Roger Weller, geology instructor ( firstname.lastname@example.org )
last edited: 9/7/11 copyright
How can minerals be identified?
Because minerals are defined as specific chemicals with a crystalline
structure, they have both distinctive chemical and physical 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.
(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 deal with the manner in which the atoms are bound within
the crystal structure or how the mineral reacts with light.
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 goes from 10 (hardest) to 1 (softest)
10- diamond -diamond hardness-Opal's Pals
5- apatite -apatite hardness-Opal's Pals
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 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.
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
2 directions of cleavage
3 directions of cleavage
4 directions of cleavage
Fracture describes a broken surface that is not cleavage.
irregular fracture- the most common type of fracture
conchoidal fracture-Opal's Pals
Flexible means that the material can be bent.
As might be expected, most mineral cannot be bent without breaking.
The most flexible mineral is
If pressure is applied slowly both selenite and stibnite can be bent.
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.
once bent, stay bent.
Metals are malleable; they can be pounded flat with a hammer without breaking.
Naturally occurring malleable metals
A rock or mineral that crumbles easily is said to be friable.
many physical properties in this category that deal with the interaction of
minerals with light.
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
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,
quartz-variety rose quartz
quartz-variety smoky quartz
quartz-variety rock crystal
Some minerals occur in a wide range of colors, sometimes even in the same specimen.
tourmaline, polished cabochons
A mineral that allows
light to pass through, almost unimpeded, is called transparent.
Window glass is transparent. Most faceted, high quality gemstones are transparent.
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.
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.
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.
adamantine (extremely shiny)- diamond
glassy (vitreous)- quartz
pearly- talc heulandite
resinous- sphalerite, amber
waxy- common opal
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.
hematite-reddish brown streak
limonite-brownish yellow streak
gold) can be separated from
(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)
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)
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.
classic crystal shapes
quartz-six-sided prism capped by a pyramid
beryl-six-sided prism with a flat top (basal pinacoid)
Some minerals are characterized by the way in which they commonly crystallize.
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 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
have densities much greater
than most common minerals.
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.
Most minerals are not even slightly
However, if there is uranium or thorium present within the mineral, it will be radioactive.
is a common
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
(sodium chloride) from
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
may produce a strange,
Marcasite produces a sulfur smell as it decomposes.
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
azurite and malachite
chalcopyrite and dolomite
orpiment and realgar