History of the Earth

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

More generally, the rock is a rhyolite porphyry – rhyolite meaning pretty high in silica (a granite-like composition) and formed at or near the surface of the earth, and porphyry meaning it has two grain sizes – a fine matrix, with larger crystals of quartz (and microcline feldspar) suspended in that matrix. This implies that there were two periods of cooling, one at deeper depths where it took the larger crystals a longer time to cool (and grow), followed by a later, quicker period of cooling, so the matrix crystallized so fast the grains are very small, but the larger, older grains are still there within the matrix.

All that cooling happened about 1,093,000,000 years ago (almost 1.1 billion) during a time called the Grenville Orogeny (orogeny means mountain-building) when what is now central Texas was amalgamated to the main part of the North American continent. The llanite was probably an aspect of the intrusions of the Town Mountain Granite, which has similar age but crystallized at greater depth. It’s part of a long belt that extends with some discontinuity to central Tennessee, through Kentucky and Ohio, then northeast across Ontario, Quebec, and into Labrador. Rocks now in southern Scandinavia were part of the Grenville mountain belt, the result of a collision between continental masses that was assembling the supercontinent Rodinia over a long period of time, from about 1,250 million years ago to 980 million years ago.

The llanite, in the form of a narrow dike, intruded older rocks toward the end of the Grenville Orogeny. The mountain belt continued into Mexico, and rocks of similar age are found in Australia and Antarctica as well as South America today. Exactly how those rocks fit into the big picture is still debated, but one version of the assembled continent of Rodinia is in the comments.

At left, one reconstruction (others exist) of Rodinia about 750 million years ago, just before it began to break up. “Rodinia” is from Russian for “motherand,” or “to give birth,” alluding to this continent’s early place in the rifting-collision cycles (called Wilson Cycles with respect to ocean basins, for J. Tuzo Wilson) that have followed. Even so, Rodinia was probably preceded by at least one earlier supercontinent, named Columbia – but that’s debated.

 

The llanite dike intrudes older Precambrian metamorphic rocks called the Valley Spring Gneiss, which have a lot of magnetite in them. The Valley Spring Gneiss is dated to about 1,375,000,000 years ago or older.

The phenocrysts (“showing” or “visible” crystals) of blue quartz in the llanite are supposed to be “beta quartz,” a high-temperature, higher symmetry form of quartz that can crystallize only above 573ºC – in fact, it cannot even exist at surface conditions and pressures, so all “beta quartz” is actually a pseudomorph (“false form”) of regular quartz that has formed as the original beta quartz cooled. This little crystal (not from Llano) is probably one such pseudomorph. (Technically, since I knew you wanted even more jargon, it’s a paramorph rather than a pseudomorph, because while the crystal structure has changed, the chemistry has not.) The essentially perfect hexagonal symmetry of this crystal marks it as a beta-quartz shape, versus the lower (trigonal) uneven symmetry typically displayed by normal quartz. It’s possible for normal trigonal quartz to have equally developed faces that appear fully hexagonal, but it’s unusual.

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.