Zinnwaldite is one of the mica minerals.
It is not a mineral anymore because The International Mineralogical Association that attempts to take care of the mineralogical nomenclature decided that there is no need for this mineral name. Possibly because it is compositionally like a middleman between siderophyllite and polylithionite. I am not sure whether the name has been discredited completely or is it ok to use the term for the compositions between the two mentioned before.
However, the sample shown below deserve to be called that way no matter what because they are from Zinnwald/Cinovec at the German/Czech border in The Ore Mountains which obviously gave name to that mineral.
Zinn means ‘tin’ in German. That does not mean that zinnwaldite contains tin but the mineral often associated with it does and is the principal ore of tin. This mineral is cassiterite.
It is compositionally similar to pink mica lepidolite (they both contain lithium and fluorine) and resembles biotite in appearance. Zinnwaldite is darker than lepidolite because it contains iron. Iron always makes the color of minerals darker. Minerals that contain lots of iron are often black (common pyroxenes and amphiboles like augite and hornblende, for example).
Zinnwaldite occurs in granitic pegmatites and greisens (often with cassiterite). It is also frequently associated with fluorite, lepidolite, spodumene, topaz, beryl, tourmaline, and monazite.
Zinnwaldite from Zinnwald. Width of sample 7 cm.