Lesson 3
Mineralogy
ENV 1203
Fundamentals of Earth Science
ENV 1207
Introductory Soil Science

Mineralogy
• Science of minerals,
their crystallography,
chemical composition,
physical properties and
genesis, their
identification and
classification.
• Georgius Agricola
De Re Metallica (1556)

Mineral
• Naturally occurring
• Inorganic elements or compound (Exceptional:
Graphite, Diamond or Calcite)
• Homogeneous solid (Ice but not water)
• Crystal or crystalline in nature – and so has an
ordered array of atoms at least in microscopic
scale
• Definite chemical composition or range of
chemical composition

Types of Minerals
Minerals
Rock
forming
Silicates
Group
forming
Non-
family
Non-
silicates
Economic
Two fundamental
characteristics
• Chemical composition
• Crystal structure

Chemical Composition
• Unit Cell: Representative unit of any substance
that contains one or an integral multiple of chemical
formula units.
M= (V × G) 10^-24 g
• Bonding:
Metallic – Varible – Native, Sulfides,
Covalent – Very strong – Semi metal and Non-metal
Ionic – Strong - Oxides, Carbonates, Sulphates, Phosphates
Van der Waals - Weak

Bonding

Coordination Number
Radius
Ratio
Arrangement
of Anions
around Cation
Coordination
No. of Cation
0.15-0.22 Corners of
equilateral
triangle
3
0.22-0.41 Corners of a
tetrahedron
4
0.41-0.73 Corners of an
octahedron
6
0.73-1 Midpoint of a
cube
8
1 Midpoint of
cube edges
12

Isostructure
• Isostructure: Same structure different chemical composition,
have a one-to-one equivalence in the structural arrangement
of their constituent atoms.
– Halaite (NaCl) and Galena (PbS)
– Berlinite (AlPO4) and Quartz (SiO2 = SiSiO4)
– Tantalite (FeTa2O6) and Brookite (TiO2 = TiTi2O6)

Isotype
• Isotype: Same structure different chemical
composition, but do not exhibit a one-to- one
equivalence of site occupancy and no
implications regarding chemistry.
– Halite (NaCl) and Calcite (CaCO3)

Isomorphic
• Same external form different chemical composition.
• First introduced in 1819 by a German chemist Eilhard
Mitscherlich, who prepared crystals of
– (NH4)H2AsO4, (NH4)H2PO4,
– KH2PO4, KH2AsO4,
• Noticed that they had the same form, and that the
interfacial angles between corresponding faces and
same interfacial angles.

Polymorphism
• An element or compound that can exist in more than one
crystallographic structure. Same atoms and/or ions in the same
proportions may assume more than one structure.
• Each polymorph has its own physical properties and a distinct
internal arrangement of its atoms and/or ions.
– Dimorphic
– Trimorphic
• Different polymorphs of the same substance are formed under
different conditions of pressure, temperature, and chemical
environment.
• Hence the presence of a given polymorph in a rock will often tell us
something about the conditions under which the rock was formed.

Polymorphism & Inversion
• Inversion/Transformation: The change from one
polymorph to another is generally termed as.

Displacive Inversions
• Immediate and reversible at the transition temperature
and pressure.
• Metastability is precluded (high temperature form can
not be preserved).
• Often such changes do not involve the breaking of
bonds between neighboring atoms or ions, but simply
their bending,
– Low quartz <<>> high quartz
– Low leucite <<>> high leucite.
• High-low polymorphs are characterized by the fact that
the high temperature form has higher symmetry than
the corresponding low-temperature form.

Reconstructive Inversions
• Some high-temperature polymorphs are quenchable as
metastable phases because these transformations require the
breaking of bonds in the structure and the rearrangement of
the atomic or ionic linkages.
• Quartz <<>> Tridymite << >> Cristobalite.
• High-temperature polymorph has a more open structure and
dynamically maintained at high temperatures
• The incorporation of foreign ions into the interstices of the
structure, often buttress the structure and prevent its
transformation to a different polymorph when the
temperature is lowered.

Inversion/Transformation

Polytypism
• Another variety of polymorphism which involves
the stacking of identical layers in different
sequences within a structure.
• Polytypes have the same unit cell lengths in two
dimensions but commonly have a different cell
length in the third dimension, the one essentially
perpendicular to the layers.
• Sphalerite-Wurtzite, the micas, and some of the
clay minerals.
• Polytypes are readily distinguished based on their
X-ray patterns.

Pseudomorphism
• When a mineral is replaced by another mineral without
any change in the external form.
• No change of substance occurs (paramorphism) - when
one polymorph changes to another without change in
external form and the replacing form is a paramorph.
• Addition and removal of some elements - in which the
later pseudomorph mineral has been formed from the
original mineral by a process of chemical change.

Importance of Pseudomorphism
• Formation of pseudomorph implies that the original
mineral was no longer stable under changed physical and
chemical conditions and thus was replaced by another
mineral suited to those conditions.
• Provide valuable evidence or information toward
deciphering the geological history of the rock containing
them.
• Indicate the nature and composition of circulating solutions
that added or subtracted certain elements.
• Estimate the temperature and pressure conditions under
which the change took place, if the stability fields of the
original mineral and of the pseudomorph are known.

Properties of Minerals
• Habit/Form
• Color
• Diaphaneity
• Luster
• Streak
• Hardness
• Cleavage
• Fracture
• Specific Gravity
• Miscellaneous

Form/Habit
• Crystal faces/planar growth surface/External
geomorphic planes are results from orderly
internal arrangement of the constituent atoms
and molecules within the mineral grain.
• Open space or favorable environment.
• In nature, most of the minerals do not show
any crystal form

Crystal Form

Crystal Form
Well developed crystals bounded by geometric
faces.
• Cube: 6 square - Pyrite
• Rhombohedral: 6 rhombic - Calcite
• Octahedral: 8 equilateral triangle - Fluorite
• Hexagonal: 6 rectangular - Berly
• Hexagonal dipyramid: 6 rectangular terminated
by pyramids - Quartz
• Dodecahedral: 12 rhombic/trapezoidal - Garnet

Crystalline Form
Lacks well developed crystal faces but a confused
aggregate of imperfectly formed crystals.
• Acicular: Needle like - Actinolite
• Banded: Narrow bands of different color - Malachite
• Bladed: Flat and elongated like a knife blade - Kyanite
• Botryoidal: A bunch of grapes - Chalcedony
• Columnar/Prismatic: Cylinder - Hornblende
• Concretionary and Nodular: Nodules – Flint
• Dendritic: Tree like – Copper
• Fibrous: Fine thread - Satin spar

Crystalline Form
• Foliated: Leaf like - Micas
• Granular: Aggregate of large or small grains - Chromite
• Lamellar: Consist of separable thin plates –
• Oolitic: Fish egg – Hematite
• Pisolitic: Globular like pea – Bauxite, Limonite
• Radiating: Aggregates of needle that radiates from a
center – Pyrophyllite
• Reniform: Rounded kidney shaped – Goethite
• Tabular: Tabletop - Microcline
• Massive: No definite Form

Color
• Actual color is got only on fresh and unaltered
surface of a mineral.
• Dark and bright color - useful diagnostic
property
– Sulphur (Yellow)
– Cinnabar (Bright Red)
– Azurite (Deep Blue)
– Malacite (Green)

Color
• Light color - fugitive for some minerals –
presence impurities create wide range of color
– Quartz (Crystal clear)
– Rose Quartz (Pink)
– Amethyst (Violet)
– Smoky Quartz (Grey)
• Presence of minute number of chromophores.

Diaphaneity
• Degree of transparency – the way a mineral can
transmits light
– Transparent – objectives can be seen through the
mineral
– Translucent – light passes through the specimen but
objectives can’t be seen through
– Opaque - light passes through
• Some thick minerals may appear to be opaque
but can be translucent on thin edges in front of
bright light.

Lustre
• Nature and quality of reflected light from a
mineral
• Should be observe on a fresh surface, cleavage
or crystal planes.
• Metallic lustre: Very shiny like broken metal,
usually opaque and dark in color – Galena,
Pyrite, Gold, Silver

Lustre
• Nonmetallic lustre:
– Vitreous: Like broken glass (common) – Quartz,
Hornblende
– Resinous: Resin – Sulphur, Sphalerite
– Pearly: Pearl, perfect seen on cleavage planes surface
– Micas, Talc
– Silky: Like silk – Tiger’s eye, Asbestos
– Greasy: Like thin layer of oil – Serpentine, Nepheline
– Waxy: Like wax – Chalcedony
– Adamantine: Like diamond – Cassiterite, Cerussite
– Dull/ Earthy: No lustre – Hematite, Kaolinite

Hardness
• The ability of a mineral to scratch something
else.
• Not resistance to breaking, resistance to
scratching.
• Moh’s Harness Scale: An arbitrary scale which
was developed by Moh that ranks the relative
hardness of minerals.

Hardness Scale
Moh’s Hardness Scale Pocket Hardness Scale
Talc 1
Gypsum 2 Fingernail 2.5
Calcite 3 Copper coin 3
Fluorite 4
Apatite 5 Broken glass 5.5
Orthoclase 6 Steel file 6
Quartz 7
Topaz 8
Corundum 9
Diamond 10

Streak
• Color of a mineral when it is powdered.
• Streak Plate: An unglazed porcelain plate
where across we drag a mineral to get the
color of powdery residue of it.
• Hardness is very close to 6 so we can not used
it for those minerals which have hardness 6+
• Streak of a mineral is usually constant even
though the color of bulk minerals are variable.

Streak
• Color of a mineral may be quite different from
each other, but streak will remain same.
• White/Light color – Non-metallic
• Dark color - Metallic

Cleavage
• Planar breakage of a mineral.
• The tendency of mineral to break in definite direction.
– Perfect: when it is difficult to break any other direction but
cleavage surface are extensive and smooth – Muscovite,
Biotite, Galena, Calcite.
– Good/Distinct: when the minerals break along cleavage
plan but can also be broken in other directions, the
cleavage surfaces being smooth but interrupted by other
fractures – Hornblende, Augite, Hypersthene.
– Imperfect/Indistinct: when the ease of fracture along the
cleavage plane is only somewhat more pronounced than in
other direction and cleavage surfaces tend to be small and
much interrupted – Olivine, Sphene, Spinal.

Cleavage
• Most minerals have no cleavage.
– 1 Directional cleavage
– 2 Directional cleavage at 90
– 2 Directional cleavage not at 90
– 3 Directional cleavage at 90
– 3 Directional cleavage not at 90
• How do you distinguish cleavage from crystal faces?
• Why does a mineral break along parallel planes of
weakness?
• How do you know you are looking at a cleavage surface
and not just an otherwise flat surface?

Fracture
• Non planar breakage in an irregular manner.
• The nature of broken surface of a mineral other than
its cleavage planes.
• It is often a highly variable property and depends upon
the nature of atomic structure beneath the fractured
surfaces.
– Conchoidal: smooth curved (concave)
– Hackly: jagged surface having sharp edges
– Fibrous: fibrous surface
– Splintery: splinter in wood
– Uneven/Irregular: rough and irregular surface. Non
diagnostic and non-planar

Specific Gravity
• How heavy the mineral is
– Light: <2.5 – Opal, Gypsum, Graphite
– Average: 2.5 – 4.0 – Quartz, Orthoclase, Calcite
– Heavy: 4.0 – 6.0 – Magnetite, Pyrite, Chromite
– Very Heavy: > 6.0 – Copper, Galena, Cinnebar
• Miscellaneous:
– Magnetism – Magnetite, Geothite
– Play of color – Labradorite
– Feel – Talc, Graphite
– Reaction to HCl – Calcite, Dolomite
– Chatoyancy – Tiger’s eye
– Striations – Quartz, Plagioclase frldspar, pyrite