Mineral evolution is a recent hypothesis that provides historical context to mineralogy. It postulates that mineralogy on planets and moons becomes increasingly complex as a result of changes in the physical, chemical and biological environment. In the Solar System, the number of mineral species has grown from about a dozen to over 5400 as a result of three processes: separation and concentration of elements; greater ranges of temperature and pressure coupled with the action of volatiles; and new chemical pathways provided by living organisms.
On Earth, there were three eras of mineral evolution. The birth of the Sun and formation of asteroids and planets increased the number of minerals to about 250. Repeated reworking of the crust and mantle through processes such as partial melting and plate tectonics increased the total to about 1500. The remaining minerals, more than two-thirds of the total, were the result of chemical changes mediated by living organisms, with the largest increase occurring after the Great Oxygenation Event.
Use of the term "evolution" In the 2008 paper that introduced the term "mineral evolution", Robert Hazen and co-authors recognized that an application of the word "evolution" to minerals was likely to be controversial, although there were precedents as far back as the 1928 book The Evolution of the Igneous Rocks by Norman Bowen. They used the term in the sense of an irreversible sequence of events leading to increasingly complex and diverse assemblages of minerals. Unlike biological evolution, it does not involve mutation, competition or passing of information to progeny. Hazen et al. explored some other analogies, including the idea of extinction. Some mineral-forming processes no longer occur, such as those that produced certain minerals in enstatite chondrites that are unstable on Earth in its oxidized state. Also, the runaway greenhouse effect on Venus may have led to permanent losses of mineral species. However, mineral extinction is not truly irreversible; a lost mineral could emerge again if suitable environmental conditions were re-established.
Presolar minerals
In the early Universe, there were no minerals because the only elements available were hydrogen, helium and trace amounts of lithium. Mineral formation became possible after heavier elements, including carbon, oxygen, silicon and nitrogen, were synthesized in stars. In the expanding atmospheres of red giants and the ejecta from supernovae, microscopic minerals formed at temperatures above .
Evidence of these minerals can be found in interstellar grains incorporated into primitive meteorites called chondrites, which are essentially cosmic sedimentary rocks. The number of known species is roughly a dozen, although several more materials have been identified but not classified as minerals. Because it has a high crystallization temperature (about ), diamond was probably the first mineral to form. This was followed by graphite, oxides (rutile, corundum, spinel, hibonite), carbides (moissanite), nitrides (osbornite and silicon nitride) and silicates (forsterite and silicate perovskite (MgSiO3)). These "ur-minerals" seeded the molecular clouds from which the Solar system was formed.
Processes After the formation of the Solar system, mineral evolution was driven by three primary mechanisms: the separation and concentration of elements; greater ranges of temperature and pressure combined with chemical action of volatiles; and new reaction pathways driven by living organisms.
Separation and concentration
The highest level in the classification of minerals is based on chemical composition. However, the defining elements for many mineral groups, such as boron in borates and phosphorus in phosphates, were at first only present in concentrations of parts per million or less. This left little or no chance for them to come together and form minerals until external influences concentrated them. Processes that separate and concentrate elements include planetary differentiation (for example, separation into layers such as a core and mantle); outgassing; fractional crystallization; and partial melting.
Intensive variables and volatiles
Allowable combinations of elements in minerals are determined by thermodynamics; for an element to be added to a crystal at a given location, it must reduce the energy. At higher temperatures, many elements are interchangeable in minerals such as olivine. As a planet cools, minerals became exposed to a greater range of intensive variables such as temperature and pressure, allowing the formation of new phases and more specialized combinations of elements such as clay minerals and zeolites. New minerals are formed when volatile compounds such as water, carbon dioxide and O2 react with them. Environments such as ice caps, dry lakes, and exhumed metamorphic rock have distinctive suites of minerals.
Biological influence Life has made dramatic changes in the environment. Most dramatic was the Great Oxygenation Event, about 2.4 billion years ago, in which photosynthetic organisms flooded the atmosphere with oxygen. Living organisms also catalyze reactions, creating minerals such as aragonite that are not in equilibrium with their surroundings.
Chronology Before the formation of the Solar System, there were about 12 minerals. The estimate for the current number of minerals has been changing rapidly. In 2008, it was 4300, but as of November 2018 there were 5413 officially recognized mineral species.
In their chronology for Earth, Hazen et al. (2008) separated the changes in mineral abundance into three broad intervals: planetary accretion up to 4.55 Ga (billion years ago); reworking of Earth's crust and mantle between 4.55 Ga and 2.5 Ga; and biological influences after 2.5 Ga. They further divided the ages into 10 intervals, some of which overlap. In addition, some of the dates are uncertain; for example, estimates of the onset of modern plate tectonics range from 4.5 Ga to 1.0 Ga. Eras and stages of Earth's mineral evolution Era/stage Age (Ga)Cumulative no. of species Prenebular "Ur-minerals" >4.6 12 Era of planetary accretion (>4.55 Ga) 1. Sun ignites, heating nebula >4.56 60 2. Planetesimals form >4.56–4.55 250 Era of crust and mantle reworking (4.55–2.5 Ga) 3. Igneous rock evolution 4.55–4.0 350–420 4. Granitoid and pegmatite formation 4.0–3.5 1000 5. Plate tectonics >3.0 1500 Era of biologically mediated mineralogy (2.5 Ga – present) 6. Anoxic biological world 3.9–2.5 1500 7. Great Oxidation Event 2.5–1.9 >4000 8. Intermediate ocean 1.85–0.85 >4000 9. Snowball Earth events 0.85–0.542 >4000 10. Phanerozoic era of biomineralization <0.542 >5413
Planetary accretion
In the first era, the Sun ignited, heating the surrounding molecular cloud. 60 new minerals were produced and were preserved as inclusions in chondrites. The accretion of dust into asteroids and planets, bombardments, heating and reactions with water raised the number to 250.
Stage 1: Sun ignites Before 4.56 Ga, the presolar nebula was a dense molecular cloud consisting of hydrogen and helium gas with dispersed dust grains. When the Sun ignited and entered its T-Tauri phase, it melted nearby dust grains. Some of the melt droplets were incorporated into chondrites as small spherical objects called chondrules. Almost all chondrites also contain calcium–aluminium-rich inclusions (CAIs), the earliest materials formed in the Solar System. From an examination of chondrites from this era, 60 new minerals can be identified with crystal structures from all of the crystal systems. These included the first iron-nickel alloys, sulfides, phosphides, and several silicates and oxides. Among the most important were magnesium-rich olivine, magnesium-rich pyroxene, and plagioclase. Some rare minerals, produced in oxygen-poor environments no longer found on Earth, can be found in enstatite chondrites.
Stage 2: Planetesimals form Soon after the new minerals formed in Stage 1, they began to clump together, forming asteroids and planets. One of the most important new minerals was ice; the early Solar System had a "snow line" separating rocky planets and asteroids from ice-rich giant planets, trans-Neptunian objects, and comets. Heating from radionuclides melted the ice and the water reacted with olivine-rich rocks, forming phyllosilicates, oxides such as magnetite, sulfides such as pyrrhotite, the carbonates dolomite and calcite, and sulfates such as gypsum. Shock and heat from bombardment and eventual melting produced minerals such as ringwoodite, a major component of Earth's mantle.
Eventually, asteroids heated enough for partial melting to occur, producing melts rich in pyroxene and plagioclase (capable of producing basalt) and a variety of phosphates. Siderophile (metal-loving) and lithophile (silicate-loving) elements separated, leading to the formation of a core and crust, and incompatible elements were sequestered in the melts. The resulting minerals have been preserved in a type of stony meteorite, eucrite (quartz, potassium feldspar, titanite and zircon) and in iron-nickel meteorites (iron-nickel alloys such as kamacite and taenite; transition metal sulfides such as troilite; carbides and phosphides). An estimated 250 new minerals formed in this stage.
Crust and mantle reworking
The second era in the history of mineral evolution began with the massive impact that formed the Moon. This melted most of the crust and mantle. Early mineralogy was determined by crystallization of igneous rocks and further bombardments. This phase was then replaced by extensive recycling of crust and mantle, so that at the end of this era there were about 1500 mineral species. However, few of the rocks survived from this period so the timing of many events remains uncertain.
Stage 3: Igneous processes Stage 3 began with a crust made of mafic (high in iron and magnesium) and ultramafic rocks such as basalt. These rocks were repeatedly recycled by fractional melting, fractional crystallization and separation of magmas that refuse to mix. An example of such a process is Bowen's reaction series.
One of the few sources of direct information on mineralogy in this stage is mineral inclusions in zircon crystals, which date as far back as 4.4 Ga. Among the minerals in the inclusions are quartz, muscovite, biotite, potassium feldspar, albite, chlorite and hornblende.
In a volatile-poor body such as Mercury and the Moon, the above processes give rise to about 350 mineral species. Water and other volatiles, if present, increase the total. Earth was volatile-rich, with an atmosphere composed of N2, CO2 and water, and an ocean that became steadily more saline. Volcanism, outgassing and hydration gave rise to hydroxides, hydrates, carbonates and evaporites. For Earth, where this stage coincides with the Hadean Eon, the total number of widely occurring minerals is estimated to be 420, with over 100 more that were rare. Mars probably reached this stage of mineral evolution.
Stage 4: Granitoids and pegmatite formation Given sufficient heat, basalt was remelted to form granitoids, coarse-grained rocks similar to granite. Cycles of melting concentrated rare elements such as lithium, beryllium, boron, niobium, tantalum and uranium to the point where they could form 500 new minerals. Many of these are concentrated in exceptionally coarse-grained rocks called pegmatites that are typically found in dikes and veins near larger igneous masses. Venus may have achieved this level of evolution.
Stage 5: Plate tectonics With the onset of plate tectonics, subduction carried crust and water down, leading to fluid-rock interactions and more concentration of rare elements. In particular, sulfide deposits were formed with 150 new sulfosalt minerals. Subduction also carried cooler rock into the mantle and exposed it to higher pressures, resulting in new phases that were later uplifted and exposed as metamorphic minerals such as kyanite and sillimanite.