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The temperatures were still relatively high: we know they must have been around 600°C, because the garnets formed alongside a high-temperature version of quartz called tridymite.

It was tridymite, a mineral that is rare on Earth and has never seen before on Mars.

When crystalline grains or blebs of quartz occur, we have a quartz-trachyte; when tridymite is abundant, as in the trachyte of Co.

It was of the texture and roughness of granite, but more heavily shot with quartz, or tridymite than any other granite he'd ever seen.

As they cooled, they sometimes fractured, and tridymite ended up growing in their cracks.