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.

colour


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.


Streak
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.

streak

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

metallic lustre

Depending on the quantity or intensity of light a mineral reflects it may appear:

Splendent
Shining
Dull


Transparency


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.


Habit
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.

quartz crystal

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.

triclinic crystal

tetragonal crystal

orthorombic crystal

monoclinic crystal

isometric crystal

hexagonal crystal

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
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 in mica

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.


Fracture
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.

conchoidal fracture

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.


Hardness
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

1
talc
 
2
gypsum
 
   
fingernail
3
calcite
 
   
copper coin
4
fluorite
 
   
penknife
5
apatite
 
   
glass
6
feldspar
 
7
quartz
 
8
topaz
 
9
corundum
 
10
diamond
 


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.

Low SG
Medium SG
High SG
Very High SG
 
(all vitreous)
(all metallic)
(metal ores)
       
gypsum
muscovite
barite
cassiterite
halite (rock salt)
biotite
malachite
galena
graphite
calcite
sphalerite
 
 
fluorite
chalcopyrite
 
 
hornblende
haematite
 
 
augite
magnetite
 
 
orthoclase
pyrite
 
 
plagioclase
   
 
olivine
   
 
quartz
   


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

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

fluorescence

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.


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