The International Mineralogical Association recognizes around 6,000 minerals, but scientists at the Carnegie Institution for Science have noted some of these can be formed through alternative physical, chemical, and biological processes. Recording each “mineral species” by each possible formation process, they identify 10,556 known “mineral kinds”.
By grouping each mineral kind by its method and timing of formation they offer an insight into why the Earth is so much richer in mineral diversity than other known worlds, and how this has grown through its history.
Professor Robert Hazen and Dr Shaunna Morrison identified 57 processes that act on available elements to produce the array of minerals we find around us, describing each and their frequency in the jounal American Mineralogist. The list includes the obvious, such as the products of lava and radioactive decay, but the authors draw distinctions between being boiled and baked (ie cooking with and without water). Even being struck by lightning features, but perhaps the most surprising category is for minerals derived from urine or bird/bat droppings (72 examples).
“Each mineral specimen has a history. Each tells a story. Each is a time capsule that reveals Earth's past as nothing else can,” Hazen said in a statement.
“More than 80 percent of Earth’s minerals were mediated by water, which is, therefore, fundamentally important to mineral diversity on this planet. By extension, this explains one of the key reasons why the Moon and Mercury and even Mars have far fewer mineral species than Earth,” Hazen added.
The work was partly funded by NASA because of the implications it holds for interpreting the development of other worlds through the minerals we can find there.
Life has left its record in the rocks as well, not just in the form of fossils, but in mineral composition. “One third of Earth’s minerals could not have formed without biology – shells and bones and teeth, or microbes, for example, or the vital indirect role of biology, such as by creating an oxygen-rich atmosphere that led to 2,000 minerals that wouldn't have formed otherwise,” Hazen said. Another sixth or so sometimes form from biological activity, but also in other ways.
The proverb about there being more than one way to skin a cat might better be rephrased (for many reasons) as there being more than one way to make a mineral. There are, for example, nine ways to turn carbon into diamonds, including in outer space and during asteroid impacts.
Maximum flexibility goes to pyrite, better known as fools gold, which can be formed via 21 different paths, both biological and lifeless, but Hazen and Morrison identify nine minerals capable of forming in at least 15 ways. Almost 60 percent of the “species” have only one known formation process.
The first minerals were dominated by 11 common elements, but subsequently, very rare ingredients contributed to the current diversity. Some 2,400 elements include at least one of 41 elements whose combined abundance is less than 5 parts per million (ie 0.0005 percent) of the Earth's crust.
Almost 300 of the minerals in the catalog are thought to be older than the Earth itself – 97 of these have only been found in meteorites, suggesting Earth lacks the capacity to form them.
The most common manner in which minerals are produced is through weathering or oxidation at or near the Earth's surface, followed by deposition of minerals dissolved in volcanic hot water.
Humanity's influence is visible as well, with more than 500 entries in the database from minerals resulting from mining – 234 from fires in coal mines alone. Materials deliberately made by humans, and not otherwise present on Earth, were excluded from the list, however.
In an accompanying paper in the same journal, Hazen and Morrison, along with two co-authors, propose that classifying minerals by their timing and method of formation offers considerable benefits. For example, they suggest one category could be minerals that have only existed since the Great Oxidation Event changed Earth's atmosphere and oceans, while another could be those that preceded the Earth becoming wet.