 3.3 and
Thus far, we have discussed the composition, structure, and
physical properties of minerals, but have not addressed how
they originate. One phenomenon that accounts for the ori-
gin of minerals is the cooling of molten rock material known
as
magma
(magma that reaches the surface is called
lava
).
Figure 3.19). Others, known
as
accessory minerals
, are present in such small quantities
that they can be disregarded.
Given that silicate minerals are by far the most com-
mon ones in Earth's crust, it follows that most rocks are
composed of these minerals. Indeed, feldspar minerals
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