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