The temperature dependence of hydrous components in minerals

A summer undergraduate research project of

Paul Magyar

OH and water molecules in the nominally anhydrous minerals can be readily detected by infrared spectroscopic methods. One of the challenges is determining when water is present as fluid inclusions rather than as bound molecules in the coordination sphere of cations. Two of the criteria we use are the temperature depencence of the  IR or NMR spectrum as a function of temperature. Water in liquid inclusions will usually freeze at or slightly below 0°C. When it does, the change in the infrared spectrum is particularly obvious.

The change is not obvious, however, in a set of minerals that have nano-inclusions (dimensions on the order of a few 10's of nm). These do not freeze in the fashion of larger inclusions and are furthermore difficult to identify because they are so small that they are essentially invisible under the optical microscope.

To characterize water in both nano- and micro-inclusions we are examining the temperature depencence of water in selected minerals that are known from scanning electron microscope studies to have small inclusions.

Figure 1, below, shows the typical temperature depencence of the OH bands in a minerals with only bound OH ions. The mineral is grossular garnet from East Africa.

Grossular OH spectrum

Click on the image to animate the spectrum as a function of temperature.