A BRIEF STUDY OF MINERALS

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A BRIEF STUDY OF MINERALS

• Presented by Linder Winter
• 2005 Hammond Coaches Clinic

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A BRIEF STUDY OF MINERALS

• Note The mineral numbers used throughout this
  presentation refer to the specimen numbers of the
  minerals in the ESES mineral set referenced in
  the Rocks and Minerals Event Rules of the
  National Science Olympiad.

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A BRIEF STUDY OF MINERALS

• This presentation is divided into three parts
  Environments of Formation, Identification of
  Mineral Shapes, and Identification of Minerals.

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A BRIEF STUDY OF MINERALS

• Part I Environments of Formation

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Minerals and Crystals

• Whether a given mineral soup forms into random
  aggregates of grains within commonly found rocks
  or beautiful crystals, both are similar in
  composition.
• This presentation approaches the study of
  minerals and their crystalline forms as if they
  are the same.

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Part I Environmentsof Crystal Formation

• Few minerals have a field of stability so
  restrictive that they can crystallize in only one
  type of environment.
• The slides that follow describe the environments
  of formation for many
• of the minerals included on the official
  National Science Olympiad list.

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Igneous Environments

• Numerous cavities and fractures are found deep
  within the Earth where crust and mantle meet. It
  is within these spaces where essen-tial
  conditions for crystal growth are found.
• Fluids composed of water, CO2, and volatiles
  (substances that give off gasses) escape from
  molten magma and flow through these fractures and
  cavities.

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Igneous Environments

• These cavities and fractures provide the space
  and the range in temperature and pressure
  required for minerals to form.
• Now all that is needed is time and lots of luck.
• Should the cavities close due to shifting of the
  crust and mantle, crystal growth may cease.
• Should the cavities reopen, volatiles with a
  different composition may become available,
  resulting in crystals of different shades of
  color or even different minerals.

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Igneous Environments Hydrothermal

• Hydrothermal, as the name implies, involves water
  and heat.
• As water percolates through the lower portion of
  Earths crust, it dissolves minerals released
  from magma.
• These hydrothermal fluids move through fractures
  in the crust. Along the way, they dissolve
  additional minerals, or combine with other ground
  water. If combined with the right combination of
  temperature, pressure, time, and space, crystals
  may form.

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Igneous Environments Pegmatite Dikes and Veins

• Large masses of concentrated volatiles may form
  as isolated floating globs within the magma in
  the upper part of the mantle.
• These volatile-rich masses may eventually cool to
  form pegmatite dikes and veins.
• Many rare and beautiful minerals are produc-ed
  within pegmatites topaz, beryl, corundum,
  tourmalines, micas and apatite.

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Specific Minerals Associated with Igneous
Environments

• 1. Within slow-forming rocks during the principal
  stage of crystallization of a body of molten rock
  (magma) olivine, amphibole, mica, feldspars,
  quartz, apatite, magnetite, diamond.

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Specific Minerals Associated with Igneous
Environments

• 2. Within hydrothermal veins formed in fissures
  as a result of precipitation from solution
  feldspars, quartz, epidote, barite, calcite,
  hematite, fluorite, galena, sphalerite,
  chalcopyrite, pyrite
• 3. Within fumarolic deposits formed as a result
  of sublimation of volcanic fumes hematite,
  pyrite, sulfur

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Specific Minerals Associated withSedimentary
Environments

• Those associated with weathering of mineral
  deposits malachite, azurite, gold, silver,
  copper
• Evaporation of water, especially sea water
  gypsum, calcite, dolomite, halite
• Activities of animal and vegetable organisms
  sulfur, aragonite, apatite, bauxite, chalcedony

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Specific Minerals Associated with Metamorphic
Environments

• 1. Formed by solid-state transformation due to
  temperatures and pressures often different from
  the original mineral-forming ones.
• 2. These solid-state minerals include garnets,
  micas, staurolite, amphiboles (hornblende),
  corundum, pyrite, graphite.

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II. Crystalline Shapes

• Identification of Crystalline Shapes
• Crystal images on the following slides are
• compliments of Wikipedia, The Free Encyclopedia

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Axis of Rotation

• An axis of symmetry is an imaginary line drawn
  through the center of a crystal around which it
  may be rotated so that each successive face
  appears similar to the one preceding it.
• Depending on the number of degrees through which
  the crystal must be rotated to produce this
  effect, a crystal may look the same from 2, 3, 4,
  or 6 different positions such a crystal is
  described as having n-fold symmetry (e.g.
  "2-fold symmetry") along that axis of rotation.

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Identification of Crystalline Shapes

• Isometric or cubic three axes of symmetry, all
  of equal length.
• Envision an axis drawn from the center of the top
  face to the center of the bottom face.
• Envision the cube rotat-ed about that axis. A
  total of four identically-shaped faces appear as
  the cube rotates.

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Identification of Crystalline Shapes

• Imagine a second axis drawn from the center of
  the left face to the center of the right face.
• Rotate the cube on that axis. Four
  identically-shaped faces will again rotate into
  view.

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Identification of Crystalline Shapes

• Finally, imagine an axis drawn from the center of
  the front face to the center of the rear face.
• A cube displays four three-fold axes, i.e. four
  similar faces for each of its 3 axes.

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Identification of Crystalline Shapes

• Tetragonal three axes of rotation, two of equal
  length and a third that is either longer or
  shorter.
• Envision three axes of rotation, only one of
  which is a four-fold axis.
• Which of the axes of
• rotation in this crystal has a four-fold axis?

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Identification of Crystalline Shapes

• Which of the axes
• of symmetry in this crystal has a four-fold
  axis?
• The longest axis as the crystal rotates, four
  similar-appearing faces appear in succession.

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Identification of Crystalline Shapes

• The other two axes are two-fold, i.e. as the
  tetragonal crystal rotates about its axis, two
  similar-appearing faces are alternately revealed.

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Identification of Crystalline Shapes

• Hexagonal four axes of rotation -- three of
  equal length and a fourth that is either longer
  or shorter.
• One axis of six-fold symmetry

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Identification of Crystalline Shapes

• Orthorhombic three axes of rotation, each
• of unequal length.
• Two-fold symmetry

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Identification of Crystalline Shapes

• Monoclinic three axes of rotation each of
  unequal length.
• When viewed in the position shown, this object
  has three axes of unequal length top to bottom
  right to left front to back.
• One two-fold axis the axis of symmetry.

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Identification of Crystalline Shapes

• Triclinic three axes of length, none
  perpen-dicular to the other.
• Three axes, all of unequal length, none at right
  angles to the others.
• No axes of symmetry

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Identification of Crystalline ShapesCoaching
Strategy

• Have participants construct their own models.
  When completed, have them identify the number of
  axes of rotation and the number of folds.
• These models may be found at (Enter this URL in
  your notebook letter case counts!)
• http//bca.cryst.bbk.ac.uk/BCA/ed/Class.pdf

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III. Mineral Properties

• LUSTER

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Luster Basics

• Luster is a subjective judgment indicating how a
  mineral appears to reflect light.
• A major distinction is whether a minerals luster
  is metallic or nonmetallic.
• Luster of non-metallic minerals may vary from
  specimen to specimen.
• Diagnostic properties reflect the internal
  structure and composition of minerals.

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Earthy LusterDirt or Dried Mud Appearance

• Specimen 23A Goethite
• Specimen 31A Kaolinite

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Vitreous Luster Glass-Like

• Specimen 9A Barite
• Specimen 21A Fluorite
• A majority of specimens on the official NSO list
  display a vitreous luster.

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Dull Luster Non-Reflective

• Specimen 29A Hematite
• Specimen 34A Malachite

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Pearly LusterPearl-Like

• Specimen 37A Opal
• Specimen 35A Muscovite

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Metallic LusterMetal-Like

• Specimen
• Silver
• Specimen 38A Pyrite

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Greasy LusterGreasy-Like

• Specimen 49A Sodalite
• Specimen 53A Talc

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Adamantine LusterBrilliant

• Specimen 50A Sphalerite
• Specimen 54A Topaz

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Resinous LusterSimilar to Resin or Sap

• Specimen 50A Sphalerite
• Specimen 52A Sulfur

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Silky LusterSilk-Like

• Specimen 56A Tremolite
• Specimen 57A Ulexite

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Mineral Properties

• HARDNESS

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Mohs Hardness Scale

• Many years ago, Fried-rich Mohs proposed his
  Hardness Scale which is still in use today.
• 1. Talc
• 2. Gypsum
• 3. Calcite
• 4. Fluorite
• 5. Apatite

• 6. Orthoclase
• 7. Quartz
• 8. Topaz
• 9. Corundum
• 10. Diamond

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Hardness Basics

• Hardness is the ability of a mineral to resist
  scratching or abrasion.
• Hardness is a very reliable physical property for
  use in the identification of minerals due to
  chemical consistency of minerals.
• The hardest minerals tend to be those with small
  atoms packed tightly together with strong
  covalent bonds throughout.

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Hardness Basics

• Hardness among individual specimens of the same
  mineral may vary slightly.
• Inconsistencies occur when the specimen is
  impure, poorly crystallized, or an aggregate of
  the same crystal.
• A scratch on a mineral is actually a groove
  produced by micro-fractures on the surface of a
  mineral.

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Hardness

• Minerals may be tested for hardness by scratching
  one mineral of known hardness against another
  whose hardness one wishes to determine.
• To limit the damage to minerals, other
  non-mineral objects of known hardness may be
  substituted for those used to test others for
  hardness.

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Hardness Test Basics

• Do not SCRATCH NICE CRYSTAL FACES! Test on
  fractured, cleaved, or inconspicuous parts of a
  mineral only.
• Ideally, hardness tests should be performed on
  only individual crystals. A massive system is
  most likely to be softer.

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Hardness Test Hints

• Make certain a scratch is a scratch rather than a
  dust trail left on a mineral after being
  scratched by a softer material. This is
  particularly true when testing harder minerals
  against porcelain plates.
• Most minerals have small differences in hardness
  according to the direction and orientation of the
  scratch.

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Hardness Test Precautions

• NEVER perform streak or hardness tests on
  minerals unless specifically given permission by
  the event supervisor to do so either verbally
  or written!
• Performing hardness tests causes harm to the
  specimens.
• The supervisor may provide scratch and/or
  hardness test data in those instances when this
  information would be helpful.

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Testing for Hardness

• Many of those event supervisors who do permit
  hardness testing of their minerals provide
  objects for participants for use as substitute
  testing surfaces.
• Fingernails 2.5
• Copper 3.5 (avoid pennies)
• Window glass 5.5
• Streak plate 6.5

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Determining Hardness

• Using fingernails, copper tubing, window glass,
  and streak plate, determine the approximate
  hardness of the following
• Specimen 4A Apatite
• Specimen 17A Corundum
• Specimen 21A Fluorite
• Specimen 22A Galena

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Determining Hardness

• Apatite 5 Scratches copper (3.5), but
  not glass (5.5)
• Corundum 9 Scratches streak plate (6.5)
• Flourite 4 Scratches copper (3.5), but not
  glass (5.5)
• Galena 2.5 Scratches fingernail (2.5), but
  not copper (3.5).

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Mineral Properties

• STREAK

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Streak Basics

• Streak is the color of a minerals powder.
• Streak, for a given mineral, is generally very
  consistent from specimen to specimen thus making
  it a valuable test for mineral ID.
• A mineral harder than 6.5 will not leave a streak
  on a streak plate due to the streak plates
  hardness of 6.5. Be careful as a streak of
  porcelain may result.

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Streak Basics

• Fortunately, most minerals with a hardness gt 6.5
  have a white streak.
• Streaks made by similarly-colored minerals may
  differ from their outward surface color.
• To test for streak, rub the mineral against a
  tile of white, unglazed porcelain.
• Note the color of the streak.

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Testing Various Specimens for Streak

• Hematite, Specimen 29A

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Testing Various Specimens for Streak

• Hematite leaves a dark red streak.

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Testing Various Specimens for Streak

• Galena, Specimen 22A

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Testing Various Specimens for Streak

• Galena leaves a gray streak.

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Testing Various Specimens for Streak

• Pyrite, Specimen 38A

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Testing Various Specimens for Streak

• Pyrite leaves a greenish black streak.

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Testing Various Specimens for Streak

• Malachite, Specimen 34A

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Testing Various Specimens for Streak

• Malachite leaves a light green streak.

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Testing Various Specimens for Streak

• Barite, Specimen 9A

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Testing Various Specimens for Streak

• Barite leaves a white streak.

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Testing Various Specimens for Streak

• Chalcopyrite, Specimen 15A

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Testing Various Specimens for Streak

• Chalcopyrite leaves a dark green streak.

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Mineral Properties

• FRACTURE

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Fracture Basics

• Fracture describes how a mineral tends to break.
• Fracture differs from cleavage. Cleavage is the
  tendency of some minerals to break along one or
  more regular, smooth surfaces and will be
  addressed later.
• Fracture occurs in all minerals, even those with
  cleavage.

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Fracture Basics

• Fracture is the way minerals break when they do
  not yield along cleavage or parting surfaces.
• Three commonly recognized fractures are
• 1. conchoidal smooth, shell-shaped
• 2. fibrous splintery
• 3. uneven rough and irregular

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Conchoidal Fracture

• Olivine 36A
• Magnetite 33A
• Conchoidal fracture is evident in the image of a
  magnetite specimen.

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Uneven Fracture

• Hematite 29A
• Hornblende 30A
• Tremolite 56A

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Fibrous Fracture

• Ulexite 57A

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Mineral Properties

• CLEAVAGE

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Cleavage Basics

• Cleavage reflects the tendency of a mineral to
  break along sets of parallel planes due to its
  internal atomic structure having regular
  directions of weaker bonding.
• Cleavage planes fail when force is applied to
  them.
• The quality of cleavage may be described as
  perfect, good, fair, or poor.
• Not all minerals display cleavage.

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Cleavage Basics

• Cleavage is generally observable only in a
  minerals crystal form, not in massive specimens.

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Determining Cleavage

• To determine whether a mineral specimen possesses
  cleavage, ask yourself
• Are there any sets of parallel planes along which
  a mineral preferentially breaks?
• If the mineral shows cleavage, how many different
  sets of parallel planes does it have?
• What are the approximate angles between the
  different cleavage directions? 30 45

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Cleavage Test Precaution

• Do not confuse crystal faces planes formed by
  the growth of a crystal with cleavages.
• Crystal faces will not be sets of parallel
  planes. They will be single surfaces.

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Biotite 11A

• Cleavage is perfect in one direction producing
  thin sheets or flakes.

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Calcite 13A

• Cleavage is perfect in three directions forming
  rhombohedrons.

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Galena 22A

• Cleavage is perfect in four directions forming
  cubes.

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Halite 28A

• Cleavage is perfect in four directions forming
  cubes.

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Mineral Properties

• Color

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Color Basics

• Silicon and aluminum rich minerals are typically
  light in color quartz and feldspar.
• Iron and magnesium rich minerals are typically
  dark in color olivine and pyroxene.

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Color Basics

• Silicon and aluminum rich minerals are typically
  light in color.
• Iron and magnesium rich minerals are typically
  dark in color.
• Which of these minerals is most likely rich in
  silicon and aluminum?

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Color Basics

• Which of these minerals is most likely rich in
  silicon and aluminum ?
• The feldspar in the upper photo is most likely
  rich in silicon and aluminum.

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Color Basics

• Which of these minerals is most likely rich in
  iron and magnesium?

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Color Basics

• Which of these minerals is most likely rich in
  iron and magnesium?
• Both augite, in the upper photo and hornblende
  in the lower photo are rich in iron and
  magnesium.

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Color Basics

• Caution!
• 1. Color can be altered by weathering.
• 2. Trace elements may alter the typical color
  of a specimen. Generally, quartz is light in
  color, but a small amount of iron may tint it a
  purplish color.
• 3. Some minerals come in a wide variety of
  colors.

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Mineral Properties

• Transparency

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Transparency Basics

• Transparency describes the capability of a
  mineral to transmit light.
• A mineral can be transparent, translucent, or
  opaque.
• Most gem minerals are highly transparent.
• Most metallic minerals are opaque.

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Transparency Descriptions

• Three terms describe the full range of mineral
  transparency
• 1. Transparent light enters and exits in a
  relatively undisturbed fashion
• 2. Translucent light enters and exits, but in
  a disturbed and distorted fashion
• 3. Opaque light cannot penetrate the surface

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Transparency Test Precaution

• Effective for use with crystalline mineral forms
  only!

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Halite 28A

• Is this specimen transparent,
• translucent,
• or opaque?

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Halite 28A

• Is this specimen transparent,
• translucent,
• or opaque?

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Calcite 13A

• Is this specimen transparent,
• translucent,
• or opaque?

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Calcite 13A

• Is this specimen transparent,
• translucent,
• or opaque?

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Topaz 54A

• Is this specimen transparent,
• translucent,
• or opaque?

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Topaz 54A

• Is this specimen transparent,
• translucent,
• or opaque?

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Amazonite 3A

• Are these specimens
• transparent, translucent,
• or opaque?

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Amazonite 3A

• Are these specimens
• transparent, translucent,
• or opaque?

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Mineral Properties

• SPECIFIC GRAVITY

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Specific Gravity Basics

• Specific gravity is the density of a mineral
  relative to that of water at 4C. It depends upon
  the chemical composition and crystal structure of
  the mineral.
• Specific gravity has no unit because it is
  derived from the density of the mineral divided
  by the density of water. Thus, all units cancel.
• A mineral with a SG of 2, is twice as dense as
  water

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Specific Gravity Basics

• Earths crust, from where amateurs are most
  likely to collect minerals, is composed mostly of
  the minerals quartz, calcite, and feldspar. These
  minerals have an average SG of 2.75.
• Non-metallic minerals tend to be of low density
  metallic minerals of high density.

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Testing for SG Hefting

• Hold a mineral of known SG in one hand and
  another of unknown SG, of about the same size, in
  the other.
• Preferably, the mineral of known SG should be
  near the average of 2.75. Compare the SG of the
  known with the unknown.

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Testing for SG Hefting

• Chances are quite good that the event supervisor
  will place three to four specimens at a station
  and ask that these be ranked according to
  specific gravity.
• Because of the variability among the minerals in
  the Science Olympiad kits, we will not perform a
  specific gravity test by hefting during this
  session.

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Testing for SG Lab Activity

• Important For the lab activity, be certain to
  select only specimens containing one mineral.
• Impure specimens containing significant amounts
  of other minerals will result in incorrect
  specific gravity values.
• Supervisors may choose objects other than common
  minerals, i.e. density blocks.

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Testing for SG Lab Activity

• Attach the mineral to a spring scale using a
  length of string.
• What is this minerals weight (D) in air?
• Suspend (totally immerse) the mineral in water.
• The letter W will represent the minerals weight
  while suspended in water.
• To calculate the minerals SG, use the formula
  D/(D-W).

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Special Mineral Properties

• Some minerals may be easily identified due to
  special properties

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Special Mineral PropertiesPenetration Twinning

• Staurolite is famous for its twinned crystals
  that form into the shape of a cross. Staurolite
  is an example of penetration twinning, the
  symmetri-cal intergrowth of two or more crystals
  of the same substance.

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Special Mineral PropertiesStriations

• Quartz, 44A striations are uniquely
  perpendicular to the crystal length and appear
  only on prism faces.
• Pyrite, 38A Striations on one side of the cube
  are perpendicular to the striations on the other
  side.

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Striations Basics

• The most common cause of striations is the
  convergence of two crystal faces. One of the
  faces is overtaken by the other but manages to
  leave its mark in the form of an almost
  imperceptible edge, or stria.
• This edging is repeated again and again as the
  mineral grows and can fill an entire face with
  these tiny edges or striations.

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Mineral Properties

• FORM HABIT

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Form and Habit

• Geometric features of a single crystal may be
  de-scribed in terms of crystal habit and crystal
  form.
• Crystal habit refers to the general shapes of
  individual crystals or aggregates of crystals,
  which are governed by the development of their
  faces. 
• Crystal form refers to the characteristic
  appearance of a crystal as determined by its
  predominate form.
• Common forms include cube, such as pyrite,
  octahedron, such as fluorite, and hexagonal, such
  as quartz.

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Form and Habit

• Copper provides a good example of crystal form
  cubic.
• Copper provides a good example of crystal habit
  dendritic (similar in appearance to a river and
  its tributaries).

113
Form and Habit

• Halite has a cubic habit that reflects the cubic
  arrange-ment of its atoms

114
Form and Habit

• Mica has a sheet-like habit that reflects its
  silicate sheet structure.

115
Image Credits

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