Minerals

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Minerals

• Characteristics and Identification

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How do we differentiate one mineral from another?

• Physical properties of any mineral can be
  identified in any of the following ways
• Color Streak
• Luster Hardness
• External Crystal Form Cleavage
• Fracture Specific Gravity Special
  Properties Chemical Tests

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What we are going to do

• In pairs you are going to take a mineral from the
  box provided.
• Each mineral is numbered and the number
  corresponds to a name.
• One member of the group will record the notes
  provided on the PPT the other will record the
  information for the mineral.
• Do not worry about being correct, do your best to
  understand the concepts.

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Luster

• This property describes the appearance of
  reflected light from the mineral's surface.
• There are two types of luster,
• Metallic looks like polished metal.
• Nonmetallic does not look like polished metal.
• Nonmetallic minerals are described using the
  following terms vitreous (glassy), pearly,
  silky, resinous, and earthy.

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Pyrite has metallic luster
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Quartz has nonmetallic luster
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Colour

• Although an obvious feature, it is often
  unreliable to use to determine the type of
  mineral.
• Colour arises due to electronic transitions,
  often of trace constituents, in the visible range
  of the spectrum. For example, quartz is found in
  a variety of colors.
• Colour of a mineral may be quite diagnostic for
  the trace element and coordination number of its
  bonding environment.

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A mineral can be many different colors. Below is
Mica.
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Hope Diamond 44.5 carats
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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.

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Gold

• When gold is run across a streak plate it makes a
  yellowish-gold color.
• That makes sense.

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Pyrite or Fools Gold

• When pyrite is run across a streak plate, it has
  a black or dark green streak.
• Pyrite is not worth much money, while gold is
  worth a lot. They look alike, so miners call it
  fools gold.

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Density and Specific Gravity

• Density - Defined as the mass divided by the
  volume
• Specific Gravity - Ratio of the mass of a
  substance to the mass of an equal volume of water
• 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

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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).

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The External form is a product of the internal
geometry and structure Perfect crystals however
are rare because of interaction with other
minerals
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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
• 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).

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Hardness
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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

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Rhombohedral Cleavage in Calcite
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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.

Conchoidal Fracture in Glass
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Special and Other Properties

• Striations - Commonly found on plagioclase
  feldspar. Straight, parallel lines on one or
  more of the cleavage planes caused by mineral
  twinning.
• Magnetism - Property of a substance such that it
  will spontaneous orient itself within a magnetic
  field.
• Double Refraction - Seen in calcite crystals.
  Light is split or refracted into two components
  giving rise to two distinct images

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