Mineralogy Geos 250: Lab 1&2 Physical Properties of Minerals: (Ex1-2 in Klein)
Physical properties vary greatly between different minerals (quartz versus calcite)
and somewhat between different specimens or crystals of the same mineral (rock
crystal versus citrine). Each of the physical properties depends either on the inherent
chemical make-up of a mineral (its formula such as SiO2, CaCO3) or upon the internal
structural arrangement of its lattice of atoms, ions or molecules (hexagonal versus
rhombohedral). Both of these factors depend on the environment in which the
mineral formed. Environment to a mineralogist means the sum of elements present
such as the major rock forming elements like: Na+ , Fe, O; the trace elements, the
phases present such as liquids, H2O, gases, silicate melts, and other chemical
conditions like acidity or oxidation state as well as the physical conditions such as
temperature, pressure, depth in the earth or strain (directed pressure). Mineralogists
have been identifying minerals by their macroscopic physical properties since
antiquity. Only recently ~100 years do we understand some of the underlying
chemical and physical basis for these behaviours as atomic theory and x-ray
diffraction were both 20th century discoveries. Use the special comparative test kits
for lustre, hardness, fracture etc. for real mineral examples of these properties and
how to tell them apart.
Lustre is the qualitative appearance of a mineral in reflected light. This depends on
the interaction of photons of visible light with the crystal field (electronic structure)
of the mineral. This is distinct from its opacity or transluscence. Metallic minerals
and some metal sulfides and oxides tend to be metallic and highly lustrous like a
silvered mirror. Silicate minerals tend to be glassy (vitreous). Clays, some salts and
zeolites tend to be earthy. Other terms pertaining to luster include: adamantine,
greasy, pearly or resinous.
Colour is a devilishly simple yet maddeningly complex property. While many
metallic elements appear silvery grey and sulfur appears in shades of yellow, there
are more than 20 common different reasons for a mineral to be coloured to even to
vary in colour by zonation across a single crystal. Other properties depending on
light include: iridescence, fluorescence, opalescence, translucence, opacity, and
asterism; each with its own particular explanation.
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Streak is a far more diagnostic property than colour. This is the colour of the mineral
when reduced to a structureless powder by grinding it to atoms on an unglazed
ceramic plate. Be careful here if you can still see small crystals under the hand lens
you need to keep streaking to grind it to dust!
The Habit of a mineral is its external crystal faces or form, the arrangement of the
crystals with respect to one another or to adjacent minerals and depends most
strongly on the environmental conditions during a mineral’s formation and its
subsequent history. This includes crystallinity and how well (euhedral) or poorly
formed (anhedral) they are in addition to shapes of individual crystals (acicular,
bladed, tabular, blocky, prismatic) or crystal aggregates (dendritic, drusy, foliated,
pseudomorphous, massive, radiating). Some minerals characteristically form twins or
epitaxial overgrowths on a particular mineral due to chemical or structural
similarities.
Density or specific gravity (compared to an equal volume of water) depends on the
chemical constituents (Fe at mass 56 is heavier than Mg at 23) and upon structural
arrangement or bond length (Diamond is denser than Graphite). Density can be
calculated from the mass and displaced volume of a mineral specimen or measured
directly by weighing the specimen in water using a Jolly Balance. The calculation is
S.G = Weight air / (Weight air – Weight water) . Most mineralogists learn to heft a
specimen and estimate its SG. A closely related property is Molar Volume (space
occupied by one mole of the chemical formula).
Hardness has to do with the bulk or directional arrangement of chemical bonds, how
long or short, how frequently spaced and how weak or strong they are. Depending
on the hardness, minerals are of different use to us. This property also affects how
they deform or break. We compare minerals to common materials (knife, fingernail)
and to Moh’s Hardness Scale (Graphite to Diamond).
Tenacity is how minerals deform or break under the application of force or directed
stress. In addition to bond strength and hardness, bond arrangement and type play a
big role in this bulk physical behavior. Different types of tenacity include brittle,
plastic, sectile, flexible (elastic) and malleable. At really strain rates (km/sec) most
minerals behave elastically and transmit sound waves.
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Cleavage and Parting are the terms for the characteristic directions along which
minerals break. You need a large well formed crystal to test this property. Cleavages
are named and distinguished by the number of different planar directions and the
angles between those planes. Later you will also learn to give a miller index
designation such as the form (100) for cubic. This property depends upon the internal
structural arrangement of ions or atoms in the mineral lattice. Minerals have
anywhere from 0 to 6 cleavages. 1 is called basal, 2 defines a prism, 3 forms pyramids,
cubes or rhombohedrons, 4 makes tetrahedrons or octahedrons and 6 makes
dodecahedrons.
Fracture is a qualitative characteristic of breakage for 3 dimensionally tough
substances that lack inherent planes of weakness. It describes the morphology of a
broken surface such as conchoidal, hackly, splintery or uneven.
Reactivity to cold dilute HCl (1-3 Molar). Most carbonates bubble or fizz (releasing
CO2 ) when streaked or powdered. Calcite fizzes vigorously even without powdering
it. Most sulfide minerals release rotten egg gas (H2S) when powdered in acid).
Arsenide minerals smell of garlic under this test.
Other special properties include: melting point, radioactivity, polarizability,
conductivity of heat or electricity and ferromagnetism. However these peculiar
properties are far more variable, require special apparatus or are restricted to
diagnosing a few special minerals.
Synthesis Questions:
Your target is to describe 8 mineral specimens on the forms at the end of the lab. To
do this, you first might want to work through these comparative exercises and answer
following questions to help you get some practice and to be able to distinguish them.
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1) Compare and explain the differences in luster between aluminum foil (or a pop
can), bauxite and corundum. (6)
Mineral/Material
Lustre
Explanation
Aluminum metal
Bauxite
(Bohmite, Diaspore, Gibbsite)
Corundum
2) Describe and account for the similar lustre (appearance in reflected light) and
colours of: galena, graphite, molybdenite and stibnite. (12)
Mineral
Lustre
Colour
Explanation
Galena
Graphite
Molybdenite
Stibnite
3) Describe, compare and account for the lustre and streaks of: albite, calcite and
halite. (9)
Mineral
Lustre
Streak
Albite
Calcite
Halite
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Explanation
4) Describe the colour and streak of siderite and limonite versus calcite(use coloured
calcite) and kaolinite and account for the differences. (12)
Mineral
Colour
Streak
Explanation for difference
Siderite
Limonite
Calcite
Kaolinite
5) Obtain well crystallized, equant specimens of: calcite, fluorite, pyrite and garnet.
Looking only! Do not scratch, mar, break or acidify these specimens! Draw and name
each crystal form. Draw and name the shape of a single representative face for each
of these. How many faces comprise each form. (16)
Mineral
Form
Face
Total#
Calcite
Fluorite
Pyrite
Garnet
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Name of shape (polyhedron)
6) Obtain well crystallized specimens of barite and gypsum (both sulfates) as well as
hematite and corundum (both oxides). Measure or estimate their specific gravities
either by displacement or with jolly balance and write the numerical values next to
their compositions (chemical formulae). Account for the differences. (20)
Mineral
Formula
Dry(g)
Wet(g)
S.G.
Comparison
Sulphates:
Barite
Gypsum
Oxides:
Hematite
Corundum
7) Obtain specimens of talc, clinochlore (chlorite), enstatite and olivine. These are all
magnesian silicates. Describe their habit and determine and account for their
differences in hardness. (12)
Mineral
Hardness
Habit
Talc
Chlorite
Enstatite
Olivine
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Explanation for difference
8) Obtain well crystallized blocky or tabular single crystal specimens of: halite,
gypsum, muscovite and galena. Carefully using the edge of a knife blade try cutting
the edge, prying or carving each and use descriptive words to contrast their different
types of tenacity. Tenacity relates to bonding types, strength and the presence of
planes of weakness or glides. Tenacity also depends on strain rate so some minerals
may be either plastic or brittle, elastic or brittle etc. (16)
Mineral
Formula
Bond Type Tenacity Description
Explanation
Halite
Gypsum
Muscovite
Galena
9) Obtain well crystallized specimens of: halite, fluorite, calcite and sphalerite. Use a
contact goniometer or your hand lens and eye to measure or estimate the interfacial
or cleavage angles for each mineral. List these along with the mineral’s name and the
name of the cleavage type for each one. Cleavages may be basal (pinacoid) or in a
single direction, prismatic (in 2 intersecting zones) or volume bounding as in cubes,
rhombohedrons etc. Finally explain why a given mineral exhibits its cleavage. (12)
Mineral
Cleavage Type
Angle
Explanation
Halite
Fluorite
Calcite
Sphalerite
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10) Find well crystallized pieces of: actinolite, native copper, and quartz. Either break
a small corner off of them or compare the shapes and surface morphologies of freshly
broken surfaces. Name each fracture type and account for why each occurs as it does
(structure and tenacity both contribute here as does bond type). Tenacity often
determines habit as minerals need to be physically broken to be exposed. (12)
Mineral
Formula
Habit
Tenacity Description
Explanation
Actinolite
Native Copper
Quartz
11) Obtain specimens of magnetite and pyrrhotite and test their magnetic properties
compared to specimens of hematite and chromite. Which ones have the strongest
magnetism and why? Ferromagnetism for most minerals is from unpaired “d-block “
electrons in Fe ions all being aligned if crystal structure allows this. In addition to
comparing the tug of a magnet (strong, weak, etc.), you can use a susceptibility meter
to get a numerical value for magnetic susceptibility. (20)
Mineral
Formula
Fe%
Crystal structure
Magnetite
Pyrrhotite
Hematite
Chromite
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Magnetism Explanation
12) Use a Geiger counter to measure comparable weight specimens of pitchblende
(uraninite), zircon , apatite and muscovite (illite equivalent clay in shale)
Radioactivity is measures as decays (beeps on the Geiger counter) per gram of
material per unit time. All three of the former ones owe their radioactivity
dominantly to 238U. For clay minerals as in shales the natural radioactivity is a
mixture of U and Th cations exchanged on the edges of broken or incomplete crystal
lattice. We use the slightly elevated radioactivity of clays to tell shales from
sandstones in borehole logs. Account for their differences and explain your radiation
exposure risks to a thumbnail or bullet sized piece of each mineral. What are the
consequences of these to human health? (12)
Mineral
Formula
Radioactivity
Explanation
Uraninite
Zircon
Apatite
Illite
13.Choose 8 different mineral specimens from as many different groups as possible
(e.g. not all sulfides because we will do these systematically by groups later) to
describe and fill out a description form for each using your test kits, hand lenses and
or the binocular scopes. Pick different groups: silicates, oxides, carbonates, sulfides,
native elements (12 points each).
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