Chalcedony and Quartz   SiO2    Hexagonal-trigonal
o Hydrothermal environment. (chapters 12 and 18)
- Lower left portion is chalcedony (yellow, length fast) in part forming zig-zag extinction bands known as Runzelbänderung. Upper right portion consists of quartz crystals (blue, length slow) that have grown from the chalcedony substrate. Thin section with XP. Red-1 plate inserted from lower right. View width is 1.5 mm.