Basic principles of infrared microscopy and its application in fluid inclusion studies of opaque minerals

Abstract

Nowadays fluid inclusion petrography and microthermometry are widely applied techniques for investigating
various geological processes. The most common minerals in the process of fluid inclusion microthermometry are
primarily quartz, calcite and other transparent minerals. However, many minerals in the 400–700 nm wavelength (visible
light) range are opaque. The IR radiation has lower energy than the visible light and therefore its energy is not large
enough to excite the movement of an electron from the valence band to the conduction band; thus the light is not absorbed
by the mineral. Consequently, the mineral is transparent in IR light. Several opaque minerals — such as pyrite, enargite,
stibnite, molybdenite, haematite, etc. — have been found to be transparent in IR light. Microthermometric studies on
these minerals have indicated that their fluid inclusions acted in such a way as to preserve certain stages of the geological
processes; otherwise, these could not have been reconstructed on the basis of conventional studies of fluid inclusions in
transparent minerals. The IR microscopy and microthermometry have recently been introduced into routine practice in
the fluid inclusion laboratory of the Department of Mineralogy at the Eötvös Loránd University. The calibration
measurements indicate that the “green house effect” on fluid inclusions in enargite — which is caused by the thermal
energy of the IR radiation — does not reach the extent of that limit which could effect the geological interpretations of
microthermometry data. This can be explained by the properties of the analytical system, given that all the instruments
have been optimized for the transmission of the IR radiation.
IR microscopy and microthermometry represent a powerful new field for fluid inclusion microthermometry and
could open up new areas for the study of various geological processes.