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Minerals II: Physical Properties and Crystal Forms
The Physical Properties of Minerals
External Crystal Form
The Physical Properties of Minerals (cont.)
Specific Gravity
Special Properties
Other Properties
Chemical Tests
Physical properties of minerals
• Based on the principles discussed during the last
lecture and above, we now know that minerals are
composed of atoms, arranged in a specific order,
with a well defined chemical composition. We
might expect then that the microscopic
variations in bond environment discussed
above, will also be manifested in macroscopic
physical and chemical properties. This is indeed
the case.
Important Physical Properties I
• Luster - This property describes the
appearance of reflected light from the
mineral's surface. Nonmetallic minerals
are described using the following terms:
vitreous, pearly, silky, resinous, and
Important Physical Properties II
• Color - Although an obvious feature, it is often
unreliable to use to determine the type of mineral.
– Color arises due to electronic transitions, often of trace
constituents, in the visible range of the EM spectrum.
For example, quartz is found in a variety of colors.
• Color of a mineral may be quite diagnostic for the
trace element and coordination number of its
bonding environment.
Hope Diamond: 44.5 carats
Important Physical Properties III
• Streak - The color of a mineral in its
powdered form; obtained by rubbing the
mineral against an unglazed porcelain plate.
– Streak is usually less variable than color.
– Useful for distinguishing between minerals
with metallic luster.
Density and Specific Gravity
• Density - Defined as the mass divided by the
volume and normally designated by the Greek
letter, rho, 
mass/volume; SI units: kg/m3 or kg m-3, but
geologists often use g/cm3 as the unit of choice.
• Specific Gravity - Ratio of the mass of a
substance to the mass of an equal volume of water.
Note that water = 1 g cm-3. S.G. is unitless.
• Examples - quartz (SiO2) has a S.G. of 2.65 while
galena (PbS) has a S.G. of 7.5 and gold (Au) has a
S.G. of 19.3.
Color and Density
Two broad categories are ferromagnesian and nonferromagnesian
silicates, which simply means iron and magnesian bearing or not. The
presence or absence of Fe and Mg strongly affects the external appearance
(color) and density of the minerals.
Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc
– Olivine - high T, low silica rocks; comprises over 50% of upper mantle
– Pyroxenes - high T, low silica rocks
– Amphiboles - esp. hornblende; moderate T, higher silica rocks
– Mica - esp. biotite; moderate T, higher silica rocks
– Garnet - common metamorphic mineral
Nonferromagnesian silicates - light color, density close to 2.7 g/cc
– Mica - exp. muscovite; moderate T, higher silica rocks
– Feldspars - plagioclase and orthoclase; most common mineral in crust;
form over a wide range of temperatures and melt compositions
– Quartz - low T, high silica rocks; extremely stable at surface, hence it
tends to be a major component in sedimentary rocks.
– Clay - esp. kaolinite; different types found in different soils
Crustal Minerals
More Zoning
Zoning in Plagioclase Feldspar
Na-rich rim
Ca-rich core
Important Physical Properties IV
• Crystal form or habit - The external
morphology of crystals generally reflect the
internal arrangement of their constituent
atoms. This can be obscured, however, if the
mineral crystallized in an environment that did
not allow it to grow without significant
interaction with other crystals (even of the
same mineral).
Chrysotile Asbestos
Belongs to the Serpentine mineral family hydrated ferromagnesian silicate.
Crystal Forms: Quartz
Intergrown cubic crystals of fluorite
Quartz Interfacial Angles
Steno’s Law (1669): Crystal face internal angles remain constant!
Macroscopic Forms and Microscopic Blocks
Crystal Forms
Unit Cells and Crystal Structure
Cubic unit cell:
smallest repeatable unit
Important Physical Properties V
• Hardness - This is the resistance of the mineral
to abrasion or scratching. This property doesn't
vary greatly from sample to sample of the same
mineral, and thus is highly diagnostic. It also is a
direct reflection of the bonding type and internal
atomic arrangement. A value is obtained by
comparing the mineral to a standard scale devised
by Moh, which is comprised of 10 minerals
ranging in hardness from talc (softest) to
diamond (hardest).
Mohs’ Hardness Scale
Polymorphism and polymorphs
• Substances having the same chemical composition
but different crystal structures.
– e.g. diamond and graphite
• Both minerals are composed of pure carbon, but
diamond is the high pressure polymorph of
• This gives rise to extremely different physical
3 mm
Natural Octahedral Diamond
Graphite & Calcite
Diamond vs. Graphite Crystal Structures
Hardness: 10
Hardness: 1-2
Fingernail Hardness (2.5) Scratches Gypsum (2)
Important Physical Properties VI
• Cleavage - Orientation and number of planes of
weakness within a mineral. Directly reflects the
orientation of weak bonds within the crystal
structure. This feature is also highly diagnostic.
• Fracture - This describes how a mineral breaks
if it is not along well defined planes. In minerals
with low symmetry and highly interconnected
atomic networks, irregular fracture is common.
Planer Cleavage in Mica
Weak Bonding Yields Planer Cleavage
Amphibole Cleavage ~120/60°
Rhombohedral Cleavage in Calcite
Conchoidal Fracture in Glass
Special and Other Properties
• Striations - Commonly found on plagioclase
feldspar. Straight, parallel lines on one or more
of the cleavage planes caused by mineral
• Magnetism - Property of a substance such that it
will spontaneous orient itself within a magnetic
field. Magnetite (Fe3O4) has this property and it
can be used to distinguish it from other nonmagnetite iron oxides, such as hematite (Fe2O3).
• Double Refraction - Seen in calcite crystals.
Light is split or refracted into two components
giving rise to two distinct images.
Calcite Double Refraction
X-ray diffraction: Laue
photographic method
Bragg Relationship
Laue X-ray photograph of Vesuvianite
Vesuvianite: Contact Metamorphic Mineral
System: Tetragonal
Hardness: 61⁄2