Big Valuable Crystals: Pegmatites (GeoShort)

Explicit content warning

07/01/21 • 18 min

What type of rock has really big minerals and lots of uncommon elements? It's a pegmatite! Pegmatites contain lots of important elements and minerals, both for our economy and for mineral collectors.
Join us for a tour of pegmatites. We give quick low-down on how pegmatites form. They contain really large mineral grains, which usually implies that they cooled slowly. But, pegmatites form from the leftover fluids from magma chambers and contain all the uncommon (or unpopular!) elements that get concentrated. Once they reach a certain concentration they form rare minerals that are concentrated in things like Lithium, Cesium, or Beryllium!
The minerals in pegmatites grow so fast because the fluid they grow in is much less viscous (more watery) than magma. This means that atoms and ions can diffuse much more quickly to the sites of mineral nucleation, allowing for massive (really really massive, up to 30 ft) crystals of minerals like spodumene and beryl to form.

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Previous Episode

The Spartans Last Stand: Geology of the Hot Gates

Have you ever watched the movie 300 or heard of the Spartans making a stand against the Persian Empire? It's an amazing story right...but it has some amazing geology behind it too.
Geology has influenced historical events in momentous ways, and here on PlanetGeo we are going to start highlighting these ways. This incredible story of the Hot Gates is just the first part of that series.
In this episode we are going to cover

Let’s start out with the words: Hot Gates. First, the Gates part of that. What formed the gates? Well it’s a fault like the Grand Teton National Park, and the tectonics of this region are really complex! We will start narrow (at the gates themselves) and zoom out to the broader region and talk about how complex plate tectonics can be on the 3D surface of the planet.
We will talk about the Hot part of the Hot Gates, and highlight some hydrothermal activity in the region. This includes hot springs, steam, and volcanoes!
Lastly, the Hot Gates don’t exist anymore. The planet is always changing, sometimes rapidly. Sea level has changed over time in this region since 480 BC, 2500 years ago. The sea level change is not due to actual sea level changing though, it is due to a totally different process: the valley is filling up with sediment that pushes the shoreline back from the cliffs that used to form the Hot Gates. This is totally normal stream behavior in areas with active mountain belts!

So, here’s the backstory. The Persian Empire was massive and was interested in invading Greece. There were several major battles here but we are focusing on one in particular where 300 spartan warriors stood their ground against at least 50,000 persians and maybe many more (history is foggy here).

They were able to fend off so many adversaries by using the local geology to their advantage, they took a stand in the Hot Gates, which were a series of three narrow passes between a cliff face and the Aegean Sea. This is a really interesting part of geoscience in that much of history is shaped by geoscience.

Next Episode

Dammed Up Again: Streams and Dams Part Two

We liked talking about dams so much we did it again! Join our discussion on streams, how the normally operate, and how dams disrupt the way that streams function.
We highlight a few things in this episode, including a recent Eos article on sediment accumulation behind dams - expertly written by Tullos et al., the Paonia Reservoir where sediment accumulation has filled in the reservoir in record time, and another article on contaminated sediment releases from the Milltown Dam and the Hudson River that have caused big problems in the past, written by James Evans.
Chris also gets his long but warranted rant on the famous Aswan High Dam, and key example of the debates around the pros and cons of dams.
Also check out our previous episode on Dams at this link or anywhere you get your podcasts.