Opal-AN

A variety of Opal-A

Opal-AN is a term for amorphous opal with a glass-like structure (Graetsch, 1994). The "A" in the name stands for amorphous; the subscript "N" is to imply its structure is network-like similar to silica glass; however, it still contains about 3-8% water, both as molecular water and bound in silanol (Si-O-H) groups.

In the study of Lee et al. (2022) opal-AN was shown to mainly comprise aggregates of differently-sized nanospheres (<5 nm) that form banded features. The opal-AG studied comprised close-packed silica nanospheres, ~400 nm in diameter.

This form of opal occurs as hyalite in volcanic and pegmatite environments where the silica deposits from the gas phase (Flörke et al., 1973) and forms globular and irregular crusts. Because of its homogeneous internal structure, it does not show opalescence and is transparent if pure. The refractive index varies slightly with water content and lies between 1.44 and 1.46 (Graetsch, 1994).