RECOGNITION AND DESCRIPTION OF MINERALS

By this point in the course you are familiar with the following minerals which make up the majority of those present in igneous, metamorphic and sedimentary rocks.

ISOTROPIC MINERALS

Halite NaCl
Fluorite CaF2
Periclase MgO
Garnet (Ca,Mg,Fe2+,Mn)3 (Al,Fe3+,Cr)2(SiO4)3

UNIAXIAL MINERALS

Quartz SiO2
Calcite CaCO3
Nepheline NaAlSiO4
Apatite Ca5(PO4)3(F,Cl,OH)
Zircon ZrSiO4
Tourmaline Boro-Silicate

BIAXIAL MINERALS

Olivine (Mg,Fe)2SiO4
Orthopyroxene (Mg,Fe)2Si2O6
Clinopyroxene (Ca,Mg,Fe,Al)2Si2O6
Hornblende Ca2(Mg,Fe,Al)5Si8O22(OH)2
Plagioclase (Na,Ca)Al(Al,Si)Si2O8
Alkali Feldspar KAlSi3O8
Biotite K2(Mg,Fe)3AlSi3O10(OH,F,O)2
Muscovite KAl2(AlSi3O10)(OH)2
Chlorite (Mg,Al,Fe)3(Si,Al)4O10(OH)2*(Mg,Al,Fe)3(OH)6
Tremolite
Actinolite
Ca2Mg5Si8O22(OH)2
Ca2Fe5Si8O22(OH)2


The last half of the text is devoted to the description of the common minerals (200 or ~10% of those known minerals) found in most rocks. However, over 3,000 different minerals have been identified and any one or more of these may be present in a given thin section in amounts from 1-2% to a single grain.

What features may be used to aid in the identification of minerals within an individual thin section?

  1.     Shape of Crystal
  2. acicular needle-like grains (actinolite, tremolite)
      bladed elongate, slender (hornblende)
    columnar elongate with equidimensional cross sections (qtz, pyroxenes)
    equant equidimensional grains (quartz, olivine)
    fibrous grains form long slender fibers (asbestos, sillimanite)
    lathlike flat elongate grains (plagioclase)
      prismatic crystal faces defined by prism (apatite)
      tabular book shape (plagioclase)

  3.     Degree of Crystallinity
  4.   euhedral grains have well formed crystal faces
      subhedral poorly formed and/or irregular crystal faces
      anhedral no regular crystal faces

  5.     Cleavage
  6. In grain mounts, straight edges on grains generally indicate the presence of cleavage. In thin section, cleavage is difficult to recognize in minerals with a low relief, under normal circumstances, by closing the aperture diaphragm cleavage is more readily observed.

    Grains at the edge of the thin section may exhibit cleavage more so than grains in the centre, as these grains will readily break along the cleavage planes as the thin section is being made.

    The angle between cleavages is useful for identifying the broad mineral group to which the unknown belongs, e.g. if the grain exhibits two cleavages that intersect at 56 - 124° then the unknown belongs to the amphibole group.

  7.     Twinning
  8. The presence and nature of the twins in a grain may be diagnostic for that mineral. A variety of common twins may be observed in thin section, with the type dependant on the individual mineral.
    1.   simple twins - two segments of one grain go extinct at different times as the stage is rotated.
    2.   contact twins - the segments are joined by a smooth twin plane separating the segments.
    3.   penetration twins - the segments are joined by an irregular contact.
    4.   polysynthetic twinning - consists of numerous twin segments joined on parallel twin planes.

  9.     Alteration
  10. The type of alteration product observed within an individual grain may be used as an aid in identifying that grain, e.g.:

  11.     Associations
  12. Knowing the common mineral associations in a variety of rock types may aid in making an "educated guess" as to the identity of an unknown mineral.

    Knowing the association for a given rock may aid in the identification of a specific mineral which might otherwise have been overlooked.

    On Pages 113-144 of Nesse there is a list of common igneous, metamorphic and sedimentary rocks and the minerals which might be expected to occur in each.