Physical Properties of Minerals
Physical properties
• Physical properties provide consistent criteria that aid in
the recognition of minerals
• These range from the simple to more involved
– Simple – color, luster, hardness, specific gravity
– More complex – optical properties, electrical properties,
magnetism, internal structure
Physical properties - Color
• Color: in minerals is caused not only by the chemical
composition, but also by atomic arrangement, impurities
and structural defects.
• Many minerals exhibit a range of colors caused by
impurities – quartz, fluorite, calcite, beryl, apatite and
tourmaline to name a few
• Some minerals are characterized by their color. Always
look at a fresh surface!
– Non-metallics:
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Blue – Azurite, Lazurite
Green – Malachite, Dioptase
Yellow – Sulfur, Orpiment, Carnotite
Orange-red – Realgar
Red – Cuprite, Cinnabar
– Metallic minerals:
• Deep brass yellow – Chalcopyrite
• Pale brass yellow – Pyrite
• Copper red - Copper
• When color changes along the different crystallographic
axes the property is termed pleochroism
• Example: gem andalusite is green in one direction and redbrown in another. Other gem quality minerals with this
property are ruby, saphire, topaz and emerald
• Changes in color may result in variations in chemical
composition
• Example: Sphalerite, ZnS, may range from yellow to black
as ferrous iron substitutes for zinc
• Differences in structure can cause color variations
• Example: gray to opaque graphite and colorless diamond
• Inclusions of other minerals in the host may cause color
variations
• Example: quartz may be colored red-brown by inclusions
of mica or hematite
Physical properties - Luster
• Luster represents the appearance of the mineral surfaces
in reflected light
• Two major categories – metallic and non-metallic.
• Native metals and most sulfides have metallic luster.
• Non-metallic luster can be described by these terms:
– Vitreous or glassy – a glass-like appearance. Quartz and Beryl
– Adamantine or brilliant – bright luster lika a polished diamond.
Diamond, cassiterite, zircon
– Resinous – like rosin. Sulfur
– Silky – luster produced by aggregates of parallel fibers.
Asbestos, some gypsums
– Waxy –literally like wax. Chalcedony
– Greasy – literally greasy. Rough diamonds
– Pearly – iridescent, like “mother-of-pearl”. Talc.
– Dull or earthy – weathered appearance. Some hematite.
Physical properties - Streak
• Represents the color of the powdered mineral
• Rub mineral across an unglazed porcelain plate (streak
plate)
• Hardness of the streak plate surface is 6 ½ so some
minerals won’t show streak.
• Notably some dark metallic minerals have streaks that vary
from the mineral color. Hematite –black mineral with redbrown streak
Physical properties - Hardness
• Hardness is the extent to which the mineral resists
abrasion.
• Time honored scale is the Moh’s Hardness Scale based on
10 minerals. Each will only scratch those with a lower
hardness number and will be scratched by those above it.
Softest is 1, hardest is 10. However we don’t carry 10
minerals around in our pockets.
• Moh’s scale and the practical equivalents
Moh’s Mineral Equivalent
1. Talc
2. Gypsum
2 ½ Fingernail
3. Calcite
3 Copper coin
4. Fluorite
5. Apatite
5 ½ Knife blade
6. Orthoclase
6 Glass
6 ½ Steel file
7. Quartz
8. Topaz
10. Diamond
Physical properties - Tenacity
• This describes the way a mineral breaks or deforms under
stress
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Brittle- Breaks or powders. Quartz, diamond
Maleable – can be hammered into sheets. Gold, silver, copper
Ductile – can be drawn into a wire. Gold, silver, copper
Sectile – can be cut with a knife like wax. Gypsum
Flexible – can be bent but doesn’t return tooriginal shape. Talc,
molybdenite
– Elastic – returns to original shape. Micas
Physical properties - Cleavage
• Tendency of a mineral to break along planes parallel to crystal
faces.
• Not all minerals exhibit cleavage, but if they do, all crystals of the
same mineral show the same cleavage
• Cleavage is described as perfect, good, fair, or poor along with the
number and direction of cleavage planes
– Fluorite – perfect to good –octahedral cleavage in 4 planes
– Galena – perfect to good – cubic, 3 planes at right <‘s
Physical properties - Fracture
• Breakage along other than cleavage planes.
• Most minerals show uneven or irregular fracture and it is not
normally used for identification.
• The possible exception to this is conchoidal fracture found in
chalcedony, obsidian, quartz.
Physical properties – Specific Gravity
• Weight of the mineral with respect to an equal volume of pure
water. i.e. SG of 3 is 3X heavier than water
• SG of Non-metallics:
– Low (< 2.2) Sulfur, Halite, Opal
– Avg. (2.6 – 3.0) Quartz, Feldspar, Calcite
– High (> 3.5) Topaz, Barite, Cinnabar
• SG of Metallics:
– Low (209 – 2.23) Graphite
– Avg. (~ 5) Marcasite, Pyrite
– High (> 7) Galena, Copper, Silver
• In general, metallic minerals will have a higher SG then non
metallics. Porous minerals and minerals with a lot of impurities will
appear to have lower values than the true SG.
Physical properties – Magnetism
• Self evident – magnetite is the example.
Physical properties – Electrical Prop.
• Piezoelectric effect present in some minerals. Pressure applied in
the appropriate direction on the crystal will cause an electric current
to be produced. Quartz and Tourmaline both exhibit this property.
Chemical Tests
• Really qualitative chemistry
• Use of blowpipe and reducing or oxidizing flame
– Reducing flame – Bright blue area of the flame removes oxygen
– Oxidizing flame – clear flame just outside the blue adds oxygen
– Iron will be yellow in oxidizing flame and pale green in reducing flame.
Chemical Tests
• Fusibility – relative ease with which the mineral melts
– Scale of fusibility:
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1. stibnite – fuses in any flame – easily ~550ºC
2. chalcopyrite – fuses with some difficulty ~800ºC
3. garnet – fuses easily in blowpipe flame 1050ºC
4. actinolite – thin edges will fuse easily 1200ºC
5. orthoclase – fuses on edges with difficulty 1300ºC
6. enstatite – blowpipe will only round edges 1400ºC
7. quartz – infusible with ordinary blowpipe flame
- Color of the flame during fusibility tests can also be used
• Violet = potassium
• Orange = calcium
• Green = copper
• Borax bead tests – when clear borax beads are touched to
powdered mineral while hot, the color is observed while hot and
then as it cools. If bead turns black, too much mineral adheared to
bead
– Green bead – chromium present
– Blue bead – cobalt present
• Tests with acid:
– Calcite effervesces in cold,dilute HCl
– Dolomite won’t effervesce in cold HCl, but will in warm acid