Show Notes
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The brownest zones of barite in this example result from dense inclusions of deep reddish material, probably hematite, in blebs smaller than a hundredth of a millimeter barely visible in the image. The larger inclusions you can see here are brassy spheres, triangular prisms, and elongate pencils, and are probably pyrite but chalcopyrite is possible. The lower right inset shows the pencil forms a little better than the main image.
The upper right inset is the entire 2.3-cm specimen. I don’t have a locality, but I attribute it to Frizington, Cumbria, England, because of the barite forms and inclusions, the associated white calcite (colored pink in places by hematite), and the presence of a few tiny hematite blades on the surface.
Zoning in crystals results from changes in the environment while the crystal is growing. In this case there were variations in the iron (and maybe copper, if some of the inclusions are chalcopyrite) content in what was otherwise a solution dominated by barium and sulfur (barite is barium sulfate).
Zoned crystals are common, although this one is kind of dramatic and I think the pencil-like inclusions, standing in ranks perpendicular to the crystal faces, are unusually weird and cool. It would be more difficult for such things to form than for a barite crystal surface to be coated with something and then have more barite deposited over that coating.
—Richard I. Gibson
More from 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.
Mineral Monday + Tectonic Tuesday. Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
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.
In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.