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.