(Opening music)
Welcome back mineralogy fans!
Almost everywhere on Earth, the rocks below
us
are made of silicate minerals. Maybe you
have some carbonates or sulfates below you,
but go down far enough, and the
basement rock is mostly silicate minerals.
and of that majority of the Earth's crust,
almost half are the mineral group we call
feldspar, a word with German roots
meaning: a field rock, with no ore.
With feldspars
accounting for half of the Earth's crust,
another quarter of the Earth's crust is
made up of the mineral quartz.
Though I like the idea that I
once came up with that the name
"quartz" comes from the sound you get when
you hit flints or
chert pebbles together, it looks like
it comes from the
Polish "twardy", meaning hard, which
transformed into the Polish
"kwawrdy", which gave the German "quarz",
which we get "quartz".
We begin with the feldspars, which all
show an appearances of non-metallic
vitreous luster. Feldspars with a
hardness of 6
will scratch a 5.5 hardness glass, but just barely.
Some force needs to be
applied to produce a noticeable scratch.
The specific gravity is about 2.55 to 2.76,
which is just
too common to be of much use. But two
good
directions of cleavage at 90-degree angles,
which causes this mineral to
form rod to blocky shaped crystals
with irregular ends,
is much more diagnostic. Striations,
these roughly parallel lines seen in the
mineral feldspar,
is another diagnostic property
of the felspar minerals, and
are actually due to what we call twinning, which
is when crystal lattices mirror each other
in two separate crystal forms.
A crystal of feldspar is really sheets of
individual crystals at opposing angles
to each other.
It gets cumbersome to say calcium-rich
plagioclase feldspar vs.
orthoclase feldspar, so we tend to
abbreviate the feldspars.
A Ca-rich vs. sodium-rich, Na-rich,
plagioclase is sometimes called Ca-plag
or Na-plag respectively,
and orthoclase
feldspar is also called
potassium feldspar, and since
the symbol for potassium is K, you'll often
hear geologists refer to this mineral as
K-spar.
So we will focus on the two end points
of the continuous plagioclase series,
and the discontinuous K-spar. The main
difference in
physical properties of these varieties
is their color,
though we must be careful here as the most
precise identification uses microscope
thin sections and chemical analysis,
but plagioclase in general is
cloudy to translucent white to grey, and
K-spar is
white to brick red with the most common
color being salmon pink.
Color ranges enough in the most common
varieties, but there are even odder
varieties, such as Labradorite,
whose twining allows for selective
reflection of certain wavelengths of light,
or amazonite, who's blue-green color may be
due to incorporation
water and lead in the crystal. Those can
show you why we must be careful with
color.
The plagioclase minerals make up around
39% of
the mineral mass of the Earth's crust.
K-spar makes up another 12%,
so the combined feldspars
make up 51%,
a little over half of the mineral mass of
the Earth's crust.
Feldspars are economically useful as a
mild abrasive, or in ceramics as a glaze.
A major use is as a fluxing agent for glass
manufacturing, in which sodium and potassium-oxides from
feldspar help to lower the melting point of
glass forming agents.
And the durability and chemical
stability of feldspars make them useful as
filler in various industrial products
like plastics, rubber, or paints.
The next most common mineral,
which often gets confused with feldspars,
is quartz, being just as abundant as,
and often found with K-spar, as we see
here.
Quartz makes up another 12% of
mineral mass bringing us a coverage of 63%,
almost two-thirds of the mineral
mass of the Earth's crust.
Like feldspar, quartz is non-metallic
vitreous luster, though it tends to be
more transparent than
feldspar, though not always. The hardness
of quartz is 7,
so it can barely scratch feldspar, and
makes a relatively deeper scratch in glass.
The most diagnostic property of quartz
vs. feldspar
is that it doesn't have cleavage. It does
have
a hexagonal pyramidal crystal habit, but
irregular to conchoidal fracture with a
total lack of cleavage.
Any flat face seen in quartz is a
crystal habit face,
not a cleavage plane. Remember,
feldspars are more blocky with two good
cleavage planes at 90 degrees.
Though quartz is most common as a
grey to white variety,
color is not very diagnostic in quartz.
It can have a little carbon (radiation damage) to make it
smoky quartz, or some
iron or magnesium to get rose quartz, or
some titanium to get amethyst quartz,
and an agate is simply all micro-crystalline
quartz crystals,
with varying impurities to give all the
colors of the rainbows that we can observe.
Though large crystals of quartz can be
spectacular,
we often encounter things like micro-crystalline, or crypto-crystalline quartz,
which make up rocks such as agate, flint,
jasper and chert. Because silicon is
chemically similar to carbon
we see wood being petrified by micro-crystalline quartz replacement of silicon
for carbon.
Tiger's eye is an asbestos mineral being
replaced by micro-crystalline quartz
and hematite, in what is known as a
pseudomorph, when one mineral takes on the
crystal habit of another through
replacement.
Quartz is economically useful for its
piezoelectric property of separating
charges when stressed, and so can be used
to gauge pressure.
Or you can pass a current through a
quartz crystal and get vibrational
differences which can be used for
frequency control in things like clocks and
watches. Historically, the Stone Age
is really the Quartz Age, as most stone
tools from this time are made of quartz.
The irregular fracture can be
manipulated into
very sharp edges, whereas feldspar
rarely has any angle sharper than 90
degrees.
In this modern world, quartz is the main
component to make glass and silicon chips,
and is also a common gemstone in
some of its unusual forms.
Sand mined for fracking operations
is made up of countless grains of quartz.
And so,
right in this episode, we have
encountered about two-thirds
of all crustal minerals discussing
quartz and the feldspars;
all within the silicate group that we are
not even close to being done with yet.
When we come back next time, we will stay
in the silicates and examine the
minerals from the mafic end of the
discontinuous series:
olivine, pyroxene, and amphibole.
