GeoEvelyn has declared this Geology-Picture-A-Day Week and I thought I’d join in. Mostly since I need a couple of things identified.
Found this chunk of aa (rubbly basalt) in the roof of a collapsed lava tube right next door to where we stayed on the Big Island of Hawaii. What are the growths/precipitates on the aa surface? They look like tiny little mushrooms but are actually crystalline. American nickel for scale.
Two possibilities occur to me- calcite as a result of reaction with atmospheric CO2- quite common on basalt, or vapor phase minerals, which are also pretty common (though less so) in volatile-rich volcanic rocks. Try fizzing one of the white spots with acid. They look kind of chalky, and I’m betting they’re calcite. Vapor phase mins often look glassy to glossy, not so chalky.