On the nature and significance of rarity in mineralogy

(1) Phase topology

Restricted phase stability in P-T-X space: The first category of mineral rarities arises because different phases have different ranges of stability in multi-dimensional pressure-temperature-composition (P-T-X) space (where composition typically refers to numerous coexisting elements). On the one hand, the commonest rock-forming minerals display wide ranges of P-T-X stability. By contrast, some rare phases, even though formed from relatively common elements, display extremely limited P-T-X stability fields and thus form only under idiosyncratic conditions (Table 1). For example, harmunite (CaFe2O4; Galuskina et al. 2014), though formed from abundant elements, has a narrow stability field (Phillips and Muan 1958), especially in the presence of silica (Levin et al. 1964, Fig. 656), and is reported from only two localities in mindat.org. Similarly, hatrurite (Ca3SiO5; Gross 1977) is listed on mindat.org from only one locality in spite of the abundance of many other calcium silicates. Hatrurite is rare because it only forms at temperatures above 1250 ^°C in a narrow range of compositions (Welch and Gutt 1959), notably in the absence of aluminum (Levin et al. 1964, Fig. 630). We also suggest that the extreme rarity of several zeolites (recorded at only one or two localities in mindat.org; Table 1), is also a consequence of their presumed highly restricted phase stability in P-T-X space. Zeolites display modular framework crystal structures with interconnected 4-, 6-, and 8-member tetrahedral rings that form a rich variety of channels and cavities, so small changes in the ratios of cations, as well as in the P-T conditions of formation, can lead to many new phases (Bish and Ming 2001; Bellussi et al. 2013; www.iza-structure.org/databases/).

Special cases of restricted mineral stabilities arise from extremes of eH and pH. For example, several native elements, including nickel, silicon, titanium, and zinc, seldom occur in Earth’s crust owing to the requirement of extremely reducing conditions. The exceptionally acidic conditions of some hot springs and weathered sulfide environments (with reported pH as low as –3.6; Nordstrom et al. 2000), also lead to rare minerals (Table 1), such as bernalite ammonioalunite, and schwertmanite [e.g., Jönsson et al. 2006; for characteristics and citations of rare minerals see Anthony et al. (1990) and references therein]. Similarly, extremely alkaline hypersaline Mono Lake, California, with pH ~10 hosts the only known occurrence of the biologically mediated mineral hazenite.

The great contrasts among stability ranges of minerals point to as yet unexplored aspects of the topological distribution of phases in P-T-X space. IMA approved minerals incorporate 72 different chemical elements that are reported as essential in one or more minerals. Furthermore, the numbers of species containing each of these elements is, to a significant degree, correlated with the crustal abundance of the element (Christy 2015; Hazen et al. 2015a). This correlation between crustal abundance and mineral diversity suggests that there exists an average “phase volume” in 74-dimensional P-T-X space, as well as a statistical distribution of smaller to larger phase volumes.

Two caveats are required. First, a continuum exists between “small” and “large” phase volumes; therefore, any division of minerals into one or the other of these two categories (as implied by the dissected cube in Fig. 2) is inherently arbitrary. Second, while it is obvious that some minerals have a greater stability range than others, no metric yet exists to quantify “volume of P-T-X phase space.” Such a metric is essential if quantitative statistical analysis of the distribution of phases in phase space is to be attempted.

In spite of these issues, it seems likely that the number of different mineral species to be found on a terrestrial planet or moon will be a direct consequence of phase topology in combination with the extent of mineral evolution on the body. Mineral diversity, including the presence of rare minerals, will reflect the total P-T-X range available on that planetary body, coupled with the statistical distribution of phase topologies. Investigations of the relationship between mineral diversity and phase space may thus prove to be of interest, both in characterizing the variety of rocky planets and in developing a deeper understanding of phase topology.

(2) Planetary constraints: Incorporation of rare elements or formation atP-T conditions rarely encountered in near-surface environments

A second category of mineral rarity arises from the improbable occurrence of certain P-T-X chemical microenvironments near Earth’s surface (Table 1). These rare minerals often have large stability ranges in P-T-X space and thus do not conform to category 1; rarity arises because those stability conditions are rarely sampled in Earth’s crust.

Several examples include minerals of Earth’s rarest chemical elements, such as tellurium with a crustal abundance of 0.005 ppm, mercury with a crustal abundance of 0.05 ppm, and cadmium with an upper crustal abundance of 0.09 ppm (Table 1; Wedepohl 1995; Rudnick and Gao 2005). Many additional rare minerals, as well as thousands of potential minerals that are known as synthetic phases but have not yet been discovered naturally, require the incorporation of two or more elements that seldom occur together and thus are far rarer than would be expected from their crustal abundances. Examples include such unusual pairings as Be-Sb in swedenborgite, Ge-Te in alburnite, and Mo-Th in ichnusaite (Table 1). A few minerals incorporate nine or more chemical elements in combinations that point to rare, if not unique, geochemical environments (Table 1).

Finally, several minerals such as diamond, coesite, and ringwoodite may form commonly at depth, where they crystallize at extremes of pressure and temperature, but those P-T regimes are less commonly sampled at Earth’s surface—an effect that is analogous to compositional regimes rarely found in Earth’s crust. As an extreme example is the perovskite form of MgSiO3, bridgmanite, which has only been described as a microscopic shock phase from a single meteorite, yet it is likely that bridgmanite is the dominant lower mantle mineral, and thus is Earth’s most important mineral in terms of volume (Tschauner et al. 2014).

Unlike the first category of rarities that arise from limited stability in P-T-X space, many of the scarce minerals in category 2 have extensive P-T-X stability ranges. Rarity emerges from the nature of cosmochemistry and the idiosyncrasies of unusual geochemical environments on Earth, as opposed to restrictions imposed by phase topology. Note that, as with the ill-defined parameter “volume of P-T-X phase space,” compositional rarity is a continuous function; some elements and their combinations are less common than others. Also, as with phase space, there exists no obvious metric of rarity for combinations of elements. It might be tempting to employ crustal abundances of elements to quantify the compositional axis (e.g., Wedepohl 1995; Rudnick and Gao 2005), but the production of rare minerals is equally dependent on the extent to which an element can be locally concentrated by physical, chemical, or biological processes—mechanisms that do not directly correlate with crustal abundances. For example, hafnium with a crustal abundance of 5.3 ppm is an essential element in only one mineral species, in contrast to uranium (2.7 ppm; >250 species), because Hf mimics Zr and thus is not easily concentrated into its own phases. Thus, no simple measure yet exists for compositional rarity, which must for the present remain a qualitative characteristic of minerals.

(3) Ephemeral minerals: Phases that do not persist under ambient conditions

A third category of mineral rarities includes numerous phases that form under varied non-ambient conditions but degrade quickly at ambient conditions. Some of these minerals may form frequently in Earth’s near-surface environment, but are nevertheless rare primarily owing to their relatively brief lifetimes.

Minerals can be ephemeral for several reasons. Phases that melt or evaporate at ambient conditions are rarely represented in mineral collections. For example, methane hydrate (nominally CH4·5.75H₂O) is well known as an abundant crystalline phase from continental shelf and Arctic drill sites (Hyndman and Davis 1992; Kvenvolden 1995), but it evaporates quickly at room pressure, or burns if set afire, and has not yet been characterized as a mineral. Similarly, the crystalline form of CO₂, which is only stable below –78.5 ^°C, is not yet known as a mineral on Earth, though it could form under Earth’s most extreme cold conditions of –94.7 ^°C (recorded from East Antarctica by NASA in August 2010) and it has been documented by remote sensing on Mars (Byrne and Ingersoll 2003). By contrast, phases such as ethanol (C₂H₅OH; freezing temperature –114 ^°C) and acetylene (C₂H₂; –80.8 ^°C) that have been proposed as plausible minerals on Titan (surface temperature –179 ^°C) are unlikely phases on Earth. Other phases that melt or evaporate under most surface conditions include acetamide, hydrohalite, and meridianiite (Table 1).

Hygroscopic phases that rapidly hydrate (Table 1) may also be more common than is reflected in mineral collections. Magnesium sulfate (MgSO₄), though well known as a synthetic compound, has not yet been found in nature. By contrast, 11 hydrated magnesium sulfate minerals have been described, including such common phases as epsomite (MgSO₄·7H₂O) and kieserite (MgSO₄·H₂O). Lime (CaO), similarly, is recorded from fewer than 10 localities, in contrast to the common hydrated daughter mineral, portlandite [Ca(OH)₂]. By contrast, several uncommon minerals are unstable in part because they readily dehydrate upon exposure to air (Table 1).

Water-soluble minerals may also be under-reported, and thus appear to be rare. More than 100 evaporite mineral species, including halides, borates, and nitrates, can persist in dry evaporite environments for many years, only to be washed away during rare rain events. Similarly, water-soluble phases that form in volcanic fumeroles may form intermittently and then dissolve with each subsequent rainstorm. Several examples of these scarce soluble fumarolic phases incorporate an alkali and/or a halogen element (Table 1). Other water-soluble phases that may be under-reported occur in a wide variety of environments, including oxidized zones of ore bodies, carbonatite lavas, alkaline massifs, coal mine waste dumps, bat guano, fossilized wood, mine fires, and high-temperature metamorphic assemblages (Table 1). Among the least stable minerals are rare species that are deliquescent—both adsorbing moisture from the air and then dissolving in it. Finally, a few rare minerals, including edoylerite, metasideronatrite, and sideronatrite, are photosensitive and gradually decompose when exposed to sunlight.

It should be noted that the description of a mineral as “ephemeral,” as with other parameters of rarity, is a relative term. The phases enumerated in Table 1 (and many more) may degrade in less than a day. However, many unstable or metastable minerals transform more slowly. Many Hg minerals, for example, are known to evaporate gradually; thus, more than half of IMA approved mercury minerals are known only from deposits younger than 50 million years (Hazen et al. 2012), in contrast to the age distributions of minerals of many other elements (Hazen et al. 2014). Similarly, borates, nitrates, and halides that are stable for thousands of years in evaporite deposits may, nevertheless, be ephemeral over time scales of millions of years. Grew et al. (2016) have found that more than 100 species of boron minerals (of 291 approved species) are known only from the Phanerozoic Eon. Thus, gradual loss of some Hg and B minerals may contribute to their relative rarity.

(4) Negative sampling biases

A significant number of rare minerals may be poorly documented because they are either difficult to recognize based on their appearance, occur only at the micro- or nano-scale, or are found in under-sampled lithological contexts. Thus, some minerals are rare because they are exceptionally problematic to recognize in hand specimen; notably a pale color and lack of distinctive crystal morphology leads to difficulty in identification. For example, Hazen et al. (2015b) noted that more than half of known sodium minerals are poorly crystalline and white or gray in color. Rare sodium minerals, thus, may be significantly under-reported, while a significant fraction of sodium minerals remains undiscovered.

At the extremes of scale, several new minerals have been discovered only as micro- or nano-phases. These microscopic minerals, and many others yet to be discovered, are likely to be more common than implied by numbers of known localities. For example, several rare tellurium minerals known only from Otto Mountain, California, have been discovered through intensive application of microscopy and electron microprobe analysis of specimens from that deposit (Housely et al. 2011; Table 1). These minerals are intrinsically uncommon, but their rarity may be exaggerated because of the technical difficulties in finding and characterizing such microscopic phases.

The application of transmission electron microscopy to the discovery of new minerals, thus far applied primarily to meteorite phases, has led to descriptions of species such as hutcheonite and allendeite, which may remain rare by virtue of the difficulty and expense of the analytical technique (Table 1; Ma and Krot 2014; Ma et al. 2014). These extraterrestrial phases, and many others awaiting discovery on Earth, are certainly volumetrically insignificant, but they may occur much more commonly than is implied by a list of their known localities. We suspect that numerous other nano-minerals await discovery, and all will be rare by virtue of their miniscule grain size.

Some minerals may seem to be rare because of their remote and/or dangerous environmental contexts. Minerals from Antarctica, deep-ocean minerals (notably those formed at sub-surface volcanic vents), and phases that grow in aqueous environments at extremes of temperature or pH, crustal environments at elevated pressures, or in volcanic fumaroles, are all from mineral-forming environments to which access is challenging and thus may be under-represented in mineral collections.

It should be noted that positive sampling biases also likely affect our perceptions of mineral rarity. Intensive searches for deposits of rare elements such as Au, Cd, Co, Ge, U, and the rare earths have undoubtedly led to the discovery of species containing these elements at more localities than comparably rare minerals of less economic interest (Hazen et al. 2015b).

Rarity in mineralogy vs. biology

The preceding taxonomy of mineralogical rarity differs in significant respects from that for biological species (compare Figs. 1 and 2). Biological species are rare if they have few individuals, are found in a narrow geographic range, and/or have a restricted habitat. These traits are not exactly analogous to the potential for formation of rare minerals, which must possess small P-T-X phase volume (category 1 above), incorporate rare combinations of elements (category 2), and/or are ephemeral (category 3). Both biological and mineralogical rarity depend to a significant degree on the nature of the environmental niches in which the species are found but, unlike evolving biological species, minerals owe their rarity to circumstances of cosmochemistry, geochemistry, and/or phase equilibria.

An additional important difference between biological and mineralogical rarity is that biological species, once extinct, will not re-emerge naturally. Rare minerals, on the other hand, may disappear from Earth for a time, only to reappear when the necessary physical and chemical conditions arise again.

Even more fundamental a difference between biological and mineralogical species lies in what John N. Thompson (2013) has called “relentless evolution.” In contrast to mineral species, biological species that do not become extinct nevertheless are constantly evolving, in some instances not so gradually, into new forms. Minerals do not evolve in this way, though an intriguing and as yet little explored aspect of mineralogy is how trace and minor elements and isotopes in common mineral species have varied through Earth history in response to changing near-surface conditions (Hazen et al. 2011). Thus, such diverse mineral groups as feldspars, amphiboles, clays, tourmalines, and oxide spinels from Earth’s Archean Eon may differ in subtle and systematic ways from those formed more recently.

Important similarities in the perceptions of biological and mineralogical rarity are the influences of sampling bias. In both domains, species that are difficult to discover by virtue of their bland appearances, small sizes, or inaccessible environments (category 4) may be much more common than are represented by reported occurrences (Zhang et al. 2014; Hazen et al. 2015b; Petsch et al. 2015).

Like biological rarities, rare minerals often display two or more of the categories of rarity, as illustrated by the various octants in Figure 2. For example, several rare copper vanadate minerals, including fingerite, mcbirneyite, stoiberite, and ziesite (Table 1), are known from the summit crater fumeroles of Izalco volcano, El Salvador, and at most one other locality (Hughes and Hadidiacos 1985). These minerals: (category 1) have extremely restricted stabilities in P-T-X space (Brisi and Molinari 1958); (category 2) they feature two elements, Cu and V, that are seldom found in combination; (category 3) they may be unstable under prolonged exposure to the atmosphere; and (category 4) they form in an extremely dangerous volcanic environment.