Mineral Evolution

The suite of minerals that makes up the solid Earth - and especially the minerals at or near the surface - has not always been what it is today. It has changed through time because the Earth itself has changed. This is the central claim of mineral evolution: that the planet’s mineral diversity is a historical record, not a fixed inventory. ^[1]

The idea cuts against the deep intuition of uniformitarianism - the principle taught in introductory geology that the Earth’s cycles (rock cycle, hydrologic cycle, and so on) repeat without directional change. That framing is a useful simplification, but it leaves the impression that the Earth now is essentially the same as the Earth always was. No important aspect of Earth’s history, in fact, simply repeats. The Earth formed once; life developed once and has evolved continuously; the tectonic regime has shifted across deep time and will shift again. Time has an arrow, and each stage opens new geochemical environments that were not available before. ^[1]

Three Eras of Mineral Evolution

Earth’s mineralogical history divides into three major eras. ^[1]

Planetary Accretion (>4.55 Ga)

The mineralogy of the primordial material from which the Earth accumulated is represented by chondritic meteorites, whose major minerals are olivine and pyroxene along with a variety of less abundant minerals and carbonaceous material. Some minerals found in chondrites are unknown on Earth because the chemical and physical conditions that produced them no longer exist here. As chondritic material accumulated into increasingly large bodies - asteroids and planetesimals - the heat generated by accretion and radioactive decay initiated melting. Meteorites that record this melting and segregation stage are pallasites: olivine crystals set in an iron-nickel metal matrix, representing the boundary between rocky mantle and metallic core in a differentiated body. ^[1]

Crust and Mantle Reworking (4.55-2.5 Ga)

This era is still ongoing. It encompasses melting, magmatic differentiation, fractional crystallisation, volcanism, metamorphism, erosion, and deposition - all the processes generating continental crust. The oceans were probably established by about 4.3 Ga, and their presence enabled the formation of hydroxide, hydrate, carbonate, and evaporite minerals. The generation of granitic continental crust - probably by melting of wet basaltic rocks - concentrated silicon, aluminium, potassium, and sodium into the upper crust and made large volumes of quartz, K-feldspar, plagioclase, micas, and amphiboles possible for the first time. ^[1]

Biologically Mediated Mineralogy (<2.5 Ga)

When life first appeared sometime between 3.5 and 4 billion years ago, the atmosphere was anoxic - dominated by volcanic gases (H₂O, CO₂, SO₂, CO, N₂) plus ammonia and methane, with no free oxygen. Early photosynthetic bacteria released O₂ as a metabolic waste product. For more than a billion years this oxygen was consumed by weathering reactions and atmospheric chemistry, but around 2.5 Ga - at the beginning of the Proterozoic - oxygen began accumulating substantially in the atmosphere in what is called the Great Oxidation Event. ^[1]

This transformed the near-surface geochemical environment. More than half of all currently known mineral species are oxidised and hydrated weathering products - minerals that are stable precisely because of the oxygen-rich atmosphere the Great Oxidation Event created. Few of these minerals would have been stable in the anoxic world that preceded it. In total, two-thirds of all known mineral species owe their existence directly or indirectly to biological processes, mostly through the environmental changes initiated by life’s oxygen production. ^[1]

The Growth of Mineral Diversity

The number of mineral species has increased systematically from roughly 60 in the chondritic meteoritic starting material to approximately 4900 today. This growth reflects the progressive diversification of bulk compositions and the expanding range of temperatures, pressures, water activities, oxygen fugacities, and pH conditions that different geological environments represent. ^[1]

The claim is not rigorously testable, however, because the rock record of the Earth’s earliest history is almost entirely absent. The oldest reliably dated rock is only slightly older than 4 billion years, and the geochemical and mineralogical record of the first 550 million years of Earth’s history is essentially missing. If a better early rock record existed, the list of early minerals would almost certainly be much longer than even the chondritic estimate suggests. ^[1]

Extinct and Endangered Mineral Species

The concept of mineral evolution introduces the possibility of extinct mineral species - minerals that once existed but can no longer form or survive. Extinction could occur in two ways. First, if a chemical element present in the early Earth was itself destroyed by radioactive decay over billions of years, any minerals made from that element would have disappeared with it. Some physicists suggest that very heavy elements (up to atomic number 126) may once have been present on Earth with half-lives of up to 100 million years; minerals built from such elements could have existed and then vanished entirely. Second, if the geological environment required to form a mineral no longer exists anywhere on Earth and no surviving samples have been preserved through normal geological processes, that mineral is effectively extinct. ^[1]

An endangered mineral species is one for which the geological environment needed for formation is no longer available, but samples from an earlier time are still preserved in the rock record. Whether such designations will prove useful in practice remains to be seen - identifying what the necessary geological environments were, and confirming that no modern analogue exists, is a formidable task.

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