Mineralogy

A major focus of attention is the relationship between the spectroscopic properties of minerals and their composition and structure. Topics include trace hydrous components in minerals, metal ion site occupancy, effects of natural ionizing radiation, and X-ray amorphous materials.
Click here for more information on Mineralogy at Caltech.

Hydrous Components in Nominally Anhydrous Minerals

Hydrogen is one of the most pervasive elements in the crust and upper mantle of our planet. It is a mobile, reactive component which can cause major changes in the chemical, physical, rheological, and electronic properties of the host phase. Much of the effort of our group is directed at investigating the possibility that hydrogen, in some chemical form (typically OH- and H₂O), can enter the structure of major minerals which are usually formulated as anhydrous. These studies rely heavily upon spectroscopic methods of analysis because they can determine both the chemical species present and, with proper calibration, the amount of hydrogen present. ... more ...

Spectroscopy of Minerals

Spectroscopic probes ranging in energy from gamma rays to microwaves play an important role in contemporary mineral studies. They are used to identify the cations which are present in a mineral, their concentration, their crystallographic site, and the identity of ions in the immediate vicinity of the target ion. Our students and postdocs have played a prominent role in the development and application of these methods. Special attention is directed at systems where the spectroscopic response is not proportional to the sum of the response of the individual components. Intervalance charge transfer processes which involve two or more cations which can exist in different oxidation states are most important. The Fe²⁺ - Fe³⁺ and Fe²⁺ - Ti⁴⁺ interactions, which are particularly important in establishing the color of many common minerals such as micas, amphiboles, and pyroxenes, have been the object of much research.

Radiation-induced Damage

Most minerals are subjected to a pervasive, long-term in situ exposure to natural, low levels of ionizing radiation such as the gamma rays from the decay of ⁴⁰K. Over geologic time, the effects of this radiation can cause substantial changes to the properties of the affected mineral. Most prominent are the electronic (oxidation state) changes which result from the ejection of electrons from valence orbitals of individual ions. These changes are often evident through color phenomena in the mineral (e.g. smoky quartz). We have been concerned with the characterization of the specific changes which occur in the radiation damage process, especially those involving transition metal cations in minerals such as beryl, feldspars, tourmalines, and zircon. In a study of tourmaline we showed that what is now pink elbaite tourmaline from the pegmatites of the Southern California batholith must have been nearly colorless at the time of crystallization (Mn²⁺) and only developed its pink color after millions of years exposure to ionizing radiation which caused the oxidation of Mn.

The Mineral Spectroscopy Server

Provides on-line information on the color of minerals; spectra of minerals (VIS, NIR, IR, Raman); Data files as ASCII (wavelength, absorbance) pairs; and an extensive reference list to optical spectra of minerals sorted by mineral and author.

Plus special topics including information on manganese oxides; desert varnish; and the ametrine variety of quartz