Hyalite Opal on Augite

Mineral Matters #404

This week’s Facebook Mineral of the Week Group’s selected theme is crystals impaled or perched on another.

Technically, this is not a crystal, but it is perched. Well, draped anyway (although it is probably shown upside down from when it formed). And also, technically, it is a valid mineral, although some doubt that it should be.

Opal is amorphous silica, and perhaps should be referred to as a “mineraloid”. This particular form used to be called hyalite opal, now opal-AN (for Amorphous Network).

Hyalite is relatively common, at least down my way, and often occurs in cavities in basalt. This specimen, nicknamed “The Pipe” (by me 😁) due to its similarity in shape to the old clay pipe bowls, comes from an old, now flooded, basalt quarry not far from where I live.

Below: Hyalite opal on augite, Miners Rest Quarry, Miners Rest, City of Ballarat, Victoria. Width of view 3.5mm.