Chalcedony and Opal

Chalcedony SiO₂ Hexagonal-trigonal and Opal SiO₂H₂O (paracrystalline)
o Hydrothermal environment. (chapters 12 and 18)
- (A) (PPL) Clear central portion (SE-NW) is layered chalcedony, formed at the expense of opaline silica (dark portion with colloform layering, NE and SW areas).
- (B) (XP) Layering and fibrous nature of chalcedony is readily evident. Fibrous, weakly birefracting nature of opaline material (NE and SW) is evident indicating that this silica is not completely amorphous.
- (C) (XP + Red-1) Fibers oriented SW-NE in central chalcedony zone are yellow with red-1 plate inserted, indicating fibers are length fast (confirming chalcedony). Note two thin layers that are blue-green indicating the silica here is either quartz, moganite, or quartzine (both length slow, normal to layering). NE and SW areas are dominantly bluish red indicating minimal birefraction and dominance of the magenta of the red-1 plate with XP. This material is may be fibrous opal-CT (lussatite). Thin section view is 3.2 mm wide.

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