GE 114 - Optical Mineralogy: Laboratory Exercise 5
Common Igneous and Metamorphic Rock-Forming Minerals
Goal:
To identify and differentiate between common or important minerals using
optical microscopy
Items to be written up and handed in are in bold type.
In the Hand Specimen Lab last week, you made flow charts or tables of physical
properties in order to differentiate between similar-looking minerals. This week, we
will focus on optical properties as a way to differentiate minerals in thin section.
I. Using optical properties such as:
Color
Optic Sign (ex: Biaxial +)
Pleochroism
Cleavage or fracture
Isotropy or Anisotropy
Parallel or inclined extinction
Uniaxial or Biaxial
Birefringence
Relief
Make a flow chart or table that distinguishes the following minerals from one
another:
garnet
amphiboles
magnetite
micas
kyanite
epidote
andalusite
sillimanite
olivine
clinopyroxene
orthopyroxene
You don’t need to write down every optical property of every mineral: just find the
distinguishing features of each mineral. You may use information from the Atlas of rockforming minerals in thin section, Nesse, or Deer, Howie, Zussman. There are also thin
sections of almost all of these minerals available in the lab for your perusal.
II. Look at Sample N14-1, “Raymond Granite,” (used to make the threshold of the
loading dock in Arms and in the stairs and entrances of Astronomy). There is a hand
sample and a thin section. This rock contains both quartz and feldspar grains adjacent to
each other. How can you distinguish quartz from feldspar in this thin section?
III. Identify the prominent mineral in ONE of the following thin sections:
TH-20 Basalt, Kilauea, Hawaii
LA 252b Schist, Eastern Vermont
R-RA-376 Metamorphosed Conglomerate, Ralston Buttes
NAT-22 Volcanic rock, North Atlantic Tertiary Volcanic Province
February 17, 2016
1
IV. Identify as many minerals as you can in ONE of these thin sections (there
should be 3-4 in each. Drawing a sketch of each mineral and listing observed properties
will be helpful.
SG-1 Flecked Schist, San Gabriel Mts.
LA 261a Massive porphyritic schist, Eastern Vermont
February 17, 2016
2