A team of researchers has inventoried and categorized all of Earth’s rare mineral species described to date.
This false-color image shows the mineral ichnusaite. Image credit: American Mineralogist / Paulo Biagioni et al.
As of January 31, 2016, the International Mineralogical Association has approved 5,090 mineral species, fewer than 100 of which make up 99 percent of Earth’s crust.
Of those 5,090, around 2,550 are defined as rare — found at five or fewer locations worldwide.
“And more than two-thirds of known mineral species, including the great majority of rare species, have been attributed to biological changes in Earth’s near-surface environment,” said team member Dr. Robert Hazen, of the Carnegie Institution.
According to Dr. Hazen and his colleague, Dr. Jesse Ausubel from the Rockefeller University, each rare mineral fits into one or more of four categories:
(i) unique conditions that created the mineral:
“In very simple terms, imagine making minerals at a kitchen stove using a pressure cooker. What results in the pot is a function of variables: temperature, pressure and the ingredients,” said Dr. Hazen, who is the lead author of a paper published online in the journal American Mineralogist.
“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 this mineral forms only in a very restricted environment with temperatures above 1250 degrees Celsius and in the absence of another extremely common element, aluminum.”
(ii) planetary constraints:
“Incorporation of rare elements, or mineral formation at pressure-temperature conditions rarely encountered in near-surface environments,” the scientists said. “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.”
(iii) ephemeral minerals:
Some minerals form under unusual conditions, but then simply melt, evaporate or dehydrate when exposed to different surface conditions.
A crystalline form of methane hydrate, for example, found in core samples from continental shelf and Arctic drill sites, evaporates at room pressure.
“As well, water-soluble minerals may also be under-reported, and thus appear to be rare,” the researchers said.
“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.
(iv) places scientists rarely sample:
In this category are rare minerals that simply come from under-sampled regions, from extreme environments such as the flanks of erupting volcanoes, frigid and remote regions of Antarctica, or 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 recognize based of their lack of bright colors or showy crystal faces. Most mineral collectors favor eye-popping specimens for their display case.
“Most mineral experts are familiar with at best a handful of the 2,550 obscure rarities,” said Dr. 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 it comes from dangerous volcanic fumeroles near active volcanoes, so is rarely collected,” he said. “Consequently, fingerite is only known from near the summit of the Izalco Volcano in El Salvador.”
According to Dr. Hazen, a mineral called ichnusaite is another true rarity — created through a subterranean mash-up of the radioactive element thorium and lead-like molybdenum, with only one specimen ever found, in Sardinia a few years ago.
“If you wanted to give your fiancé a really rare ring, forget diamond. Give her Sardinian ichnusaite,” he said.
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Robert M. Hazen & Jesse H. Ausubel. 2016. On the Nature and Significance of Rarity in Mineralogy. American Mineralogist, vol. 101, doi: 10.2138/am-2016-5601CCBY
Source link: https://www.sci.news/geology/earths-mineral-kingdom-03635.html
