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 a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Sunday, October 18, 2015

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 


  1. Richard,
    Thanks so much for your History podcasts, etc. New listener....stone mason, spelunker,amateur geologist.
    In a band, of old real geologists, named "Sixling Twin".
    Terry Waggener
    Old Rag Mountain