Overview and images of pegmatite as a rock type are here: Pegmatite
Pegmatites are coarse-grained igneous rocks. Here is a sample from Norway which is composed of three minerals: spessartine (manganese-bearing garnet group mineral), muscovite (light-colored common mica), and white feldspar.
The sample was originally described simply as “spessartine”. It is obvious why, such a nice specimen which shows so many large garnet crystals is by no means common and it is easy to neglect the other components. I like individual minerals as well but perhaps even more I like the assemblages of minerals (which we call rocks) and the stories associated with them. That’s why I try to understand what could be the other minerals to see the broader picture.
The only instrument I have used to analyse the sample is my pair of eyes but I have no doubt that the greenish gray flaky mineral is muscovite. Muscovite is a very common mica, especially in pegmatites. The white mineral is a little trickier. It looks like feldspar but which one? White feldspar like this could be Na-rich plagioclase (albite) but K-feldspars (orthoclase, for example) may be very similar. I do not believe it could be Ca-rich plagioclase because muscovite is usually associated with felsic rocks which host K-feldspars and sodic plagioclase.
I think it is plagioclase because on the other side of the rock I saw that the crystal is in some places composed of narrow lamellae which reflect light differently when looked at a certain angle. I try to look for that if I suspect that the mineral might be plagioclase. It probably isn’t pure albite because these lamellae are not present in near-pure end-members of the plagioclase series. So, it could be oligoclase for example which is the next mineral in the plagioclase series after albite. In albite, up to 10% of the sodium is replaced with calcium. The percentage is 10…30 in oligoclase.
Spessartine (red), muscovite (gray), and plagioclase (white) form this beautiful pegmatite from Ljosland, Norway. The width of the view is 9 cm. The specimen belongs to the Museum of Geology of the University of Tartu.
Another side of the same sample. The width of the view is 6 cm. Polysynthetic twinning is visible in the lower right which indicates that the feldspar is plagioclase.