Myrmekite is a metasomatic texture comprised of rods of dendritic quartz in a crystal of plagioclase. This texture forms as plagioclase replaces K-feldspar, releasing SiO2. Because the anorthite component of plagioclase requires less silica than K-feldspar , silica is released and crystallographically oriented quartz microcrystals form. This process is common and often deuteric.
The above example is one of the least spectacular examples one could find, but it's in my favorite granite (the 75 Ma Butte Granite).