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IMPERIAL COLLEGE Twins Powered By Docstoc


1. Crystal habit and crystalline aggregates

The habit of a crystal is its general shape produced by the relative
development of its different crystallographic forms. The habit is a function of
the conditions of crystallisation and the crystal structure and thus can be
highly variable.
       The form of mineral aggregates is obviously a function of the habit of
the individual crystals making up the aggregate.

          Crystal Habit                       Crystalline Aggregate
          Equidimensional                     Granular
          Fine grained equidimensional        Massive or compact

          Prismatic (or columnar)

          Bladed 
          Acicular                             Parallel or radiated
          Fibrous 

          Platy                                Radiated
2. Twinned Crystals

Contain two or more parts of the crystal which are physically continuous but in
different crystallographic orientations, symmetrically related to each other.
Generally classified into simple or contact twins, repeat or polysynthetic twins
and interpenetrant twins. They form during crystal growth /transformation or
by deformation
3. Density

Function of packing density of atoms in the crystal structure AND the
mass of individual atoms.

4. Hardness

The resistance of minerals to mechanical pressures
Mohs Scale of hardness
1.   Talc                6.   Orthoclase
2.   Gypsum              7.   Quartz
3.   Calcite             8.   Topaz
4.   Fluorite            9.   Corundum
5.   Apatite            10.   Diamond.
Fingernail 2-3; Copper coin 3.5; knife blade 6

5. Cleavage

Many minerals possess a tendency to split along one or more planes
determined by the crystal structure. The ease with which a mineral
may be cleaved, and the number and relation of the planes are
valuable aids to identification
Among minerals characterised by a single perfect cleavage are the micas.
The cleavage can be quantitatively described as:

Perfect         e.g. mica, calcite, galena
Good            e.g. feldspars
Distinct        e.g. chalcopyrite
Indistinct      e.g. olivine
6. Fracture

Unlike cleavage planes fracture surfaces are uneven and are not
parallel planes throughout the crystal nor are they related to the
crystal faces.
Described qualitatively, e.g. conchoidal, even, uneven, splintery.

7. Properties dependent on light

1. COLOUR          Some minerals have a distinctive colour due to their
composition or physical structure. These are known as idiochromatic
minerals, eg. malachite - green; native sulphur - yellow.
Other minerals may show a wide variety of colours according to impurities
present or slight differences of composition. These are known as
allochromatic minerals, e.g. varieties of quartz, fluorite.

STREAK is the colour of the mineral in a powdered form, and is determined
by scraping the mineral on a streak plate of unglazed porcelain.
The streak is usually constant for a particular mineral, e.g. iron oxide minerals
- haematite Fe203 (red streak), magnetite Fe304 (grey streak)
2. LUSTRE is the appearance of the surface of the mineral in reflected light
that is largely independent of the colour of the mineral.
The following kinds are distinguished:

      a) Metallic lustre - shown by the native metals and by the metallic
         sulphides and oxides, e.g. pyrite (FeS2), haematite (Fe203).
      b) Non-metallic lustre - many different varieties are recognised.
          i) Vitreous - lustre of glass, eg. quartz.
         ii) Resinous - e.g. zinc blend, opal.
         iii) Pearly      - e.g. dolomite, talc.
         iv) Silky         - fibrous minerals, eg. asbestos, satin spar gypsum
          v) Adamantine - e.g. diamond
         vi) Greasy            - e.g. weathered halite.
         vii) Dull or earthy - minerals with no lustre, eg. kaolinite.

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Description: IMPERIAL COLLEGE Twins