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