Thread by @astrotweeps

Peridot - Story Thread #2 (the siege dragged on ...)

Let's start with the color, because peridot has a really nice color. Not an emerald green or forest green, peridot is sort of greeny/yellow. It has a characteristic golden glow.

(pic from Smithsonian Inst. Geogallery)

The color is a great match for the spirit Chartreuse or say Absinthe (cough, ahem). Since I make gemstone stuffs for fun (as in pic), it is very appealing to me to imagine the Earth is filled with gems (however inaccurate that might be.)

But I really enjoy the idea that touching peridot is touching the inside of a planet. Olivine is not uncommon around volcanic craters (depending on magma composition.) Some, like the Kilborne Hole in New Mexico, are famous for their olivine (Pix BLM and wikipedia).

Kilborne Hole is a maar - a volcanic crater formed largely by an explosion of super heated steam. This is one of the few places on Earth where big olivine crystals are found right at the surface. Larger crystals have been taken by rock hounds, but there are still sand sized bits.

To tie this back in to previous threads, the astronauts also trained at Kilborne (and in other volcanic areas in AZ) in the early seventies. When we visited, we did find some of the basalt rocks had splashes of green glassy material.

But some spectacular samples were retrieved here in the past. Some of the "mantle xenoliths" brought up by volcanism are from 40 miles down. This xenolith shown here is encrusted with forsterite olivine. (Pic Wikimedia Commons, Rob Lavinsky, iRocks.com–CC-BY-SA-3.0 )

So if olivine is so common, and if stuff like that xenolith can be found on the surface, you might ask why we don't see this stuff all over, and why is peridot so expensive, anyway? One reason is that for a gemstone, peridot is not very 'hard.'

On the Mohs hardness scale, peridot measures 6.5 to 7 (Diamond is hardest at 10, with emeralds and sapphires at 9. Soft materials have are low, like talc at 1 and amber at 2.) So there are harder materials in the environment that can scratch peridot, assisting in its break down.

Places that might have a lot of olivine in general, don't usually have large-gem like crystals. Hawaii, for example. There is plenty of volcanism in Hawaii, and there are wide areas where the lave rocks are studded with tiny flecks of olivine.

Um. Lava. Not "lave."

Anyway, the black basalt holding the crystals breaks down faster than the olivine. The tiny flecks of olivine are liberated. And if the conditions are right, these olivine grains can collect into a pile of green sand.

I'm sure many of you have been to this location - the Green Sand Beach on the Big Island of Hawaii. A green sand beach may form in any area where the nearby rocks are rich in olivine, and where the shape of the coastline can protect the sand. (Wikimedia Commons CC 2.0)

I just CAN'T find my pic of us frolicking on that beach. Bah. Instead here is a close up view of green sand particles. As long as the sand is replenished at the same rate it weathers or is washed away, the beach remains.

(Pic - Green Sand from Mahuna Beach, Wikimedia Commons)

As a public service - please do not damage or take rocks from any of these locations. If you are there as a geologist w a permit to take small samples, then ok. But as a visitor, always leave it as you found it. Let's protect these wonders; they belong to the world (literally).

Hawaii has beaches of so many colors, red, white, black - but on this beach you get to walk barefoot on piles of tiny, glittering green gemstones. I felt a little like a dragon with a hoard of gems ... (but of course I left them for other little dragons to find, in their time.)

As noted, olivine is a common mineral in the solar system, even if not often found in glittery gem style. But it can be! Just as the Earth was once molten, so were other bodies. They had their own differentiation stage that allowed denser materials to sink and less dense to rise.

You can see the results of this in pallasite meteorites. Meteorites are of course stones found on Earth that are actually extraterrestrial. The pallasites might be the most beautiful of these, combining different amounts of metal (Fe/N) and glorious olivine.

It was thought that pallasites came from the interiors of asteroids where the olivine mantle met the Fe-N core. But recent studies have show that things are not so straightforward. Pallasites are mysterious. Here is Imilac, encrusted with golden crystals. (Pic - aakova CC 2.0)

The best as far as I'm concerned are the pallasites with the exact mix of olivine and metal that they look like stained glass. This is the Esquel meteorite (wait I need a napkin, I'm drooling.) (Pic from Bistrosavage, Creative Com. CC 2.0) The universe is gorgeous at all scales.

To tie this all back around - how do those meteorites get here? They are liberated from asteroids by impacts; impacts that might have actually shattered the parent asteroid into pieces. Pieces whose orbits eventually cross Earth's. It's a very convenient delivery service!

To finish this thread by waxing poetic - Something as simply beautiful as peridot is found in abundance in Earth's mantle, and is one of the most basic minerals in space rocks. Of course, the Earth is really just a big space rock, itself. All these areas of science are connected.