The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Friday, January 8, 2016

Episode 375: Garnets






Garnet is the birthstone for January, but there really isn’t a single mineral named ‘garnet.’ It’s a group of at least 15 minerals of differing chemistry, but only six are common. They are three aluminosilicates combined with iron, magnesium, or manganese, and three calcium silicates in combination with either iron, aluminum, or chromium. The definition of a mineral is something with a specific chemistry and a definite crystalline structure.

All six garnet minerals have essentially identical crystal structure. They are in the isometric or cubic crystal system, meaning their three crystal axes are perpendicular and the same length – that’s what isometric means, same length. But garnets seldom form cubes. The molecules are of sizes that result in many-sided forms, including dodecahedrons, with 12 faces, and trapezohedrons with 24 faces, and more complicated shapes, all of which tend to make garnets almost spherical. Often, they look like little soccer balls in the rock.

The word garnet is from an early English word that meant dark red, as many garnets are, but it may ultimately be derived from the words pomum granatum, Latin for pomegranate – because pomegranates have bright red seed covers similar in appearance to common garnets.

And they’re pretty. Red, purple, yellow, brown, orange, bright green for uvarovite, the chromium garnet – and they’re hard, usually around 7 or higher on the Mohs hardness scale. This makes them great for two things: gemstones and abrasives, and they’ve been used for both since the Bronze age.

By far the greatest use of garnets, by volume, is as abrasives. The US uses about 190,000 tons of garnets every year, with about 17% of that mined in New York’s Adirondack Mountains and in Idaho. The rest, 83% in 2014, was imported mostly from Australia and India and a bit from China. India produces about half the world’s industrial garnet, with China second with a third and Australia 15%. The US is a distant fourth with only 2% of world production.

You benefit from garnets every day, even if not directly in the rough surface of an emery board or sandpaper. Waterjet cutting is used to cut and shape metals from steel to aluminum as well as plastics and glass. Besides their use as abrasives, about a fifth of garnet consumption in the United States goes to filtration in water wells.

The value of industrial garnet in the US is about $9 million. Garnets are used for gemstones, but because they are so common, their value is nothing like that of diamonds, sapphires, rubies, and others, most of which are also harder than garnet and much rarer. But garnets can still make beautiful jewelry.

Do I have any garnets in my collection? Yup. Hundreds of thousands of them, but most of them are microscopic, in a rock with a blue mineral called glaucophane from the California Coast Ranges. The biggest one I have is maybe an inch or so across, from Ontario, but at Gore Mountain, New York, there are garnets as much as two feet across in what is possibly the largest garnet deposit in the world. They are in metamorphic rocks related to a continental collision about a billion years ago, called the Grenville Orogeny.

So happy birthday to everyone with a January birthday. Have a garnetiferous day!

—Richard I. Gibson

Saturday, November 28, 2015

The Cenozoic Episodes



Running time 3 hours

We are up to the Cenozoic Era in the monthly episodes. This one combines the 31 episodes from December 2014, covering the Cenozoic, into one episode, and it completes the packaging of each month of episodes from the original series.

As usual for this monthly compilation, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest in this project. If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

For 2016, I’m going to TRY to have more frequent new programs than I did in 2015. Some upcoming topics, many suggested by you listeners, include fluorescence in minerals, questions about the oxygen crisis back in the Precambrian, the Tepuis in South America, and more. Thank you!

—Richard I. Gibson

Saturday, October 24, 2015

The Cretaceous Episodes



Running time 2 hours 30 minutes

We are up to the Cretaceous Period of the Mesozoic Era in the monthly episodes. This one combines the 30 episodes from November 2014, covering the Cretaceous, into one episode.

As usual for this monthly compilation, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions – or contact me by email at rigibson@earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson