mineral identification
Basic Introduction to Mineral Identification
Colour
Streak
Lustre
Transparency
Form
Habit
Cleavage
Fracture
Hardness
Density
Magnetism
Effervescence
Bierfringence
Fluorescence
Taste
Odour
Sparkle
Feel
Conductivity
Acid Reaction
Tenacity
see also... identification tables of
common metallic and non-metallic minerals
Colour
The colour of a mineral is a result
of the mineral's light absorbing and light reflecting properties.
These may vary greatly in vitreous minerals with the presence of traces
of impurities. Colour is therefore not always an indication of
identity in a vitreous specimen, although it is a more reliable
indicator with opaque minerals.
An excellent example of the above is quartz. Six different varieties of quartz are each a different characteristic colour despite having identical chemical compositions (SiO2):
Rock Crystal - colourless
Amethyst - purple
Citrine - yellow, to orange-brown
Smokey Quartz - brown or grey
Rose Quartz - pink
Milky Quartz - white
It is also worth remembering that
completely different minerals may be the same colour.
The streak of a mineral is the colour of it's powder when rubbed along an unglazed porcelain plate (streak-plate) and may be different from the colour of the mineral itself.
Powder may also be produced by scratching the mineral with a knife. The streak of any given mineral is consistent for that mineral despite any differences in colour. The six different varieties of quartz above all have the same white streak.
Lustre
The mineral's appearance due to the amount and quality of light reflected from it's surfaces. Depending on the quality of light a mineral reflects it may appear:
Adamantine - the lustre of
diamond
Vitreous - the lustre of
broken glass, e.g.. quartz
Subvitreous - as vitreous,
less well developed
Resinous - the lustre of
resin, e.g.. amber and opal
Pearly - the lustre of pearl
Silky - the lustre of silk
in fibrous minerals such as satin spar gypsum
Metallic - the lustre of metal
Submetalic - as metallic,
poorly displayed
Depending on the quantity or intensity of light a mineral reflects it may appear:
Transparency- If an object can be seen with a
clear outline through a mineral then that mineral is transparent.
- If an object viewed through a mineral can be seen with a
indistinct
outline then the mineral is said to be subtransparent.
- If a
mineral cannot be seen through, but is transmitting light then that
mineral is said to be translucent.
- A mineral that does not transmit light is termed opaque.
Form
The form of a crystal is dependant upon the conditions under which it grew. For example growth may have occurred outwards into a melt unhindered or it may have been restricted by the presence of other solid matter. The following terms are used to describe form:
Crystallized
- the mineral
occurs as well developed crystals
Crystalline - the mineral
occurs as an aggregate of confused, imperfect crystals which hindered
each other's formation during growth. These minerals often have a
granular, sparkling appearance due to light reflected from the small
crystal faces.
Cryptocrystalline or Microcrystalline
- crystals are very small and are hidden from the naked eye, but show
under a microscope.
Glass - random arrangement of
atoms; no crystal structure. A substance which cooled so rapidly
that crystals did not have time to form. The result may be
thought of as a stiff, brittle supercooled liquid.
The habit of a specimen (the shape of it's crystals) is greatly affected by the conditions under which the crystals grew. It is quite common for a mineral to have many different habits. The terms used to describe a specimen's habit are split into two groups; (1) the habit of crystals, (2) the habit of crystal aggregates.
1.
Crystal
Habits
Prismatic - crystal is
elongated along one axis
Tabular - Broad, flat crystals
Acicular - needle-like
crystals
Bladed - the shape of a knife
blade
Fibrous - fine thread fibres
as in asbestos and satin spar gypsum
Foliaceous - composed of thin
separate leaves (lamellae) as in mica.
Lamellar - separable into
individual plates or lamellae
Reticulate - cross-mesh
pattern
Scaly - small plates
Individual crystals may be
described by their shape i.e.. cubic, hexagonal
elongated (prismatic),
lozenge, rhombohedral,
octohedral, etc.
2. Crystal
Aggregate Habits
Amygdales - are spherical
aggregates infilling vesicles in 'amygdaloidal' rocks.
Massive columnar - such as in
stalactites and stalagmites aggregates.
Nodular - such as flint
nodules in chalk
Granular or Saccharoidal
- grains, may range from coarse to fine. Saccharoidal means
'sugar-like'.
Mammilated - similar to
reniform (below), but more spherical outer surfaces. e.g..
malachite.
Reniform - 'kidney-shaped'
rounded outer surface. e.g. haematite
A mineral with no crystal or
aggregate shape is a glass.
Cleavage is the tendency of a mineral to split in certain preferred directions when struck. These directions are parallel to sheets of atoms in the mineral's atomic lattice.
Cleavage is described in terms of: (1) the ease of cleavage, (2) the number and orientations of cleavage planes. For example:
Gypsum has 'easy' cleavage
in one direction
Calcite has good cleavage in
three directions parallel to its rhombohedral habit and is therefore
said to have rhombohedral cleavage.
Fluorite has a cubic habit, but it
has four cleavage directions which cut across it's corners to leave an octahedral
core. Therefore fluorite has octahedral cleavage.
The fracture of a mineral is how it breaks other than along cleavage planes. The fracture may be described as:
Conchoidal - a
'shell-like', convex or concave fracture displaying curved fracture or
undulation rings concentric to the point of impact and lines or
fractures radial from the point of impact, as in quartz, flint and
obsidian.
Even - a flat fracture, as in
chert
Uneven - a rough fracture
surface. This is the most common type of fracture.
Hackly - jagged sharp ridges,
such as in native copper.
The hardness of a mineral is measured on Moh's scale. The scale lists hardness values from 1 to 10. The numbers may be treated as relative values except for diamond; i.e. fluorite(4) is twice the hardness of gypsum(2). Diamond(10) is about ten times the hardness of corundum(9). Each value has a corresponding mineral of thathardness. Therefore the hardness of a mineral can be tested relative to the minerals on Moh's scale by scratching them with those minerals and other household items of known hardness.
Moh's scale of
hardness
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Density
The relative density of a mineral is its mass divided by it's volume. The specific gravity of a mineral is it's mass divided by the mass of an equal volume of water. In the field it is adequate to simply 'heft' a specimen to determine whether it is of low, high or moderate weight compared to it's size.
Silicates and other non-metallic minerals are the least dense with SGs of 2.5 to 3.5
Metallic minerals are denser with SGs from 5 upwards (typically 5 to 8). Gold has an SG of 19 to 20.
SGs of
some
common minerals
Vitreous minerals are usually
'light' and metallic usually dense, but be aware that there are always
exceptions to the rule.
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Magnetism
Magnetite and pyrrhotite are magnetic and will be
affected by a bar magnet. Other iron minerals are magnetic to a
lesser
extent, but cannot be tested by an ordinary magnet in the field.
Large
iron-bearing masses may affect the orientation of compass
needles. A petrology lecturer described how he once stopped for
lunch on a large
magnetite-bearing outcrop and then set off in completely the wrong
direction and wasted the rest of the day.Is this mineral
magnetic (try using a compass), or is it attracted by a magnet? This
property is characteristic of Magnetite.
Effervescence
This one is most popular with the kiddies as well as the new geology student (welcome). When some minerals are exposed to acids, they begin to fizz. This is a great method you can use to identify the mineral calcite (see TASTE). You can also use this one to detect the presence of calcite in rocks.
Birefringence
This is also known as double refraction. Birefringent minerals split the light into two different rays which gives the illusion of double vision in this Iceland Spar Calcite.
Fluoresence
Some minerals display
what is called the phenomenon of photoluminescence. This basically
means that they "glow" when exposed to UV light (black light). The
above mineral (opal) is demonstrating fluoresence. Also, the mineral
Fluorite is often strongly fluorescent. Do you see a connection?
Fluorite-Fluoresence
Taste
This will quickly
identify the mineral halite (salt). If you are new to this process you
must use this one with caution, as you never know what the unknown may
be. Often, you may need to resort to this method (until you more fully
understand other identifying traits) to differentiate halite from
calcite. If you do taste the sample (especially in a class environment)
you should realize that it has been handled by and probably tasted by
hundreds of others.
Odour
Pyrite, sphalerite and chalcopyrite give a sulphurous
'rotten egg' smell
when struck or rubbed on a streak plate. haematite and limonite
may give off
an 'earthy' smell (the smell of damp earth) when breathed upon.
Sparkle
Pyrite sparkes when struck with a geological hammer. I have
also
experienced this effect with haematite.
Feel
Cryslalline minerals will feel rough. Talc and
serpentine often feel
unctuous (greasy) or soapy. Graphite and satin spar gypsum may
feel smooth, unctous or soapy. Graphite is a good conductor of
heat and will therefor feel cold.
Conductivity
Graphite is also a good conductor of electricity (it is used a brushes on
electric motors), but this property would not be tested in the field.
Acid Reaction
Carbonate minerals react with dilute hydrochloric acid:Calcite effervesces strongly in dil. HCl
Malachite also reacts strongly
Dolomite reacts weakly in warm dil. HCl or if scratched
to produce a little powder prior to applying the acid
Siderite reacts weakly
Tenacity
Tenacity describes how the mineral behaves when subjected to deformation: Brittle - The minerals breaks or crumbles easily, such as fluorite.- Ductile - the mineral can be drawn into thin wires.
- Elastic - the mineral can be deformed by force, but returns to
it's original shape when the deforming force is removed.
- Flexible - the mineral can be bent or deformed by force and remains deformed when the force is removed.
- Malleable - Can be flattened into sheets with hammering, such as
native gold, silver and copper.