Chemistry Reference
In-Depth Information
Observations with the
petrographic microscope
Rocks and minerals are observed as parallel-sided thin sections through the
petrographic microscope, a transmit light microscope using polarized light.
By convention, the thickness of a thin section is 30 micrometers (
m). The
thin section may be thicker for some specific uses, such as the study of fluid
inclusions. The opaque minerals are studied using the reflected light micro-
scope. The use of these two types of microscopes requires specific training
and experience.
The purpose of such observations is to identify the minerals forming the
rock, study the relationships between these minerals and the structure of the
rock and thus reconstruct its own history.
The first observation of the phenomenon of double refraction was
due to Bartholin around 1698 (Touret, 2006). But it was the discovery
of the polarization of light by Malus (1807) that provided to this author
an explanation of this phenomenon (1808). In the early nineteenth cen-
tury Brewster (1781-1868), studying fragments of minerals under the
microscope, identified the position of the optical axes, defined uniax-
ial and biaxial minerals, observed for the first time fluid inclusions and
was able to measure the indices (of the minerals and the fluid inclusions)
with an accuracy better than 0.05. The first thin sections date from Nicol
(1815) and Oschatz (1852). But it is mainly due to the work of Sorby
(
On the Microscopical Structure of Crystals
, 1858, first observation of
fluid inclusions in thin section) and the development, by Frankenheim, of
the petrograpic microscope in 1860 that this device actually entered into
the methods of petrography. The late nineteenth century saw the develop-
ment of the petrographic microscope (Ernst Karl Abbe, Camille Sebastien
Nachet, Emmanuel Bertrand) with the technological development of the
microscope and the systematic description of minerals in thin section. The
German school was first prominent: Zirkel (
Microscopische Petrography
,
1876), Vogelsang (
Die Kristalliten
, 1875), Rosenbuch (
Microscopische der
Mineralien und Gastein Physiography
, 1873-1877), Becke (
Becke line
,
1890). Russian Fedorov perfected the universal stage (1883). The end of
the century is dominated by the French School (Ecole des Mines, Museum
μ
