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
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.
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
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.
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
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
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
lozenge, rhombohedral, octohedral, etc.
2. Crystal Aggregate Habits
Amygdales - are spherical aggregates infilling vesicles in 'amygdaloidal'
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.
- many minerals form crystals as they cool. There are six systems of
crystals based on their geometry.
For each system listed below there are some examples of minerals belonging
to that system. "click on any link to see an image of the mineral"
Orthorhombic Minerals include: barite group minerals as well as
sulfur, staurolite, olivine, andalusite,
members of the aragonite group minerals, marcasite,
topaz, brookite, enstatite, anthrophyllite, sillimanite, zoisite,
adamite, danburite, cordierite, wavellite
"click on any link to see an image of the mineral"
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: the ease of cleavage
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
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
Moh's scale of hardness
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.
Vitreous minerals are usually 'light' and metallic usually dense, but be
aware that there are always exceptions to the rule.
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.
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
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.
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
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.
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.
Pyrite sparkes when struck with a geological
hammer. I have also
experienced this effect with haematite.
Crystalline 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.
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.Graphite
is a good example
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 describes how the mineral behaves when subjected to deformation:
Brittle - The minerals breaks or crumbles easily, such as
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.
Carbonate Minerals are based on the salt calcium carbonate.
Calcium Carbonate ionic bonds between the calcium +2 cations and the
carbonate -2 anions. There are strong, covalent bonds within the CO32-
The carbonate minerals have a structure that is similar to the cubic close
packed structure found in halite (NaCl) where the Na cations are replaced
by divalent cations (Ca, Mg, Fe, Mn, Sr, Ba, Pb, etc.) and the Cl anions
are replaced by CO32- polyatomic trigonal planar ions. Think of the ions
as being located on two face-centered cubic lattices that interpenetrate
Minerals in the carbonate group differ do not have true cubic close packed
symmetry because anions and the cations differ significantly in size. They
are distorted from this type of crystal form. Limestone, a
sedimentary rock, becomes marble from the heat and pressure of metamorphic
events. Calcite is even a major component in the igneous rock called
carbonatite. Dolomite, CaMg(CO3)2,
is a common sedimentary rock-forming mineral that can be found in massive
beds several hundred feet thick. They are found all over the world and are
quite common in sedimentary rock sequences. All dolomite rock was
initially deposited as calcite/aragonite rich limestone, but during a
process call diagenesis the calcite and/or aragonite is altered to
All carbonates have some water solubility and dissolve readily in acidic
water. They dissolve in acidic water and can recrystallize from the water.
Metal ions are frequently trapped in the lattice spaces during
This leads to carbonates with a variety of colors and crystal forms.
Carbonic acid-rich water forms caves in limestone. When the water table is
high, carbonic acid-rich water dissolves the limestone (calcite). Later
when the water table drops, a void filled with air is formed. Smaller
amounts of water rich in Ca2+ and HCO3- may continue to flow through the
void. These waters decrease the CO2 partial pressure in the atmosphere of
the cave and aqueous CO2 is released into the gas phase. This increases
the pH and drives the precipitation of calcite and formation of
stalagmites, stalactites and other cave features. Salts containing the
carbonate anion decompose with loss of carbon dioxide. This is an
endothermic reaction and produces metal oxide materials. The carbonates
are more stable, in general, with larger cations.
Halides are formed by combining a metal with one of the five halogen
elements, chlorine, bromine, fluorine, iodine, and astatine.
Many of these compounds will dissolve in water and are often associated
with evaporating seas. Halite (NaCl) or rock
salt is the most common.
Other halides minerals include: Fluorite CaF2or calcium fluoride and Sylvite KCl or potassium
Silicates are the largest, the most interesting, and the most complicated
class of minerals by far. Approximately 30% of all minerals are silicates
and some geologists estimate that 90% of the Earth's crust is made up of
silicates. With oxygen and silicon the two most abundant elements in the
earth's crust, the abundance of silicates is no real surprise. The basic
chemical unit of silicates is the (SiO4) tetrahedron shaped
anionic group with a negative four charge (-4). The central silicon ion
has a charge of positive four while each oxygen has a charge of negative
two (-2) and thus each silicon-oxygen bond is equal to one half (1/2) the
total bond energy of oxygen. This condition leaves the oxygens with the
option of bonding to another silicon ion and therefore linking one (SiO4)
tetrahedron to another and another, etc..
The complicated structures that these silicate tetrahedrons form is truly
amazing. They can form as single units, double units, chains, sheets,
rings and framework structures. The different ways that the silicate
tetrahedrons combine is what makes the Silicate Class the largest, the
most interesting and the most complicated class of minerals.
The simplest of all the silicate subclasses, this subclass includes all
silicates where the (SiO) tetrahedrons are unbonded to other tetrahedrons.
In this respect they are similar to other mineral classes such as the
sulfates and phosphates. These other classes also have tetrahedral basic
ionic units (PO4 & SO4) and thus there are
several groups and minerals within them that are similar to the members of
the nesosilicates. Nesosilicates, which are sometimes referred to as
orthosilicates, have a structure that produces stronger bonds and a closer
packing of ions and therefore a higher density, index of refraction and
hardness than chemically similar silicates in other subclasses.
Consequently, There are more gemstones in the nesosilicates than in any
other silicate subclass. Below are the more common members of the
nesosilicates. See the nesosilicates' page for a more complete list.
Sorosilicates have two silicate tetrahedrons that are linked by one oxygen
ion and thus the basic chemical unit is the anion group (Si2O7) with a
negative six charge (-6). This structure forms an unusual hourglass-like
shape and it may be due to this oddball structure that this subclass is
the smallest of the silicate subclasses. It includes minerals that may
also contain normal silicate tetrahedrons as well as the double
tetrahedrons. The more complex members of this group, such as Epidote,
contain chains of aluminum oxide tetrahedrons being held together by the
individual silicate tetrahedrons and double tetrahedrons. Most members of
this group are rare, but epidote is widespread in many metamorphic
environments. Below are the more common members of the sorosilicates. See
the sorosilicates' page for a more complete list.
In this subclass, rings of tetrahedrons are linked by shared oxygens to
other rings in a two dimensional plane that produces a sheet-like
structure. The silicon to oxygen ratio is generally 1:2.5 (or 2:5) because
only one oxygen is exclusively bonded to the silicon and the other three
are half shared (1.5) to other silicons. The symmetry of the members of
this group is controlled chiefly by the symmetry of the rings but is
usually altered to a lower symmetry by other ions and other layers. The
typical crystal habit of this subclass is therefore flat, platy, book-like
and display good basal cleavage. Typically, the sheets are then connected
to each other by layers of cations. These cation layers are weakly bonded
and often have water molecules and other neutral atoms or molecules
trapped between the sheets. This explains why this subclass produces very
soft minerals such as talc, which is used in talcum powder. Some members
of this subclass have the sheets rolled into tubes that produce fibers as
in asbestos serpentine.
Below are the more common members of the phyllosilicates. See the
Phyllosilicates' page for a more complete list.
This subclass is often called the "Framework Silicates" because its
structure is composed of interconnected tetrahedrons going outward in all
directions forming an intricate framework analogous to the framework of a
large building. In this subclass all the oxygens are shared with other
tetrahedrons giving a silicon to oxygen ratio of 1:2. In the near pure
state of only silicon and oxygen the mineral is quartz (SiO2). But the
tectosilicates are not that simple. It turns out that the aluminum ion can
easily substitute for the silicon ion in the tetrahedrons up to 50%. In
other subclasses this substitution occurs to a more limited extent but in
the tectosilicates it is a major basis of the varying structures. While
the tetrahedron is nearly the same with an aluminum at its center, the
charge is now a negative five (-5) instead of the normal negative four
(-4). Since the charge in a crystal must be balanced, additional cations
are needed in the structure and this is the main reason for the great
variations within this subclass. Below are the more common members of the
tectosilicate subclass. See the tectosilicates' page for a more complete
Sulphate minerals include the sulfate ion (SO42−) within their structure.
The sulfate minerals occur commonly in primary evaporite depositional
environments, as gangue minerals in hydrothermal veins and as secondary
minerals in the oxidizing zone of sulfide mineral deposits. Common
examples include gypsum (CaSO4·2H2O) and
anhydrite (CaSO4) in evaporitic
sediments; barite (BaSO4), which is
deposited from hydrothermal fluids; and anglesite (PbSO4), an
alteration product of lead sulfide ores.
Sulphide minerals include the sulphide anion within their structure S-2
Common examples include the important metallic minerals: Sphalerite
(Zinc Iron Sulfide), Galena (Lead
Sulfide), Molybdenite (Molybdenum
Sulfide), Pyrite (Iron Sulfide), Chalcopyrite
(Copper Iron Sulfide)
The vast majority of sulfide minerals are components of hydrothermal
sulfide ores. Some sulfides of Fe, Ni, Cu, and Pt are associated with
processes of magma formation in ultrabasic rocks. Sulfide minerals may be
of sedimentary origin, or they may be exogenous, having been deposited
from surface solutions under the action of H2S—for example, in
coal-bearing strata and in oxidation zones of sulfide deposits.
Non-metallic Minerals and their Properties
"click on a link to see an image of the mineral"
N.B. most (if not all) minerals with colour listed as 'colourless' may
be tinted almost any colour by the presence of trace impurities.
often in limestones as a relacement minerals, also in
metamorfphic deposites, ironstones and as both thin veins and
cement in sandstones eg. the New Red Sandstone iron ore and used
as a pigment in paint 'Red Ochre'