Mineral Identification Lecture
Mineral Identification Lecture
last edited: 3/22/18 copyright
2018-R.Weller
transparent gypsum
(selenite)
How can minerals be identified?
Because minerals are
defined as specific chemicals with a crystalline
structure, they have both
distinctive chemical and physical properties.
Mineral identification video
chemical properties
Geologists rarely use
chemical properties of minerals for identification purposes,
unless it is a rare
mineral.
The one exception is the use of hydrochloric acid for
distinguishing between
calcite
and
dolomite.
These two minerals
fizz (produce gas bubbles) when cold
hydrochloric acid is applied to them.
Calcite fizzes profusely and quickly, while
dolomite fizzes slowly.
video of reaction
of calcite to HCl
(The rate of
reaction is also controlled by the type of surface; porous substances
have
greater surface area and thus produce more bubbles).
physical properties
Physical properties
deal with the manner in which the atoms are bound within
the crystal structure
or how the mineral reacts with light.
hardness
Hardness is one of the
most common properties used to identify minerals.
Hardness refers to the
ability of a mineral to resist being scratched or its ability
to scratch other
materials.
Mohs Scale of Hardness video
Mohs Scale of Hardness goes from 10 (hardest)
to 1 (softest)
10-
diamond
-diamond
hardness-Opal's Pals
9-
corundum
8-
topaz
7-
quartz
6- feldspar
5-
apatite
-apatite
hardness-Opal's Pals
4- fluorite
3- calcite
2- gypsum
1 -talc -talc hardness-Opal's Pals
Glass and steel are
about 5.5 on this scale and a fingernail is about 2.5.
toughness
Toughness is not the
same thing as hardness.
Toughness refers to the ability to take a beating
without breaking.
There are many materials, such as diamond, that are very
hard but are not tough;
they are brittle.
Many physical properties of
minerals are based on what happens when you try
to damage a mineral with force.
cleavage
fracture
flexibility
elasticity
malleability
friability
ductility
cleavage
Because the atoms in
some minerals are arranged in distinct, widely separated planes,
the mineral
will break along smooth, flat surfaces parallel to these planes of atoms.
The directions of these flat surfaces are called cleavage planes and the smooth
flat
surfaces created by cleaving are cleavage surfaces.
1 direction of cleavage
mica
mica and gypsum cleavage video
topaz
2 directions of cleavage
3 directions of cleavage
halite (salt)
halite (salt) cleavage video
calcite
calcite cleavage
video
galena
4 directions of cleavage
fluorite
diamond
diamond cleavage video
fracture
Fracture describes a
broken surface that is not cleavage.
fracture-Opal's
Pals
video on fracture
irregular fracture-
the most common type of fracture
conchoidal fracture
sulfur
conchoidal
fracture-Opal's Pals
fibrous fracture
splintery fracture
flexibility
Flexible means that the material can be bent.
As might be expected, most mineral cannot be bent without breaking.
The most flexible mineral is
mica.
If pressure is applied slowly both
selenite
and
stibnite
can
be bent.
elasticity
A mineral is elastic if it can be bent
without breaking and when released,
it will snap back to its original shape.
Again, as expected, most minerals are not elastic.
Mica is flexible and elastic.
Selenite
and
stibnite,
once bent, stay bent.
malleability
Metals are malleable; they can be pounded flat with a hammer
without breaking.
malleability of gold video
Naturally
occurring malleable metals
copper
friability
A rock or mineral that
crumbles easily is said to be friable.
-friability-Opal's Pals
appearance
There are
many physical properties in this category that deal with the interaction of
minerals with light.
color
transparency
luster
streak
fluorescence
color
White light is made up of a wide range of
frequencies of light.
Each frequency represents a different color.
The color of a mineral is determined by the collection of light frequencies that
are not
absorbed by the mineral.
The color of a mineral is determined by
the frequencies of light that are either reflected
off the surface of the
mineral or those frequencies which pass through the mineral without
being
absorbed.
Many minerals possess
distinct colors which aid in the identification of a mineral.
However,
impurities, such as the presence of iron oxides, can alter the color of a
mineral,
often drastically.
yellow minerals
sulfur video on sulfur crystals
red minerals
crocoite
realgar
vanadinite
blue minerals
green minerals
purple minerals
fluorite purple fluorite video
pink minerals
brown minerals
black minerals
graphite video on graphite pencils video on graphite
white minerals
colorless minerals
Some minerals occur in a wide range of colors, sometimes even in the same specimen.
multicolored
minerals
fluorite
tourmaline, polished cabochons
transparency
A mineral that allows
light to pass through, almost unimpeded, is called transparent.
Window glass is transparent. Most faceted, high quality gemstones are
transparent.
-transparent-Opal's Pals
transparent minerals
gypsum-variety selenite , really transparent gypsum
When some light can
come through thin pieces of a material and it has a cloudy
appearance the
mineral is translucent.
-translucent-Opal's Pals
translucent minerals
corundum-variety ruby
amazonite cleavage
When no light can
pass through even a thin piece of a mineral, the material is opaque.
Minerals with a metallic luster are usually opaque.
-opaque-Opal's
Pals
opaque minerals
luster
Luster describes the appearance of the
surface of a mineral.
The greatest difference between groups of minerals
is that some have a metallic luster
while most minerals have a
non-metallic luster.
video on luster
metallic luster
non-metallic lusters
adamantine (extremely shiny)- diamond
glassy (vitreous)- quartz
silky- asbestos
pearly-
talc
heulandite
resinous-
sphalerite,
amber
waxy- common opal
dull- brucite
earthy- kaolinite
streak
Streak is the term used for the color of the
powdered form of a mineral.
The powder is obtained by dragging the mineral
across a rough, white ceramic tile.
The three common iron oxides all produce different colored
streaks.
mineral streak video
hematite-reddish brown streak
limonite-brownish yellow streak
magnetite-black streak
Native gold
(real
gold) can be separated from
pyrite
(fools gold) by a streak test.
Native
gold leaves a yellow streak, while pyrite leaves a greenish black streak.
fluorescence (visit
Fluorescent
Minerals) (also,
fluorescence-Opal's Pals)
mineral fluorescence video
Ultraviolet light is just like regular light
except that it has a higher frequency and cannot be
seen by the human eye.
Certain minerals have the ability to absorb ultraviolet light and the
emit light
at a lower frequency within the visible spectrum; this property is called
fluorescence.
To observe this
phenomenon, you need a source of ultraviolet light; these are commonly called
black lights. The room needs to be darkened because the effect is often
not very strong.
Only a small number
of minerals are fluorescent. The most fluorescent of all minerals come
from Franklin, New Jersey.
fluorescent minerals (the following examples were observed under ultraviolet light)
crystal
form
(visit Crystals
and Crystal Models)
If minerals are
allowed to crystallize in an unrestricted environment, they will form geometric
shapes with flat surfaces; these forms are called crystals.
The flat
surfaces run parallel to planes of atoms located within the crystalline
material.
Since each mineral is
a unique chemical compound, each mineral prefers to grow in specific forms.
These crystal forms can be used to identify a mineral.
crystalline structure video
classic crystal shapes
quartz-six-sided prism capped by a pyramid
beryl-six-sided
prism with a flat top (basal pinacoid)
galena-cubic crystal
fluorite-cubic crystal
magnetite-octahedron
spinel-octahedron
calcite-scalenohedron
pyrite-pyritohedron
garnet-dodecahedron
garnet-trapezohedron
mineral habit
Some minerals are characterized by the way in which they commonly crystallize.
acicular
(needle-like):
natrolite,
cuprite-variety chalcotrichite
botryoidal (grape-like):
malachite,
azurite,
siderite,
mimetite,
marcasite
drusy (mass of small, sparkly crystals): azurite, sulfur
radiating: wavellite, brochantite
banded (layered): malachite, fluorite, quartz-variety agate
arborescent (tree-like): copper
dendritic (plant-like): pyrolusite/psilomelane
density
Density is the measure of the mass of an
object compared to its volume.
Density can be expressed as grams per cubic
centimeter or kilograms per cubic meter.
Density can be calculated by
first measuring the volume of an object by the amount of
water it displaces in a
container and then compared to its mass.
Many ore minerals are noticeably denser than common rock.
Specific gravity is
another way of describing the density of a mineral.
Instead of specifying
mass per volume, the density of the mineral is compared to the density
of water,
which is established as 1.0.
Normal rock materials, like quartz and
calcite, have specific gravities in the 2.6 to 2.7 range.
Minerals such as
cinnabar,
scheelite, and
galena
have densities much greater
than most common
minerals.
density-Opal's
Pals
video
on density
magnetism
There is only one mineral that is both commonly occurring and strongly magnetic, magnetite.
If you tie a string
to a small, very strong magnet an drag it along the ground, it will often
collect small, black particles of magnetite.
video on magnetism and magnetite
magnetic-Opal's
Pals
radioactivity
Most minerals are not even slightly
radioactive.
However, if there is uranium or thorium present within the
mineral, it will be radioactive.
Carnotite
is a common
greenish-yellow mineral.
radioactivity-alpha decay-Opal's Pals
Does it burn?
There is only one
mineral that readily burns, a transparent to translucent lemony-yellow mineral
known as
sulfur.
The fumes from burning sulfur smell like a burnt match.
sulfur dioxide-Opal's Pals
taste and smell
Tasting minerals it not recommended; you might be unknowingly licking an arsenic mineral.
Taste can be used to
distinguish
halite
(sodium chloride) from
sylvite
(potassium chloride);
sylvite
has a more bitter taste than halite (table salt).
halite and sylvite-Opal's Pals
Most minerals do not
smell. A fresh broken surface of
chrysocolla
may produce a strange,
sweet
smell.
Marcasite produces a sulfur smell as it decomposes.
mineral associations
Many minerals can be
identified by the company they keep with other minerals because
they form in the
same type of environment.
minerals commonly occurring together
orpiment and realgar
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