Minerals in the News: Calcite… and Invisibility!

The other night, I was scrolling through my feed reader (and honestly, I was trying to go to sleep) when I saw this story: Using Special Crystals, Researchers Make a Paper Clip Invisible. After reading a headline like that, it’s a little harder to go to sleep, especially when you find that your life-long dreams of rendering paper clips invisible are within a whisker of becoming reality.

The suggestion that these were “special crystals” in the article’s headline was somewhat surprising, given that they were described as calcite, which is one of the most common minerals in the world.  This consternation is borne out by the article’s content, which states that to render an object invisible:

“In both experiments, researchers had to finely tune their crystals—they’re technically composite crystals, as the researchers basically glue together two crystals with opposite crystal orientations—then placed them over small but entirely visible objects (MIT used a small metal wedge the size of a peppercorn; Birmingham went bigger, concealing a paperclip). “

In other words, naturally occurring calcite crystals were modified and used to precisely refract visible light. Clearly, this is more difficult than simply putting one Iceland spar on top of another and seeing something disappear (try it for yourself the next time that you have a couple readily to hand – there are obviously other mitigating factors). But it is a remarkable discovery.

Calcite Under a Green Laser

An Iceland Spar calcite crystal is subjected to the light from a green astronomical laser. Note the beam path in the crystal. Photo Credit: Personal Collection.

The optical properties of some crystals of calcite, specifically, the iceland spar rhombohedral crystal, are well-documented and thoroughly understood. When calcite crystallises in this particular form (one expression of the trigonal hexagonal scalenohedral (32/m) form), the planes within the crystal cause light to be refracted. Depending on the power of the light source, a projected beam of light fired through calcite, like that of a green astronomical laser, can result in the beam being spread out at regular intervals having been refracted along the crystal’s internal planes. When the crystal breaks light travelling through its structure, this is known as double-refraction, or birefringence.

The property is also well demonstrated by simply placing the crystal over some text, and noting the optical effect:

Text from a label doubly-refracted by an Iceland Spar calcite crystal. Photo Credit: Personal Collection.

A fossil impression of a trilobite head and upper thorax, Cambrian Era, House Range, Utah. Photo Credit: Personal Collection.

Calcite is also known to have acted as a component in the eyes of trilobites, a now-extinct arthropod species which dominated the planet for approximately two hundred and fifty million years, from the Cambrian through the end of the Permian. These complex lenses are one of many interesting features of this fascinating and long-lived group of creatures. Interestingly, the use of calcite in optical structures persists into the modern day, in the brittle star species Ophiocoma wendtii.


The rhombohedron is one particular expression of the crystal form of calcite.  Others are representative of varying conditions of temperature and pressure under which the crystals have formed.  For example, a specimen like this one from Dal’Negorsk, in Russia, is not only differently crystallised, but faintly fluorescent:

Calcite crystal cluster, 7 x 4.5cm, Dal'Negorsk, Russia. Photo Credit: Personal Collection

Calcite crystals on matrix, Somerset, England, 7 x 3.5 cm. From the same cave system which produced the "Flos Ferri" calcites (qv). Photo Credit: Personal Collection.

And, interestingly, this English calcite from the same cave system in Somerset which produced Flos Ferri aragonites exhibits an unusual expression of the 32/m form. In this case, though, the calcite is not fluorescent, for reasons which I will try to describe at length in a future posting. Interestingly, English fluorites from more northerly counties, including Durham and Cumbria, are famous for their fluorescence, the regional geology being significantly different. Again, fluorite will be the topic of another, future posting.

Calcite crystals overgrowing earlier (orange-brown) dogtooth calcite crystals. Overall size 6.5 x 5cm, Reynolds County, Missouri. Photo Credit: Personal Collection.

As I mentioned, calcite is one of the most common minerals in the world, and it occurs in a number of very interesting forms. In northern missouri, it is also one of the few minerals to be found in the local sedimentary rock.  In fact, it is common throughout the state, being found in quantity in the lead and zinc deposits of the Tri-State Area, in the Pennsylvanian-era limestones of the north, and in the east, in Reynolds County and elsewhere. To find that such a material now has an added utility and scientific value is interesting and gratifying, in the least.


Minerals in the News: Molybdenite

The lovely flat metallic hexagonal crystal habit of molybdenite, in this attractive specimen from the Moly Hill Mine.

Molybdenite is a molybdenum sulfide mineral found around the world. One of the best locations in North America is the Moly Hill Mine in Canada, which is a source of beautiful It has now appeared in the news as a new breakthrough material with potential applications in semi-conductors and nanotechnology.

It appears that, due to its nearly two-dimensional crystal structure ( see above ), molybdenite may be even better in ultra-thin applications than silicon, which forms three-dimensional crystal lattices. Additionally, this new structure will use a hafnium oxide layer, which is simply a bonus step in mineral-nerd cool, as halfnium only occurs in a handful of comparatively rare minerals (to be exact, I count all of three on Mindat, of which one, hafnon, is the halfnium analogue of zircon and thorite, and is definitely on my short list of “species to collect, urgent”… but I digress).

Diagram depicting the integration of molybdenite into a transistor. Image Credit: EPFL

For more, this article from Science Daily (“New Transistors: An alternative to silicon and better than graphene) provides an excellent overview. And while I’m not sure that I care for the title of this article ( because guess what? I had heard of molybdenite already ) but here’s the article from the Discover Magazine blog. Have a look!

Frontiers in Mineral Identification: Rendering Your Laboratory Obsolete?

One of my diversions in the past few years has been teaching myself how to do chemical analysis of mineral specimens in my collection. With the correct reagents, some glassware, acids, and a decent burner, it’s possible to tease out a lot of the fundamentals of a mineral’s chemistry just by performing various procedures and observing the results. Books like Orsino Smith’s Chemical Analysis and Determination of Minerals give a sense of the state of the art dating to the 19th century. When you move beyond the basic physical properties of minerals, this can be an enormous aid in identification (about which topic I’ll write more in a later entry).

In the 21st Century, of course, these methods of chemical analysis seem somewhat quaint, somewhat dated. And with the advent of X-ray fluorescence analyzers, these techniques may well seem positively mediaeval. Or will they?

An XRF scanner in use, from the Thermo Scientific website.

As discussed in the most recent issue of Rock & Gem magazine (September, 2010) by noted author Stephen Voynick, the advantages of the new generation of hand-held X-ray fluorescence (XRF) scanners are speed, portability, and ease of use. XRF devices take advantage of the fact that when an X-ray strikes an atom, electrons are dislodged, and in order to regain electrical stability, the electrons are replaced by other electrons which release fluorescent X-rays, the energies and frequencies of which are unique to each element. With the appropriate detector, these energies and their frequencies can be measured – that’s where the XRF scanner comes in (please read Voynick’s article for a more detailed description – link to follow when it’s available online).

Of course, there’s a price for such technological advancement – in this case, an individual unit from Thermo Scientific‘s Niton Analyzers range, sells for about $42,000. Which means that, while undoubtedly useful, my home laboratory is still substantially cheaper. And the units do have their limitations – the model tested by Voynick, calibrated for geochemical analysis, could identify only 29 elements, and nothing lighter than chlorine (atomic number 17). Not that having an instant analysis of the percentage of heavier metals in a sample is anything to sniff at. But obviously, it would be hoped that as the price drops, the number of elements readily identifiable will increase.

It’s not really a choice, of course – not for me. I’ll continue to work on identification in the traditional way: hardness tests, specific gravity, streak, chemistry… but eventually, who knows? XRF scanners could eventually become an invaluable part of hobbyist mineralogy. In the meantime, there’s still the pleasure of spending time looking at something interesting and trying to figure out just what it is… After all, that’s why most of us started out in this hobby.