SPECIMEN: Olivine-rich Basalt

LOCATION: Hawaii

FORMULA: Olivine: (Mg,Fe)2SiO4

MOHS: Olivine: 6.5-7 Basalt 5-6 (we could represent this with a green and a gray circle for each of them?)

Olivine is a super abundant mineral in our planet’s upper mantle, but these big crystals of it are much harder to come across on the surface. And the clue to how these olivine crystals reached your hands is in the gray material around them: Basalt. Big olivine crystals like this only really make it to the surface if it’s flung upwards in volcanic eruptions.

The presence of olivine tells us about the volcanic eruption itself, too. Olivine is heavier than most of the stuff you’d find in a magma chamber, so it usually settles to the bottom. But if the volcanic eruption happened quickly and with a lot of force, the olivine gets hurled out before it can sink. So finding basalts full of olivine is a pretty lucky break. Maybe that’s why olivine is green?