Relief can be an excellent diagnostic property. Relief refers to the way a mineral appears to "stand out" when viewed in PP light.

Sometimes we refer to a mineral as having high relief or low relief. Minerals with high relief have sharp grain boundaries, show fractures and cleavages well, and sometimes appear to stick out above other minerals in the thin section. Relief is a relative property that depends on a mineral's index of refraction. So, we may talk about a mineral having higher relief or lower relief, to emphasize contast with other minerals in the thin section. If a thin section contains minerals that all have about the same index of refraction, none will stand out above the others, regardless of whether the index of refraction is high or low.

A few minerals, such as calcite, exhibit variable relief when the stage is rotated. This is because their indices of refraction depend on the vibration direction of the light passing through the mineral.


Quartz and Feldspar

Here we see quartz. plagioclase and, less obviously, K-feldspar are also present. All three exhibit about the same relief (because they have about the same index of refraction) and so there is little relative relief visible in these views



The large greenish brown to tan grains are biotite. The clear to very light green (PP) blocky mineral above and to the left of the biotite (and also just below the biotite) is orthopyroxene. It has higher relief than the biotite and so appears sharper and stands out from the biotite in the PP view. Cordierite is also present; it has relief intermediate between the biotite and orthopyroxene.



The large grain at the center of this view is diopside. In the PP view, the diopside can be seen to have significanlty greater relief than surrounding calcite, (stained pink to aid identification). The large grain of diopside also exhibits well developed cleavage and, in the XP view both the dopside and calcite can be seen to be twinned.


Sillimanite and Quartz

This sample contains needles of sillimanite (somewhat fractured), quartz, biotite and magnetite. The sillimanite has higher relief than surrounding quartz, and so stands out and has sharp grain boundaries (PP). In the XP view, the quartz shows low order white-gray interference colors, while the biotite and sillimanite higher order colors.