MOLYBDOMENITE-P21/c is a new mineral from an old classic locality, described by the youngest student investigator ever to do so. EEPS graduate student Tom Zhang talks about his discovery and the classic technique he used.
The El Dragon Mine, located in Potosi, Bolivia, opened in the early 1970’s and was abandoned in the late 1980’s, yet is still considered the richest and most complex selenium mine in the world. It is also the type locality for more than 20 selenium-bearing minerals. Tom Zhang's discovery of molybdomenite-P21/c, though not a truly unique type, provides a rare natural counterpart to a synthetic version that was, ironically, created the same year Zhang was born.
Reported in the September issue of The Canadian Journal of Mineralogy and Petrology, molybdomenite-P21/c is a lead selenite (PbSe4+O3) polymorph- a different crystal form for the same chemical composition – of the original type mineral discovered in Argentina in 1882, and twin to a recently synthesized version developed in 2001.
Zhang’s mineral and its synthetic sibling, designated as alpha-molybdomenite, also differ from the original molybdomenite (designated beta-molybdomenite) by the temperature regimes from which they form and can change. Knowing the form is important because these materials have novel physical properties that allow their use as a semiconductor in visible-infrared photodetectors, displays, transistors, thermoelectric devices, and photovoltaics. Structural differences associated with temperature may impact how these materials can be utilized.
The discovery was made in 2022 from material Zhang purchased from an old collection. As Zhang tends to collect minerals from special localities, he acquired numerous specimens to investigate the possibility of finding something new.
“I'm a collector, so I know how to look at minerals, and if I know something is common, then I will also know which one is special. So even if there are colorless minerals, like dolomite, anglesite, and cerussite from El Dragon, I can tell the difference when it’s something I haven’t seen before. I went through all the crystal-bearing samples, but I found nothing that looked new to me. Then an idea came to my mind, what about if I break those already small pieces to smaller, then try to find some new or fresh vugs, then maybe there may be some new stuff hiding.” says Zhang.
The piece he broke open was just a couple centimeters across to start with, and a vug containing the new mineral crystals was even smaller, less than one centimeter across.
“I didn't want to break it too small, because otherwise you may break those minerals, right? So once I looked at this sample, it looks like the perfect environment or space for a new mineral. I immediately figured out that the three little pale-yellow crystals in the center had a different looking [crystal] habit.”
A Raman scan later, Zhang was certain he had something not reported from this locality. Raman is a spectroscopy technique that is non-destructive and can be performed on material in situ. A laser stimulates vibrations in the mineral’s crystal lattice, resulting in a spectrum whose peak positions can be compared against a database of known minerals.
“I thought this may be molybdomenite, but the peak position was slightly offset from previously published data. To get further confirmation whether it's a new or not, you need to know the crystals most basic structure. That's why we need to use single crystal X-ray diffraction.”
Single crystal X-ray Diffraction requires patience and a steady, skilled hand.
Zhang brought two devices with him – goniometers - that are used to conduct X-ray diffraction studies of minerals. A goniometer is a device that either measures an angle or allows a mounted object to be rotated to a precise angular position. One of them (pictured) was a gift from his former University of Arizona advisor and world-renowned X-ray mineralogist Hexiong Yang, given in honor of his accomplishment.
In this case, the goniometer was used to mount a single crystal at the precise angle needed to direct the X-rays to obtain specific measurements of the spacings between the atoms that make up the crystal’s lattice. The simplest dimensions that can be measured from this technique are known as a crystal’s unit cell.
“This is why only few people find most of the new minerals. It's not only that you have to know the scientists who can make the measurements, it also requires that you know how to prep the sample. If you cannot prep the sample, you cannot get the analyses right, and you are not going to be the one who gets to report it,” says Zhang.
According the Zhang, he can prepare samples in as little as 10 minutes, although doing so required a great deal of practice. Having mounted more than a hundred for X-ray diffraction prior to this one, his advisor was pleased. “He was shocked, but he was very happy, because he found someone who can really do it well,” says Zhang.
Mounting a crystal is an intricate, multi-step process, each with a high probability of failure.
The crystal is mounted onto the tip of a glass fiber, about 50 microns in diameter and a few cm long, that is handmade by heating larger diameter glass rod to melting, and then stretching it to almost hair-like thinness. It needs to be thin enough to hold a similar sized crystal but long enough to manipulate and attach to a bronze holder that fits on top of the goniometer.
He starts by making several glass fibers, and practices mounting other crystals, such as the black one in the picture. “You also have to know how to prepare and select the crystals before they are mounted. Because everything is so small, all the steps are done looking through a microscope. The microscope I was using at the University of Arizona was more than 20 years old,” laughed Zhang.
The crystals are size sorted and gathered as a group with one of the fibers covered with a bit of sticky resin, then transferred to a microscope slide. The resin must be removed using alcohol so that crystals can then be seen individually. Once Zhang determines which crystal is the target, a new fiber is used with new resin to mount the crystal. The most nerve wracking yet necessary step is using additional alcohol to remove any resin that may be stuck to the crystal, without unmounting the crystal from the tip of the fiber. This is where the final modification of the crystal is made for the analysis, all while it is attached to the fiber. “Sometimes you need to cut the crystal into a nice shape, like little cube or rectangle, while it is attached. You cannot allow it to be too big, and you cannot make it too small, because it's not going to give you a good result.” Zhang went on to say that this 30+ year old technique does have some modern easier mounting techniques, yet those techniques don’t always provide as precise a set of values for the crystal’s atomic positions.
Once the crystal has been modified to Zhang’s satisfaction, the resin is removed completely and the crystal is remounted on a new glass fiber using superglue, which also must be done precisely such that no superglue comes between the incident X-rays and the crystal.
“I cut the glass fiber to make the tip into a sharp needle.” The tip is then dipped into a droplet of superglue. “If you have too little, then it won't hold the crystal. If you put too much, then your results are noisy, yet either way it may move or drop and you can lose a crystal.” Zhang says that he painstakingly holds the fiber on the tiny crystal between 10-20 seconds and hopes that he gets good attachment to the glass fiber.
Discovering natural occurrences of industrially developed compounds is quite rare.
It turns out that Zhang’s molybdomenite crystals were a little more complicated to measure because they are polycrystalline. Yet Zhang was able to orient his sample such that he was able to get all the data necessary to show that it was a new natural version of the synthetic alpha-molybdomenite.
“After we got the structure, we were able to map the largest unit cell. The unit cell is unique to each mineral, even for polymorphs which share the same chemistry. Once you have the unit cell, you can compare it to the inorganic chemistry database to see if there is a match, or in this case, a synthetic analog.”
While the result was not entirely what Zhang was hoping for, the existence of the synthetic analog provided some additional interesting information.
As part of creating synthetic molybdomenite, tests of its properties and observations of its behavior with respect to environmental factors are reported. Of particular interest to Zhang was the stability of the various forms with respect to temperature change.
As it turns out the makers of the synthetic form observed an irreversible phase transformation from alpha- to beta-form of molybdomenite between 315 and 355 °C, thus providing a lower limit for the natural conditions from which Zhang’s molybdomenite was formed and implying that the host rock did not experience any additional heating.
“Material science has maybe 10s of millions of man-made chemicals. And the good thing for us is, if there wasn’t a synthetic mineral, it is actually harder to publish, because you need more evidence,” concluded Zhang.