BASIC MINERALS AND
HARDNESS SET
Hardness: Specimens 1 to 9 are listed in order of the
Mohs
hardness scale. Try an experiment. Scratch one mineral with another and
then do
it in reverse order. You will find that the lower numbers are softer
than the
higher numbers. Go ahead. Scratch some minerals. That is what they are
here
for. There are some common materials that you can also use for hardness
testing: A fingernail is hardness of 2½, a copper penny (before 1983)
is 3, a
steel knife is 5, glass is about 5½, and a steel file 6½.
Note: Not every mineral is present
in every box. We sometimes
run out and have to substitute other minerals. Check the name and not
the
number.
Safety
tips:
If you are scratching glass,
put the
glass plate on the table; don’t hold it in your hand.
Use thick pieces of glass (like a Ward’s
scratch plate or a pickle bottle), not thin things like glass slides.
•___ When
doing a hardness test with a nail, be careful not to stick yourself or
anyone
else.
•___ Don’t put
any mineral in your mouth.
•___ Always
wash your hands after handling minerals.
1.
TALC,
hardness of 1 - Hydrated Magnesium Silicate. This mineral is usually
found in
metamorphosed oceanic crust. It is so soft that your fingernail will
scratch
it. Ground talc is often used as “baby powder.”
2.
GYPSUM,
hardness of 2 - Hydrated Calcium Sulfate: A very common mineral that
precipitates as seawater evaporates. The most common use is for making
sheet
rock wallboard.
3.
CALCITE,
hardness of 3 - Calcium Carbonate: A very common mineral that typically
is
precipitated by organisms like clams which use it to make their
skeletons. When
large amounts of these biological products accumulate and get cemented
together, the rock is called limestone. Calcite may be dissolved and
re-precipitated
by hot ground water as large and beautiful crystals.
4.
FLUORITE,
hardness of 4 - Calcium Fluoride: Typically found in beautiful crystals
in
hydrothermal deposits (200 degrees C and 5000 PSI) along with metallic
ore
minerals. Ancient peoples discovered that fluorite (“flowing stone”)
would make
many ore minerals melt easily. This mineral shows 4 directions of
cleavage.
5.
APATITE,
hardness of 5 - Calcium Phosphate with some fluorine: Apatite comes
from both
organic and inorganic sources. Many living creatures (like sharks) make
their
skeletons and teeth out of hydrated calcium phosphate. Their
accumulated
remains are mined for fertilizer. Inorganic apatite is found in igneous
rocks.
6.
MICROCLINE,
hardness of 6 - Potassium Aluminum Silicate: Microcline is one of a
family of
minerals called feldspars. They are found in igneous rocks and weather
to clay
minerals. Note that there are two directions of cleavage at nearly
right angles
(micro-cline = little angle) to each other.
7.
QUARTZ, hardness 7 - Silicon dioxide:
Quartz is the most common mineral on the continents of the earth. Sand
is
primarily quartz that has eroded from somewhere else, like granites,
and has
been concentrated by rivers. When minerals cement the sand together,
the rock
is called sandstone. Quartz can be
recognized by its
hardness (it will scratch glass) and its lack of cleavage. Broken
quartz looks
just like broken glass, and is just as sharp. It grows in hexagonal
crystals
with striations on the sides. Many common forms of quartz (agate,
chalcedony,
flint, chert, jasper) are made of tiny crystals, the result of rapid
crystallization.
8.
TOPAZ,
hardness 8 - When nicely colored (blue, yellow, pink)
and transparent, topaz is a gemstone. Aluminum fluohydroxysilicate:
Topaz is found in pegmatite veins associated with granites. A pegmatite
is the
last material to crystallize as a large igneous body cools. Pegmatites
frequently contain rare and beautiful minerals like topaz. Our
specimens are
rounded, water-worn pebbles collected from a stream in Brazil, so they
do not
show crystal faces. Topaz has a single direction of weakness, called a
cleavage
plane, which shows up if a pebble is broken.
9.
CORUNDUM,
hardness 9 – Aluminum oxide: Corundum is found in silica-deficient,
high
temperature metamorphic rocks or in aluminum-rich igneous rocks. If
silica is
present, corundum will not form because aluminum silicates are much
more
stable. The diagnostic test for corundum is its hardness. You will find
it
harder than anything in this box—harder than any mineral but diamond.
Minor
impurities can color corundum red (ruby) or blue (sapphire).
10. RHODONITE
Manganese silicate: Rhodonite (hardness 5 ½ to 6) is usually found in
metamorphic rocks where the heat and pressure create an environment in
which
the manganese dissolves out of low concentration sources like shale. It
can
form large masses of beautiful pink stone.
You might
mistake this mineral for the other common pink mineral, rhodochrosite
(hardness
3 ½ to 4), except that rhodonite is much harder. Try a hardness test
and see. Rhodonite
is a manganese ore and is cut as a semiprecious gemstone.
11. MUSCOVITE
Potassium aluminum silicate: Muscovite is found in igneous and
metamorphic
rocks. The distinguishing characteristic of this mineral is thin,
flexible
sheets which can be split off. It has a hardness of 2 to 2½.
12. SIDERITE
Iron carbonate: Siderite can be found in sedimentary rocks where it
precipitates from seawater as a cement. It
also is
found in hydrothermal ore veins where it forms in masses with good
cleavage
like the sample you have. This mineral is harder than calcite (hardness
3) and
softer than fluorite (hardness 4). Try it.
13. CALCITE
crystals on matrix. Calcium carbonate: Look at your specimen for broken
tips on
the ends of the crystals. You will see 3 directions of cleavage. Test
the
hardness to see that this mineral is harder than gypsum and softer than
fluorite.
14. AZURITE,
blue from Morenci copper mine in Arizona. The green is malachite.
Copper
carbonate: Azurite and malachite are common copper minerals. A hardness
test is
difficult because the azurite is a fluffy
mass of
crystals, which crush when you try a hardness test. Both azurite and
malachite
are important copper ores.
15. CELESTINE
from Lampasas County, Texas.
It
occurs in cavities in limestone. Strontium sulfate: The red color in
fireworks
is from strontium. A distinguishing characteristic for this mineral is
that it
is heavy. You can do a specific gravity test to find that this mineral
is 4
times the density of water, but first just try a “heft test.” Take a
piece of
celestine in one hand and a similar sized piece of calcite or quartz
(density
of 2.9) in the other and jiggle them. Compare it with siderite.
16. QUARTZ,
variety amethyst - from Rio Grande do Sul, Brazil - from cavities in basalt. Silicon
dioxide: The
purple color is from iron. If you were to wrap this crystal in 10
layers of
aluminum foil and heat it for 2 hours at 400 degrees F, it would change
color
to yellow as the iron changes oxidation state.
17. GYPSUM,
“roses” from the Chihuahuan desert of
northern
Mexico. Calcium sulfate: Gypsum grows in many crystal shapes. This kind
grows
in sand that is saturated with “hard water,” and the calcium sulfate
precipitates as the water evaporates. The white deposits on your
showerhead are
mostly gypsum. You will see that the hardness is the same no matter
what shape
the crystals are in.
18. PYRITE,
known as "fool's gold," from the silver mines of Peru. Iron sulfide:
diagnostic properties are its brassy gold color, its brittleness, and
when
present, its cubic crystals. Once a minor iron ore, most pyrite is
mined today
for its trace content of precious metals such as gold and silver.
19. STILBITE
occurs in cavities in basalt - from Siberia, Russia, Nova Scotia,
Canada, and
India. Hydrous sodium calcium aluminosilicate.
These crystals grow rapidly. Artificial zeolites
are
used in ion-exchange applications such as water purifiers.
20. CHRYSOCOLLA
is a hydrous copper silicate - from an Arizona copper mine. The
blue-green
color and the botryoidal growth (lots of tiny, radiating crystals that
make a
bubbly-looking mass) are typical. Chrysocolla’s
nice color
and luster make it a semiprecious gemstone.
21. ARAGONITE
- twinned crystal from Clarendon, Texas. Calcium
carbonate,
like calcite, but orthorhombic instead of hexagonal. Unless you
have
crystals, it is almost impossible to tell calcite from aragonite. Some
organisms, like oysters, make aragonite hard parts.
22. GARNET
crystal from Malawi in Africa. Aluminosilicate
group
with extremely variable cation
substitution: Garnets
occur in many rock types. Though only slightly harder than quartz,
garnets are
pretty tough. Because they break with a conchoidal
fracture (like glass), making lots of little sharp pieces, crushed
garnets are
used to make sandpaper. When clear and a pretty color (red, orange,
green)
garnet is faceted as a gemstone.
23. WAVELLITE,
green sprays on gray Big Fork chert. - From Arkansas. Hydrated
aluminum phosphate: a secondary mineral (formed by breaking down
phosphates or
Al-rich metamorphic rocks). Its bright green, lustrous,
radiating,
fibrous crystals make it a sought-after specimen, but it has little
economic
importance.
24. VANADINITE,
tiny orange to red crystals from Arizona. Lead chloro-vanadinate:
a secondary mineral formed by oxidizing lead ores in deserts. (Wash
your hands
after handling this mineral even though it is not very soluble in
water.)
25. BERYL,
small six-sided (hexagonal) crystals from New Hampshire. Beryllium
aluminum
silicate: Beryl is really hard (H=8), right up there with topaz.
Transparent,
colorful members of the beryl family (green – emerald, blue –
aquamarine, pink
– morganite) are prized as gemstones. Beryl is the only important
source of the
element beryllium.
26. ZOISITE,
pink with epidote (green), quartz (white to gray), Baja California.
Hydrous
calcium alumino-silicate: forms in
pegmatites and
metamorphic rocks that form when high Ca rocks are subjected to
considerable
pressure. Crystals radiate from central points, indicating rapid
growth.
27. ADAMITE
on brown goethite, Mapimi, Durango,
Mexico. Zinc arsenate: a secondary mineral
formed by oxidizing zinc sulfides and other primary ore minerals. A minor ore of zinc. The yellow varieties are
fluorescent
and phosphorescent. (Wash your hands after handling this mineral, even
though
it is not very soluble in water.)
28. KYANITE
in QUARTZ from Brazil – Blue blades in a milky quartz matrix. Aluminum
silicate: formed from clay that is metamorphosed at high temperatures
and pressures.
Besides its blue color, the most diagnostic feature is that it is
harder when
scratched across the crystals than when scratched parallel to them. A
knife
will make a scratch along the crystals but not across them.
29. HEMIMORPHITE,
white to gray crystals in brown to black goethite, Mexico. Hydrous zinc
silicate: Naturally colorless, hemimorphite is often colored by minor
impurities. Even though it is a silicate, it is pretty soft, H=4.5 to
5.
30. BARITE
SAND ROSE or nodule, from Norman, Oklahoma. Barium sulfate: Barite is a
white
mineral. The red sandstone in which they are found colors these
“roses.” A
small amount of barium gets “stuck” to the clay minerals that make up a
marine
mud deposit. When diagenesis transforms
that mud into
shale, some of the barium is expelled into the ground water. That
ground water
will be forced through a permeable sandstone layer on its way to the
ocean. The
barium will encounter sulfate from seawater and be precipitated as
barite. Try
a “heft” test and note that barite (density of 4.3) has a much greater
specific
gravity than quartz (density of 2.9) or calcite.
Other minerals you may encounter:
31. SPHALERITE,
dark brown to black, bright cleavage and dull crystal faces, with
yellow
chalcopyrite. Zinc sulfide: Sphalerite is the primary ore of zinc. It
has good
cleavage and a substantial heft.
32. Calcite
–
cut pieces of calcite onyx, Mexico. Calcium carbonate: This is another
form of
calcite that is deposited by dripping water in a cave.
33. SMITHSONITE,
pinkish bubbly crusts, Mexico. Zinc carbonate: Minor amounts of cobalt
color
this specimen pink.