The real, rare stu
Forget diamonds and rubies. These are the world’s rarest gems.
SCIENTISTS have inventoried and categorised all of Earth’s rare mineral species described to date, each sampled from five or fewer sites around the globe. Individually, several of the species have a known supply worldwide smaller than a sugar cube.
These 2,550 minerals are far more rare than pricey diamonds and gems. But while their rarity would logically make them the most precious of minerals, many would not work in a wedding ring setting. Several are prone to melt, evaporate or dehydrate. And a few, vampire- like, gradually decompose on exposure to sunlight.
Their greatest value to humanity lies in the tell- tale clues they offer about the sub- surface conditions and elements that created them, as well as insights into the planet’s past biological upheavals. In fact, rare minerals represent Earth’s truest distinction from all other planets, according to authors of a paper in the journal American Mineralogist.
Scientists Robert Hazen of Carnegie Institution and Jesse Ausubel of Rockefeller University say that knowing fully the mineral signature of our life- supporting planet – understanding the distinct combinations of circumstances that create rare minerals – also hints at what an inter- planetary probe might find.
Their paper, On the Nature and Significance of Rarity in Mineralogy, establishes the first system for categorising rarities in the mineral kingdom and provides mineralogists with a framework that parallels one used for understanding rare plant and animal species.
Not so rare
The authors note the irony that precious gems and other minerals highly valued by humankind – including so- called “rare earth” minerals required to make electronics – do not meet the definition of rare as far as planet Earth is concerned.
“Diamond, ruby, emerald, and other precious gems are found at numerous localities and are sold in commercial quantities, and thus are not rare in the sense used in this contribution. Uses of the word ‘ rare’ in the context of ‘ rare earth elements’ or ‘ rare metals’ are similarly misleading, as many thousands of tonnes of these commodities are produced annually,” they wrote.
There are 5,090 recognised mineral species. Of these, fewer than 100 make up 99% of Earth’s crust. Around 2,550 are defined as rare – found at five or fewer locations worldwide. More than two- thirds of the known mineral species, including the great majority of rare species, have been attributed to biological changes in Earth’s near- surface environment. Each rare mineral fits into these categories:
> Unique conditions that created the mineral
“Imagine making minerals at a kitchen stove using a pressure cooker,” says Hazen. “What results in the pot is a function of variables temperature, pressure and the ingredients, and is one or more of just 72 chemical elements that make up Earth’s mineral kingdom.”
Some minerals are rare because, even though they form from the commonest of ingredients, they must be cooked at exquisitely controlled conditions. For example, the mineral hatrurite is formed from three of Earth’s most abundant elements, calcium, silicon, and oxygen, but only in temperatures above 1,250° C and in the absence of another extremely common element, aluminium.
By knowing the idiosyncratic combination of circumstances involved in a rare mineral’s creation, scientists can deduce what elements are or are not present at a specific depth and in some cases, such information as acidity at that level.
> Planetary constraints
Other minerals are extremely rare because their ingredients are almost never found concentrated in Earth’s crust. Thus, such scarce chemical elements as beryllium, hafnium and tellurium form relatively few minerals and most species are rare.
> Ephemeral minerals
Some minerals form under unusual conditions – extreme cold or dry environments, for example – but then simply melt, evaporate or dehydrate when exposed to different surface conditions. A crystalline form of methane hydrate, found in core samples from Arctic drill sites, evaporates at room pressure.
More than 100 mineral species can persist in dry environments for many years, only to be washed away during rare rain events. Among the least stable are rare mineral species that adsorb moisture from the air then dissolve in it. And a few, like edoylerite, metasideronatrite and sideronatrite gradually decompose on exposure to sunlight.
> Places geologists rarely sample
Some rare minerals simply come from under- sampled regions, from extreme environments such as the flanks of erupting volcanoes, frigid and remote regions of Antarctica, and the deepest reaches of the oceans. Other minerals that may be much more common than are represented in mineral museums include a host of species that are difficult to recognise based on their lack of bright colours or showy crystal faces. Most mineral collectors favour eye- popping specimens for their display case.
Also, some minerals occur only at the micro or nano- scale. A number of rare minerals known only from Otto Mountain in southern California, for example, have been discovered recently through the use of hightech instruments.
Most mineral experts are familiar with at best a handful of the 2,550 obscure rarities, says Hazen, citing the mineral fingerite from El Salvador as “a perfect storm of rarity.” Fingerite forms under extremely restrictive conditions, from rare elements. It is water soluble and disappears when rained upon, and comes from dangerous volcanic fumeroles near active volcanoes, so is rarely collected. Consequently, fingerite is only known from near the summit of the Izalco Volcano in El Salvador.
Rare minerals, the authors say, are key to understanding the diversity and disparity of Earth’s mineralogical environments. They are valuable in understanding Earth as a complex evolving system in which fluid- rock interactions and biological processes lead to new mineral- forming niches.
“Another possible contribution of rare minerals, though as yet speculative, relates to the origins of life. While most origins- oflife scenarios incorporate common minerals such as feldspars or clays, a number of uncommon minerals, including species of sulfides, borates, and molybdates, have also been invoked,” says Hazen.
“We live on a planet with remarkable mineralogical diversity, featuring countless variations of colour and form, richly varied geochemical niches, and captivating compositional and structural complexities. Rare species, comprising as they do more than half of the diversity of Earth’s rich mineral kingdom, thus provide the clearest and most compelling window into the complexities of the evolving mineralogical realm.” – American Mineralogist
Nevadaite is formed under very restricted environmental conditions from scarce elements: vanadium and copper. The crystals are colourful but microscopic, and the mineral is only known from just two locations: Eureka County, Nevada, and a copper mine in Kyrgyzstan.
1 Ottoite, known only from Otto Mountain, California, incorporates the extremely rare element tellurium and is microscopic and very difficult to spot in the field.
2 Amicite is formed from very common elements but requires extremely precise conditions of temperature, pressure, and composition. It is known from only two sites.
3 Ichnusaite, created through a subterranean mash- up of the radioactive element thorium and lead- like molybdenum. Only one specimen has ever been found, in Sardinia.