The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages (link in index at right), and a few new episodes were posted from 2015-18. Beginning in May 2019, I'm adding short entries to the blog (not as podcast episodes, at least not for now, sorry!) mostly taken from the Facebook Page posts. Thanks for your interest!

Sunday, May 19, 2019

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

Friday, May 17, 2019

An extension of the Mid-Continent Rift?

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.
All that is interesting, I guess, but the thing has much broader implications. If it is a dike – which is likely in my opinion – that suggests that it formed at a time when extension, pulling apart, was the dominant stress in this area. Dikes can form under compression, but it’s a lot easier for them to intrude if the rocks are pulling apart, opening up cracks into which magma can force itself.
The northeast-southwest orientation is also intriguing, because it points pretty much dead on at a possible branch of the Mid-Continent Rift, a pull-apart feature that runs from Kansas through southeast Nebraska, northeast across Iowa, up into Minnesota, and into Lake Superior. It’s a 1.1-billion-year-old break in North America – a break that failed to completely dismember the continent, but just formed a long narrow trough filled in many places with dense, magnetic basalt. Kind of like the Red Sea today, but not as linear.
This dike in the Texas panhandle isn’t trivial – it’s at least 45 miles (70 kilometers) long. There are additional similar features on trend with it in Kansas. My interpretation is that it represents a far away expression of the extension and intrusion related to the Mid-Continent Rift System. This possible relationship is shown in a map below.
The depth to these rocks in the panhandle is probably only 3500 or 4000 feet, but to my knowledge there is no drilling to those depths in this area, so we don’t actually have rocks to validate this interpretation. But I’d bet a beer that you’d find a reversely polarized dike of basalt or similar lithology, containing a decent amount of magnetite, that solidified around 1.1 billion years ago.

—Richard I. Gibson

Wednesday, May 15, 2019

Thulite


Thule was the far north in Greek and Roman literature, often identified with Scandinavia. Thursday was named after Thor, the Norse god of thunder. Whether these pink minerals are orthorhombic thulite or monoclinic clinothulite would take analysis that I haven’t done, but either way they contain trivalent manganese to give the pink color. 

The white mineral that contains them is scapolite, specifically meionite (I HAVE had an x-ray analysis of it), a different calcium-aluminum silicate. The outcrop, a couple miles up the Delmoe Road from the Pipestone exit, is mostly scapolite, in a zone that is the continuation of the boundary between the Butte and Rader Creek Plutons of the Boulder Batholith.