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