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.
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.