Here’s a rock I’ve been dying to talk about! This little guy is orthoclase feldspar! You might be familiar with the feldspar family, as it contains gemstones such as sunstone, moonstone, labradorite, and amazonite. What’s really, really interesting about this piece of feldspar is its shape. This cool formation is called a Carlsbad Twin. Let me tell you about it!
Feldspar crystals often form as flat-topped, six-sided prisms which are strongly skewed sideways. Here’s an example of another feldspar in my collection, amazonite, which shows off this crystal habit really clearly!
The two crystals in my piece of orthoclase are making that common feldspar shape! But you’ve probably also noticed how they appear to be mirror images of each other, and also growing through each other.
That’s Carlsbad twinning!! It’s a type of penetration twinning, which means the crystals appear to be penetrating or passing through each other. But looks can be deceiving; what’s ACTUALLY happening here is much cooler. These crystals are conjoined twins! One hasn’t penetrated the other; they’re actually sharing some of their molecules like human conjoined twins might share organs.
This can happen because of the shape of feldspar’s crystal lattice - the orderly arrangement of its molecules.
The crystal lattice won’t match up perfectly throughout the entire structure, but it WILL match up right along the C axis, where we flipped it. Along this line the grid of molecules aligns, and our twin crystals can share them. The place where lattice points are shared is called the composition surface.
Simply put, molecules don’t know what shape they’re making on a macro level. The shapes they make are by complete accident, because that just happens to be what shape you get when you arrange molecules in that kind of grid. The only thing the molecules know is how they can connect to other molecules. It doesn’t matter to them which direction the crystal is growing in, or which direction the crystal lattice is facing. If they can connect up in a way that completely flips the crystal lattice, well… they don’t know they’re doing that! They’re just doing what molecules do!
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