Raymond Dedeyne and Rik Dillen (Belgium)
This is a story in two parts about two fake mineral specimens of a double salt, written by two authors with the same initials…
You can find fake mineral specimens all the time at mineral shows, but also on location. A good example is Morocco, where unbelievable quantities of fake amethyst and other geodes are sold along the roadside. Any collector, who knows something about minerals, will easily recognise such fakes and may, on somewhat closer inspection, discover lots of glue, even with the naked eye.
Two years ago in Gent, the first of our two authors (Raymond) bought, at a show, an attractive specimen consisting of several, very well-formed crystals (up to 2.5cm wide) on a matrix of gypsum crystals. At first sight, the crystals looked black, but, on closer examination, they appeared deep red-violet because octahedrical crystals of this sort are “penetration-twins” (crystals that have the appearance of passing through each other in a symmetrical manner). The seller claimed that the locality where it was “found” was Rudna in Poland.
It is difficult to blame the man too much, as he told Raymond from the beginning that the crystals were man-made. He explained that he had found the piece himself in a Polish salt-mine that, after exploitation, had been filled with liquid waste from an alum mine. Therefore, the crystals would have been formed by the evaporation of water, which lead him to label the specimen as “alum”, based on the origin of the liquid waste.
Usually, Raymond doesn’t collect man-made specimens, but this piece looked very attractive and the price was no obstacle. Also, the story intrigued him and he wanted to find out what the correct chemical composition.
The mineral appeared to be soluble in water and the resulting solution was blue-green. It was also soluble in diluted acids without any effervescence or gas production. Therefore, he concluded that it was not carbonate or sulphide. To make sure, he decided to use heavier artillery, namely an X-ray fluorescence spectrometer, to determine the composition (limited to elements heavier than sodium). It turned out that the major elements were chromium, potassium and sulphur. That immediately rang a bell! Chromium alum, with formula KCr(SO4)2.12H2O, is a chemical product used in various industrial processes (dyes, photography, textile and leather processing, and so on). This product also forms octahedrical crystals that you can grow artificially up to sizes of several centimetres.
It followed that the seller had told the truth in part – the crystals were manmade. But part of his story could not be true. The chromium alum crystals could not have grown in a flooded mine. Rather, the crystals probably grew in a water-logged cellar or on a leaking ceiling. And, where did all that water-soluble chromium comes from? The story that he found the specimen himself in a mine just didn’t make sense!
Recently, more and more such rubbishy stuff has been appearing at mineral shows. Typical examples are those spectacular specimens labelled “hemimorphite”, “zinkite” or with other fancy names. In fact, these are zinc-oxide aggregates from the chimneys of badly working, zinc-metallurgical installations.
The second author of this double story (Rik) went through another, more or less analogous story. A few years ago, he bought a collection from a club member, who had unfortunately passed away. This individual had been a very experienced and systematic mineral collector, and Rik ended up keeping almost half of his specimens for his own collection. One of the specimens that intrigued him immensely was a 10cm by 10cm item with very large, purple-violet, half-transparent, octahedrical crystals.
The label clearly identified it as being fluorite, but no locality was mentioned (which was a surprising in this otherwise splendid collection). Chromium alum immediately came to mind, because Rik had grown chromium alum crystals using the chemicals from a “Merit” chemistry toy set. It was difficult to imagine that the former owner of the piece would not have known that this was an artificial specimen, as the collection consisted of over 1,000 mineral specimens, systematically labelled with both name and locality.
However, on ultrasonic treatment in water, the crystals did not dissolve … so it could not be chromium alum. Nevertheless, they possessed a number of characteristics that pointed in that direction. The crystals displayed a soft, satin lustre that did not appear very natural. The crystal form (penetrating octahedrons) and the colour were far too spectacular for fluorite, and Rik found no similarity to any other naturally occurring mineral. In addition, for centimetre-sized crystals, the specimen was quite exceptional. Some colleagues of his were also convinced it was chromium alum, but this was contradicted by the fact that it was not soluble in water.
Eventually, Rik broke one crystal off the matrix and treated it ultrasonically in water in a reaction tube. And guess what? It quickly dissolved. Conclusion – it was chromium alum after all.
Subsequent careful examination suggested that the individual crystals on the matrix had been treated very carefully with a water resistant varnish. Therefore, as long as they stayed on the matrix, they could not be affected by water and the deception could not be discovered. It was only after breaking off the crystal that the unvarnished fracture could be attacked by water. Patient observation with a stereomicroscope then revealed traces of the varnish (with microscopic air bubbles being apparent).
It was a real work of art and had really puzzled Rik, who has 30 years of experience as a mineral collector and scientist. Taking into account the difficulty he had had in finally unmasking the crime, the disguise was almost perfect!
I should point ou that, if you create such crystals (and why not?) and then sell your creations, you must state that they are “chromium alum crystals, grown artificially”.
Our thanks to Chris Young for supplying the photo of the synthetic octahedrical crystal of chromium alum by Eric Johnson.
This article first appeared in Dutch in Geonieuws 31(6), 158-160 (2006). Geonieuws is the magazine of the Mineralogische Kring Antwerpen.