Cone-in-Cone

The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.

It’s not certain how these things form, but some kind of systematic displacement because of microscopic crystal growth variations is probably the favored idea. The variations might be because of clay content (which in my specimen might help account for the dark color), or because of changes in volume when aragonite (chemically identical to calcite, calcium carbonate, but a different crystal structure with a different volume) changes to calcite which can happen during diagenesis, the process of sediment solidifying to rock.

Cone-in-cone might also result from pressure variations, either before or after the rock becomes solid. Pressure variations that might depend on the clay content could produce micro-fractures in the calcite that make the individual crystalline material slide consistently to make the cones. This more structural interpretation might be supported by the fact that my specimens are from a seam of calcite about 3 or 4 inches thick that was within thicker, stronger rocks.

Bottom line, the features are caused by some kind of microcrystalline displacement, but exactly how this happens is not settled.

This specimen is from near White Sulphur Springs, Montana. Collected in 2004.

Link: Cone-in-Cone on Wikipedia

—Richard I. Gibson

Epitaxy

Epitaxy, from Greek words meaning “upon” or “above” and “ordered arrangement,” in minerals means crystals of one (or the same) mineral growing in a particular crystallographic position on another (or the same) mineral. It happens because the molecular spacings and orientations happen to be similar, allowing, even encouraging the crystal structure of the second, later mineral to mesh with that of the first. Some mineralogists might say that epitaxy requires the two minerals to be different minerals, but I do not – just two distinct generations of crystallization.

My example here is calcite, calcium carbonate – the sharp brown crystals are rhombohedrons, and the stuff is probably brown because it may be slightly iron-bearing (but it’s not siderite, iron carbonate). The clear crystals sit preferentially upon the corners of the rhombohedrons. I’m pretty sure, but because calcite makes a myriad of crystal forms I’m not certain, that the rhombohedral corner of the brown crystals represents the basal pinacoid position in those crystals, and the complex saucer-like colorless second-generation crystals are poised there on their own basal pinacoids. The two pinacoid surfaces have the same molecular geometry, so the two different generations of crystals – brown and colorless – joined there. 

The colorless crystals show a bunch of different forms, prisms, rhombohedrons, and probable scalenohedrons, along with the likely pinacoids.

This is all in a geode about 5 centimeters across, from Mt. Sterling, Illinois. The little crystals in the photo enlargements are about 1.5 millimeters across. I actually have both halves of this geode, although they were acquired from different dealers at different mineral shows a year or so apart.

Epitaxy isn’t especially unusual in the mineral world, but unless the minerals are in a particular crystallographic orientation, we’d probably just call one mineral growing on another an encrustation, or overgrowths or some similar word. 
—Richard I. Gibson

Thulite

Thule was the far north in Greek and Roman literature, often identified with Scandinavia. Thursday was named after Thor, the Norse god of thunder. Whether these pink minerals are orthorhombic thulite or monoclinic clinothulite would take analysis that I haven’t done, but either way they contain trivalent manganese to give the pink color. 

The white mineral that contains them is scapolite, specifically meionite (I HAVE had an x-ray analysis of it), a different calcium-aluminum silicate. The outcrop, a couple miles up the Delmoe Road from the Pipestone exit, is mostly scapolite, in a zone that is the continuation of the boundary between the Butte and Rader Creek Plutons of the Boulder Batholith.

Artinite and the Coast Ranges of California

The mineral is artinite, a magnesium carbonate that is an alteration product of high-magnesium serpentinites of the Diablo Range in central California. It’s from New Idria, a mercury (and other) mining district named for the historic Idria mercury mines now in Slovenia, which have been mined since 1490.

Tectonically, the New Idria area of California is in a fenster (German for window), a hole in a thrust sheet that is related to the collisions that produced the California Coast Ranges. Serpentine’s mechanical properties are such that it can flow – very slowly on human scales – and in places like Mt. Diablo and New Idria, it has risen buoyantly like a salt dome to breach the thrust sheet and bring the underlying serpentine to the surface.

The magnesium-rich serpentinite is part of the Coast Range Ophiolite – a word meaning “snake rock” for its often greenish color and scaly texture. Ophiolites are usually bits of oceanic crust that have been emplaced within or upon continental crust, and that’s what happened here. 

Fuchsite

Fuchsite is a chromium-bearing variety of the mica muscovite, K(Al,Cr)₃Si₃O₁₀(OH)₂ . Chromium gives the green color to the muscovite.

This specimen was collected by Theresa Schwartz and given to me by Susan Vuke. Theresa reports it was a clast in a pediment near Little Antelope Valley, northwest of Harrison, Montana. That leaves the question open as to where it formed and came from, but a source in the Tobacco Root Mountains is most likely. The pediment gravel itself was probably deposited fairly recently, the last 50 million years or so, a result of the uplift of the Tobacco Roots. There are at least two chromite occurrences regionally nearby, 3 miles southwest of Silver Star (in the Highlands Mountains), and 5 miles southeast of Sheridan (southwestern Tobacco Roots) (Chadwick, 1941 Montana Tech thesis).

A reasonable idea for the protolith is a well-sorted, clean, quartz sandstone, with a very few resistant detrital chromite grains that during metamorphism had the chromium mobilized to find potassium and aluminum impurities from wherever you want to form the mica. The chromite could have come (with obvious challenges and difficulty) from the Stillwater Complex, the chromite- and platinum-rich ultramafic deposit on the north flank of the Beartooth Mountains, supposing some fluvial system to transport the sand that became this quartzite. That could have been a Precambrian river system, eroding either from what is now the Beartooths to the east or from the minor chromite occurrences to the west.

Fuchsite was named in 1842 by Karl F. Emil von Schafhäutl in honor of Johann Nepomuk von Fuchs [May 15, 1774 Mattenzell, near Bremberg, Lower Bavaria, Germany - March 5, 1856 Munich, Germany] professor of chemistry and mineralogy at University of Landshut and curator of the mineralogy collection. (from MinDat)

Barite with inclusions

Beginning with this post, the blog is "blog only" - no more podcasts, at least not for now.

The brownest zones of barite in this example result from dense inclusions of deep reddish material, probably hematite, in blebs smaller than a hundredth of a millimeter barely visible in the image. The larger inclusions you can see here are brassy spheres, triangular prisms, and elongate pencils, and are probably pyrite but chalcopyrite is possible. The lower right inset shows the pencil forms a little better than the main image.

The upper right inset is the entire 2.3-cm specimen. I don’t have a locality, but I attribute it to Frizington, Cumbria, England, because of the barite forms and inclusions, the associated white calcite (colored pink in places by hematite), and the presence of a few tiny hematite blades on the surface.

Zoning in crystals results from changes in the environment while the crystal is growing. In this case there were variations in the iron (and maybe copper, if some of the inclusions are chalcopyrite) content in what was otherwise a solution dominated by barium and sulfur (barite is barium sulfate).

Zoned crystals are common, although this one is kind of dramatic and I think the pencil-like inclusions, standing in ranks perpendicular to the crystal faces, are unusually weird and cool. It would be more difficult for such things to form than for a barite crystal surface to be coated with something and then have more barite deposited over that coating.

—Richard I. Gibson

Episode 389 Vanadium

Vanadium is a metal, and by far its greatest use is in steel alloys, where tiny amounts of vanadium improve steel’s hardness, toughness, and wear resistance, especially at extreme temperatures. As I reported in my book What Things Are Made Of, more than 650 tons of vanadium was alloyed with iron to make the steel in the Alaska Pipeline, and there’s no good substitute for vanadium in strong titanium alloys used in jet planes and other aerospace applications.

Vanadium isn’t exactly one of the well-known elements, but in terms of abundance in the earth’s crust, most estimates indicate that there’s more vanadium than copper, lead, or tin. But it’s difficult to isolate, and it wasn’t produced chemically as a chloride until 1830, when Swedish chemist Nils Sefström named it for the Norse goddess of beauty, Vanadis, perhaps better known as Freyja. It wasn’t until 1867 that pure vanadium metal was isolated by British chemist Henry Roscoe, whose work on vanadium won him the name of the vanadium mica roscoelite.

As a mineral collector, I’m attracted to vanadinite, lead vanadate, because it forms beautiful hexagonal crystals, often bright red and so abundant from one lead-mining area of Morocco that excellent specimens can be had without mortgaging your house. Some vanadinite crystals are like perfect little hexagonal barrels, and others can form needle-like spikes around a central crystal, making the whole thing look like a cactus with caramel-orange spines.

Some of the vanadium for making steel alloys comes from primary mined vanadinite, but much more was once produced as a by-product of phosphorous manufacture, because it’s commonly associated with phosphate rock. And today, a lot of the world’s vanadium comes from refining crude oil and from fly ash residues, which are products of coal combustion. I got curious about why vanadium metal is so closely connected with these organic deposits.

Crude oil actually has lots of trace elements in it, including metals like gold, tin, and lead, but by far the most abundant are nickel and vanadium, as much as 200 parts per million nickel and 2000 parts per million vanadium in some crude oils, especially heavy, tarry oils like those found in Venezuela. In some oil, the nickel and vanadium can add up to 1% by weight of the oil, an incredibly huge amount. Refining Venezuelan crude gave the U.S. a lot of vanadium back in the late 20^th century. But why is it in there?

Oil and coal are both the result of decaying and chemically changing plant matter. Forget dinosaurs; virtually all oil, natural gas, and coal comes from plants – usually marine algae for oil and gas and more woody, land-based vegetation for coal. There’s a class of organic molecules called porphyrins. I’m no organic chemist, but these complex hydrocarbon molecules, made of carbon, hydrogen, oxygen, and nitrogen have boxy ring-like structures with open space in the centers. Chlorophyll and hemoglobin are related chemicals, both of which contain metals in the middle of the structure, magnesium in chlorophyll and iron in hemoglobin. The vacant holes in the centers of porphyrins in crude oil are ideal for trapping metal molecules, and apparently vanadium, in the form of a VO2 ion, is one of the easiest to trap because of its molecular size and electronic valence.

The vanadium comes from the original oil source rock, so there’s quite a range in vanadium content around the world. Heavy oils, like the tars in Venezuela, hold more than fluid oils like those in Saudi Arabia. This has more or less been known since at least the 1920s, and today the vanadium and other metal contents of oils are being used to characterize the original source rocks even when those source rocks no longer exist or are no longer what they once were.

The United States has had no mine production of vanadium since 2013 and even then we were 94% dependent on imports. Today 100% of our vanadium is imported, and we also produce some vanadium from imported crude oil and ash. More than 90% of the world’s vanadium is mined in China, Russia, and South Africa, although the US imports much of what it needs from the Czech Republic and Canada as well as Russia. We also imported enough ash and refining residues to account for 9000 tons of vanadium in 2015, mostly going as I said to making steel alloys. A new emerging use is in high-capacity storage batteries, where vanadium compounds make the electrolyte. These batteries have potential uses for renewable energies such as wind and solar power, and although in 2015 and 2016 several companies were working on prototype designs, they’re still pretty expensive batteries.

Way back in 1971 when I was a teaching assistant for the Indiana University Geologic Field Station, on one mapping project we went to the Mayflower gold mine south of Whitehall, Montana. I collected a bunch of rocks with interesting looking sparkly crystals – some of which I’ve only recently gotten around to really studying. I gave a talk at the 2017 MontanaBureau of Mines and Geology Mineral Symposium on minerals from there that turned out to be vanadium-bearing, including vanadinite, although it’s probably an arsenic-rich variety, and stranger minerals like descloizite, a lead-zinc vanadate, tangeite, calcium-copper vanadate, and some others. I even think there are some tiny bits of roscoelite, the vanadium mica named for the chemist who first prepared vanadium metal.  

Even more exciting for me are some tiny, millimeter-sized red-orange crystals in the specimens I found at the Mayflower Mine. All I knew for a long time was that I couldn’t figure out what they were. By looking at their crystal shapes and properties, I narrowed it down to two very strange and very rare minerals – gottlobite, a calcium-magnesium vanadate, and calderónite, a lead-iron vanadate. Both of these minerals are so obscure I didn’t really seriously imagine I had actually collected one of them. But, thanks to an analysis by Stan Korzeb, the economic geologist at the Montana Bureau of Mines and Geology, it turned out that I did indeed find calderónite, 32 years before it was described as a new mineral in 2003. Stan’s analysis in January 2018 used EDX, or energy-dispersive x-ray spectroscopy, a technique that gives not only the elements present in a mineral, but their relative proportions, which allowed Stan to calculate the chemical formula. The lead-iron vanadate calderónite he found is intergrown with descloizite, a lead-zinc vanadate. This probably indicates changing iron-zinc concentrations in the fluids that precipitated the minerals. This represents just the 11^th documented calderónite occurrence in the United States and the second in Montana. Stan identified the first in Montana in the fall of 2017.

It’s an obscure mineral, and the crystals are tiny, but it made this mineral collector’s day.

—Richard I. Gibson

Link:

USGS Mineral Commodities - Vanadium (PDF) 

Episode 384 Kyanite

Today’s topic is three minerals with the same chemical formula: Kyanite, Andalusite, and Sillimanite.

 

How can three things with the exact same chemical formula, Al₂SiO₅, be different minerals? Many of you probably recall that besides a distinct chemical composition, a mineral has a definite crystalline structure. And these three minerals each have completely different crystallography.

The basic reason for the different crystal structure is that the chemicals aluminum and silicon, arrange themselves differently depending on conditions of pressure and temperature. Kyanite forms at relatively low temperatures over a wide range of pressures while sillimanite crystallizes at relatively high temperatures, generally above 700º C over a similar range of pressures to kyanite. Andalusite develops in a more limited temperature-pressure field, call it medium temperatures but always relatively low pressures.

All that variety happens under metamorphic conditions, when rocks are undergoing lots of changes such as those that happen when continents collide, or when subduction scrunches some parts of the crust against others. So that means these minerals are usually found in metamorphic rocks, and in fact they are called index minerals for the particular conditions that they represent.

Kyanite is probably the most familiar of the three. It’s often a beautiful blue color, making long, lath-like crystals, so it’s popular with collectors. Kyanite also has a nearly unique, and diagnostic property. Whereas most minerals have a particular hardness, kyanite has two. On the Mohs hardness scale, kyanite is 5 in the direction along the length of the crystals, but 7 across them. Together with the color and crystal habit, this makes kyanite pretty easy to identify.

Andalusite and sillimanite are less common. But andalusite also makes interesting crystals, especially when carbon gets included in the growing crystals. That can produce a distinctive elongate four-armed cross, a variety called chiastolite that is sometimes polished to make jewelry. Sillimanite certainly can also make nice crystals, but I guess I’ve led a sheltered life, or maybe I just haven’t mapped enough metamorphic rocks. I’ve never seen a large sillimanite crystal in the wild, just fibrous, wispy, almost feathery coatings in metamorphic rocks like schist and gneiss.

So these things are cool collectible minerals and they help geologists figure out the pressures and temperatures that formed rocks, helping unravel the geologic history of the places where they are found. But they also have economic value.

Kyanite and andalusite especially are mined to make mullite, another aluminum silicate that’s pretty rare in the natural world but pretty common as a synthetic material made from kyanite. Toilet bowls, which you might call porcelain, are more or less mullite. Most of it is made from a clay mineral, but kyanite can be added to improve its toughness and stability. And small amounts of kyanite go to making abrasives in things like automobile brake shoes. But by far the greatest use of kyanite and andalusite is in making mullite for refractories – ceramics that retain their strength and remain chemically inert at very high temperatures. Furnaces, kilns, and crucibles in the iron and steel industries are often constructed with mullite bricks, and steel making consumes something like 70% of all the aluminum silicates produced worldwide.

The United States is the world leader in producing kyanite. It’s mined at four places in Alabama and Georgia, where the metamorphic rocks of the Appalachian Mountains contain abundant reserves. US mine production of kyanite, at about 100,000 metric tons a year, is more than we need, so we export about a third of what we produce – one of only a handful of mineral commodities that the US is self-sufficient in. The total value is around $30 million a year. South Africa produces more andalusite than the US produces kyanite, so it’s the world leader in producing this stuff, and India and Peru are the only other significant commercial producers of aluminosilicates in the world.

Price and production of kyanite is sensitive to the world economy because of variations in the steel industry, but for the past few years the price of kyanite in the US has been fairly steady at around $300 per ton. Kyanite mines in the US employ about 150 workers, and mullite plants account for about 240 more.

Kyanite’s name is from the Greek word kyanos, meaning blue. Think “cyan.” Andalusite was originally described from specimens thought to be from Andalusia, in Spain, but actually from a nearby province. But the name stuck. Benjamin Silliman, a geologist at Yale and founder of the American Journal of Science, gives his name to Sillimanite.

—Richard I. Gibson

Kyanite - USGS mineral commodities
Gigapan image of kyanite

Episode 378: Pegmatites

If you’ve listened to this series for any length of time, you know that geologists, like most scientists, are fond of jargon. They use specialized words as shortcuts for particular meanings, like ‘gabbro,’ which is easier to say than “a coarse-grained igneous rock with considerable iron and magnesium, often dominated by the minerals pyroxene and plagioclase.” I try to explain jargon terms in this podcast when I use them, since my goal really is to help understanding.

Today’s jargon word is pegmatite. It’s from a Greek word meaning “to stitch together,” and the crystals in pegmatites often do show a complex interlocking texture. But the key thing about pegmatites is that the crystals in them are big. Sometimes really big.

There isn’t really a legal definition in geology for “big,” but pegmatites usually have crystals bigger than one or two centimeters, and some would use 2.5 centimeters, one inch, as an arbitrary cut-off. But it isn’t just size that matters.

Pegmatites often, but not always, have a more or less granitic composition, so they often contain large crystals of quartz and especially feldspar. But they often form late in the history of an igneous granite body, and because those late-stage magmas contain superheated water, they can mobilize and concentrate elements that solidify into minerals at relatively low temperatures as the molten granite cools. Minerals containing lithium, boron, tantalum, and rare earth elements are pretty common in pegmatites.

The crystals grow to large sizes because they have longer times to grow – that’s the basic difference between rocks like granite and rhyolite, which have essentially the same composition, but granite cools slowly and has large crystals while rhyolite cools more quickly and has a very fine-grained texture. Think of pegmatites as the slowest of all, slow enough that in some cases crystals tens of feet long can form. At the Etta Pegmatite in the Black Hills of South Dakota, spodumene, a lithium silicate, grew into huge, log-like crystals up to 42 feet long. The mine there was an important source of lithium for years.

Tourmaline is another fairly common mineral, actually a group of minerals, found in pegmatites. It’s a complex boron silicate that often makes long, black rod-like crystals, but it’s sometimes beautiful green, pink, and other colors, even sometimes zoned from inside to outside like a watermelon. Tourmaline group minerals are hard, around seven on the Mohs harness scale, so the gemmy colored varieties are sometimes made into jewelry.

Besides their value as sources for large, collectible mineral specimens, pegmatites in some places are valuable economic resources, like the spodumene containing lithium I mentioned a minute ago. Rare earths, beryllium, and tantalum are often found in pegmatites, ultimately finding their way into things like cell phones, automobile brake shoes, and capacitors in computers.

In addition to containing large crystals, pegmatites can be big themselves. Some of the pegmatites in South Dakota are more than a mile long, but just last fall I visited a little one in the hills east of Butte, Montana, where I live. That one was no more than a meter, three feet, across, but it did have pretty cool feldspar crystals in it more than six inches long.

—Richard I. Gibson

Pegmatites of the Black Hills

Episode 376: US Mineral Dependency

  

Welcome to the History of the Earth podcast where we discuss all things geological. I’m your host, geologist Dick Gibson. There’s been a long hiatus in my production of this podcast, and I hope to rectify that with some new episodes. Today’s topic for Episode 376 is U.S. mineral dependency.

It’s a common misconception, perhaps an expression of “American exceptionalism,” that the United States is self-sufficient in most or all of the mineral commodities we use in our stuff every day. Nothing could be further from the truth.

In my book, What Things Are Made Of, published in 2011, I documented the uses for everything from arsenic to uranium, and where the U.S. gets its supply of each. At that time the U.S. was self-sufficient, a net exporter, of only 19 of the eighty-plus commodities the U.S. Geological Survey tracks. Today, in 2017 the situation is no better – the count is down to 15 items for which the U.S. is a net exporter.

Some of those items are low-value but interesting things like boron, kyanite, and diatomite. Only three, gold, iron ore, and molybdenum, are high-value metals.

A lot of the mineral commodities we import are obscure, but the vast majority of technologically active Americans use them every day. For example, indium, critical to making flat-panel displays for televisions and computers, is all imported, mostly from Canada and China. Zinc is a well-known metal used mostly in galvanizing iron to prevent rust and in brass and bronze for everything from door knobs to saxophones. In the U.S., 82% of it is imported, mostly from Canada and Mexico.

What about that vital bomb-making element, uranium? Hardly any is used to make bombs in the United States any more, but it helps generate electricity in 61 commercial nuclear power plants across the nation, and almost 20% of our electricity comes from nuclear plants. Where does that uranium come from? Only about 4% of it is mined in the United States. The rest is imported, with Canada and Kazakhstan providing close to half, and Russia, Australia, and Namibia supplying most of the rest.

How about something as common as a flashlight battery? Dry-cell batteries are made with zinc, carbon, and a pasty electrolyte of ammonium chloride and manganese dioxide. All the manganese used in the U.S. is imported, with more than two-thirds of the manganese ore we use coming from Gabon in Central Africa. By far most of the nearly 700,000 tons of manganese the U.S. consumes goes to steel alloys, where it helps make the steel resistant to abrasion and stronger in impacts. That makes it a common alloy in bicycle frames and mining tools.

Or consider common salt. Even though there are 64 plants in 16 states, with Kansas leading the way in production, the United States still imports about a quarter of all the salt we use. About half the salt consumed goes to highway deicing, but a third or so is used to make a wide variety of chemicals, including plastics like polyvinyl chloride or PVC. Food processing and common table salt amount to just 3% of the salt used in the U.S. Chile is the largest source for salt imports, with Canada and Mexico second and third.

The point of my book What Things Are Made Of and this brief set of examples is simply to help you recognize the profound level of globalization that exists in everyday products Americans use.

Thanks for listening, and I hope you’ll join us next time for another episode of the History of the Earth. 

—Richard I. Gibson

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

Episode 374. Twins

This week, our topic is twins. Twinned crystals.

When two, or sometimes more, crystals of the same mineral share volumes or surfaces within the crystal lattice, they grow together in interesting ways. One common kind of twin is essentially a reflection across a plane, a plane that is shared by both crystals. They can end up growing perpendicular to each other, or at a specific angle in what are called contact twins. One common type of contact twin is in the common mineral quartz, silicon dioxide, when two crystals grow at almost right angles to each other, sharing a common plane. This kind of twin is called a Japan Law twin because they are found frequently in quartz crystals from Japan, although they were first described from localities in the French Alps.

Sometimes multiple crystals grow together. Aragonite, calcium carbonate, crystallizes in the orthorhombic crystal system, which means it has three unequal but perpendicular crystal axes that form the basis of the molecular crystal lattice. Think of a shoe box, with three different edge lengths all at right angles to each other, that’s an orthorhombic crystal lattice. But in aragonite, sometimes particular planes within the crystals serve as common surfaces for crystallographically distinct crystals, and they grow together to form near-perfect hexagons. This can be confusing when you’re trying to identify a mineral, like aragonite, which you know is not hexagonal but there you have these nice hexagons. They’re multiple twins, actually composed of three separate crystals of aragonite grown together in a pseudohexagonal form.

The other primary type of twin is called a penetration twin, where two distinct crystals share not just a plane, but an entire volume within the combined twin crystal. They end up looking like the two crystals penetrate each other. Probably the most famous example of a penetration twin is the mineral staurolite, an iron-aluminum silicate. It’s also orthorhombic and usually forms little box-like crystals, typically like the shoe box but with one dimension often a lot thinner than the thinnest dimension of a shoe box. If two crystals share the center of the box, you end up with twins that look for all the world as if one crystal of staurolite has penetrated through the other. You can get two different angles in this sharing that make forms that look like crosses. The two crystals can cross at a 60-degree angle or at 90 degrees, making what are called cruciform – cross-shaped – twins. Such twins in staurolite are relatively common.

Fluorite, calcium fluoride, is another mineral that often forms beautiful penetration twins. Fluorite is isometric, meaning it has three perpendicular but equal crystal axes. In terms of fluorite crystals, this means that instead of that shoe-box shape, you often get perfect cubes where all the edges have the same length. If two cubes grow together and share a volume of molecules, you get twins that can be pretty cool.

There are quite a few different twin laws, and they really are “laws” to the extent that these kinds of intergrowth can only follow specific patterns, all controlled by the geometry of the crystal and the size and arrangement of the molecules in it.

Thanks for your interest!

—Richard I. Gibson 

Episode 370: Pseudomorphs

Malachite pseudomorph after cuprite showing octagon and dodecahedron faces.
From Chessy, France. Specimen is just over 1 cm in maximum dimension.
Photo by Richard I. Gibson.

Today for Mineral Monday my special guest is Kyle Eastman, mineral collector and geologist, here to discuss with me some special minerals called pseudomorphs. These are “false form” minerals that have the shape of one mineral but are actually something else because of replacement, substitution, encrustation, or some other process.

We discuss some classic pseudos from Chessy, France and Corocoro, Bolivia, as well as one of Kyle’s favorite skarns from Arizona.

Running time, 19 minutes.

—Richard I. Gibson

The Mineral Episodes

This episode is a package containing all the previous episodes of the podcast related to minerals and mineral deposits. There are 16 of them, and the running time is about an hour and thirty minutes. Thanks very much for your interest.

—Richard I. Gibson