Let us introduce you to The Sky Lantern, another rare rough diamond specimen from ALROSA's diamond deposit. This one is a rather complex rough diamond cluster; a shoutout, if you will, to a sky lantern made out of paper fragments.
You might have seen this one-of-a-kind rough diamond in a previous issue of ALROSA's magazine. You must be curious as to why the rough diamond looks the way it does. Well, get comfortable: you're about to hear all about this rare specimen. The Lantern is a diamond twin grown according to a certain law;; this type of twinning often occurs in a spinel. Basically, it is an intergrowth of two crystals at the beginning of their growth. Twinning results in reflection along a common twinning symmetry axis or plane, something that a single rough diamond would lack.
Two separate crystals share some of the same crystal lattice points in a symmetrical manner; their orientation toward crystallographic axes and planes is subject to strict concentration. It all becomes more clear if you imagine a corner of a building formed by the meeting of two walls. The contact of the crystal structures does not compromise the integrity, forming a single crystal with two mirrored twins.
In this case, two cubic crystals make up The Sky Lantern. However, natural dissolution made it so that the edges of the crystals became flattened; you'll have a hard time recognizing their former shape. Don't get us wrong, it's possible, but not at once.
These crystals were mentioned as early as the 20th century, with Alexander Fersman, a well-known rough diamond crystallographer and academician, looking into penetration of twin cubic crystals.
Der Diamant, a monography published in 1911 by the 28-year-old Fersman, who graduated from University of Moscow and began postgraduate work at Heidelberg University in Germany and his mentor Victor Goldschmidt, features images of similarly shaped rough diamond intergrowths, though at the time it was only Brazilian alluvial deposit specimens that they could access and analyze.
Just take a look at our Sky Lantern and Fersman's drawing: the resemblance, frankly, is uncanny. Their external morphology is very similar; this is why it looks like Fersman used our crystal as a model for his drawing.
The components of the diamond twin are virtually cubic crystals mirroring each other in the twinning plane (111) (a). Another way to explain it is to show these crystals rotated with respect to each other by 180° about the axis, i.e., the 3-fold axis of symmetry [111] (b).
This close resemblance (and a striking one, at that) implies that Arkhangelsk and Brazilian twinned rough diamonds were formed similarly, and therefore share properties.
A study into the external micromorphology of The Sky Lantern revealed that the rough diamond had had a way more complex structure even before the dissolution.
The Lantern is, virtually, two cubic crystals mirroring each other in the twinning plane (111). Another way to explain it is to show these crystals rotated with respect to each other by 180° about the [111] axis, i.e., the 3-fold axis of symmetry. This term is used in crystallography when it comes to a crystal repeating itself three times in a 360° axis rotation.
It is called a spinel law, describing a twinning crystal of cubic symmetry, occurring mostly in spinels, rough diamonds, fluorites, magnetites, loparite, etc.
Unlike simple contact twins, a twin crystal in which two individuals are joined along a plane, each segment of penetration twins is isolated by its own vertical and horizontal boundaries, following the general law of symmetry. In penetration twins, the individual crystals have the appearance of passing through each other.
The Sky Lantern is complicated by additional crystal blocks, a factor that complicated the structure greatly; we had to perform additional decoding in order to interpret the symmetrical structure.
If you look closely, you might see a complex cluster of facets along the horizontal line. Looks a bit like an accordion, doesn't it? This is because there was an additional parallel penetration, resulting in two more segments of facets surrounding the composition plane. We believe the twinning surface had to do with this patchwork of additional facets; it grew, causing parallel shifts and symmetrical twinning deformations of the crystal structure.
It also resembles the petals of the buds of two flowers mirror-oriented to each other. The mirror plane generated their rapid growth in opposite directions. By this it caused symmetrical shifts and deformations in the crystal structure.
The twin crystal clusters dissolve in the same fashion as single crystals, making way for rounded, curved shapes; there are twenty-four convex curved facets formed in place of the edges of the cube during natural dissolution, in other words, it is a transformation into a tetrahexahedroid. In the end, we have evolution from planar rough diamond crystals into rounded polyhedrons with convex surfaces. Protruding crystal peaks and edges are more prone to dissolution, making their surfaces the most curved. It causes a geometric pattern of etch pits on the relics of the flat faces. The Sky Lantern shows all these typical signs of dissolution.
Years went by, and similarly shaped curved rough diamond crystals were found in alluvial deposits in South Africa, Namibia, India, South Australia and in the Russian Urals. It is still unclear how they appeared in the first place, though. There is also a matter of Yakutian kimberlite pipes in Russia: no crystals of the like have ever been found there, while there have been plenty in Yakutian alluvial deposits in the north-east of the Siberian platform, between the Lena and Anabar rivers. Arkhangelsk kimberlite pipes are unique in that regard.
The morphology of curved rough diamonds from Arkhangelsk kimberlite pipes and alluvial deposits from other regions is determined to significant dissolution (and, consequently, loss of primary weight) of the crystal during its transportation to the Earth's crust surface in the melt saturated with fluids, a mixture of liquid and gaseous magma components, numerous studies suggest. When it comes to rough diamonds, fluids are aggressive: they are able to dissolve them in mere hours. This might be why only a small percentage of kimberlite pipes contain rough diamonds: they simply do not have time to reach the surface from depths exceeding 100–150 km (60–90 miles).
Geologists are still out on how the rich alluvial deposits in the north-east of the Siberian platform were formed, as only a few dozen of the more than a thousand kimberlite bodies discovered contain economic concentrations of diamonds, with the rest basically barren.
Anecdotal evidence suggests they came from a completely different source. For example, researchers found so-called fluidisites in the Urals; these carbonate rocks mainly form veins and veinlets in limestones and sandstones.
Regardless, it seems clear that there are no rough diamond-rich kimberlite pipes in the northern alluvial deposits of Yakutia; otherwise, they would have been long unearthed as rough diamond exploration companies focus greatly on hunting for kimberlite pipes. Besides, available evidence suggests there were no Paleozoic pipes that may have completely disintegrated since.
On the right is a rough diamond from Brazil that was mentioned by Alexander Fersman, a well-known rough diamond crystallographer and academician. On the left is Sky Lantern that looks similar but has a way more complex structure.
It is no wonder geologists have become fascinated with Brazilian penetration twins and Arkhangelsk Sky Lantern, as both boast similar morphology and therefore, origin. The Sky Lantern makes a great contribution to the scientific world, and it paves the way for new rough diamond exploration opportunities; it is promising both in theoretical and practical terms. Perhaps these exploration opportunities are a fresh alternative to the kimberlite pipes of the rough diamond-rich Arkhangelsk region; it is, after all, the place where our star was born.
The text was first published in SCIENCE First Hand, No.3/4 (92) 2021.