Discovering a that a Mineral is Harder than a Traditional Diamond

 

    Cubic diamond has long been considered one of the hardest minerals to have been discovered. The structure contains only carbon with each carbon atom being bonded to four others in the lattice with carbon-carbon bonds being so strong, it is not surprising that cubic diamond is one of the hardest materials on the planet. Other naturally occurring materials, such as moissanite approach diamond’s hardness, but not quite. But diamond’s reign as the hardest material has come to a close as scientists have determined that that another mineral actually holds the title for being the hardest mineral.

            In a recent article in Forbes Magazine, which is based on a paper published in Physical Review B, a mineral known as lonsdaleite was found to be a harder material than the traditional cubic diamond. The mineral is more commonly known has hexagonal diamond, since it is also comprised only of carbon atoms, but instead crystallizes in a hexagonal instead of a cubic crystal system. It was long predicted by computational studies that this hexagonal diamond should be harder than traditional cubic diamond, but, as of yet, this theoretical finding could not be reconciled with experiment. Such conformation of hexagonal diamond’s physical properties remained difficult due to the rarity of the material: it was only found to be produced in meteorite impacts, and for short periods of times in a synthetic lab. That is until a team of research scientists at Washington State University’s Institute for Shock Physics synthesized the materials long enough to characterize and measured the properties of this hard mineral.

            Based on the study, lonsdaleite proved to indeed be stiffer and stronger than regular diamonds that are ubiquitous today. But how was this rare mineral synthesized? The hexagonal diamond was made through impact chemistry/physics where gunpowder and compressed gas was used to fire graphite disks at 15,000 miles per hour towards a transparent material. The impact rapidly changed the graphite into the hexagonal form of diamond. Following this, the materials physical properties were measured through the use of lasers and sound. Since sound travels faster in stiffer materials, its stiffness could be measured. Based on their measurements, it was proved that indeed hexagonal diamonds are indeed stiffer than their cubic counterpart, and, since stiffness correlates to hardness, they are also harder.

            Such a material has a wide range of industrial applications, like in diamond saws and drill bits for instance, but with the existence of hexagonal diamonds being only for short periods of time, implementation is obviously not possible. However, as the science continues to advance, the presence of another form of diamond could become as universal as cubic diamonds is in today’s culture. This study not only provides evidence of chemists and physicists continuing to make extraordinary findings in the realm of materials, but also the ability of scientists to continually push the boundaries of current understanding.

References: 

https://www.forbes.com/sites/davidbressan/2021/03/31/scientists-create-crystal-stronger-than-diamond/?sh=a244fd53d89c

Volz, T.; Gupta, Y. Elastic Moduli Of Hexagonal Diamond And Cubic Diamond Formed Under Shock Compression. Physical Review B 2021, 103 (10).

Bundy, F.; Kasper, J. Hexagonal Diamond—A New Form Of Carbon. The Journal of Chemical Physics 1967, 46 (9), 3437-3446.

Proceedings of the Royal Society of London, Series A: Mathematical and Physical and Engineering Sciences (1913) 33, (*) (p. 277-277)