An evolutionary system of mineralogy, Part VII: The evolution of the igneous minerals (>2500 Ma)

Native elements

Graphite (C), which has been documented as a primary igneous mineral in a few alkaline rocks and carbonatites, is the only native element that occurs in any significant abundance.

More than 20 platinum and platinum group element alloys (e.g., Ru, Rh, Pd, Os, Ir, and Pt), as well as gold, silver, and their alloys, notably with copper, are important economic resources in some layered igneous intrusions and ultramafic/mafic lithologies (Kutyrev et al. 2021; Berdnikov et al. 2022). However, these phases, though at times economically important, are volumetrically minor. Several native metals (e.g., Al, Fe, Ti, Zn) representing extremely reduced environments also occur as a trace phase in some ultramafic lithologies, though their paragenesis is a matter of some controversy (Dobrzhinetskaya et al. 2009; Xiong et al. 2017, 2020; Griffin et al. 2018).

Sulfides

Numerous sulfide minerals are associated with igneous rocks, notably in the context of economically valuable late-stage hydrothermal deposits (Guilbert and Park 2007). Sulfide and silicate melts tend to be immiscible (Lester et al. 2013; Savelyev et al. 2018), so crystallization of O- vs. S-bearing minerals often follows parallel pathways, at times with S- and O-rich melts separated by density.

Pyrite (FeS₂), the most common igneous sulfide, is reported as an accessory phase in 288 (i.e., 16%) of the rocks in Online Materials^[1] Table OM3. Pyrite is thus by far the most frequent S-bearing primary phase in igneous rocks. The host lithologies span the range from granites and their pegmatites to intermediate, mafic, and ultramafic rocks, as well as varied alkaline rocks and carbonatites. This unusual versatility parallels the observation that pyrite displays the largest number of different paragenetic modes of any mineral (Hazen and Morrison 2022).

We record pyrrhotite (Fe₇S₈) from 64 igneous rocks, spanning a range of lithologies similar to that of pyrite. In addition, we record chalcopyrite (CuFeS₂; 38 occurrences), galena (PbS; 37), molybenite (MoS₂; 11), and sphalerite (ZnS; 30) as suspected primary phases from varied igneous rocks, including granites and their pegmatites, alkaline rocks, and carbonatites.

At least 40 other sulfides, along with dozens of volumetrically minor arsenides, antimonides, and sulfosalts, appear to be primary igneous phases (Online Materials^[1] Table OM2). However, in many instances it is uncertain whether these minerals associated with igneous rocks are primary igneous phases, as opposed to arising from later hydrothermal and/or alteration processes.

Oxides

Oxides are ubiquitous accessory phases, as well as occasional major phases, in the entire range of igneous rocks. More than 100 species, most of them volumetrically trivial, have been reported (Online Materials^[1] Table OM1). The most common simple oxide is rutile (TiO₂), which is reported in 100 of the igneous rocks from our study (Online Materials^[1] Table OM3), most often in granites, syenites, and carbonatites. The TiO₂ polymorphs, anatase (20 occurrences) and brookite (6), are less common, occurring primarily in granitic rocks and carbonatites (Bowles et al. 2011). An added complexity with the TiO₂ polymorphs is recognizing primary vs. secondary occurrences. For example, brookite may occur as an alteration product of titanite, whereas brookite may alter to rutile.

Additionally, among the more common simple oxides are baddeleyite (ZrO₂), which we list from 37 carbonatites and alkaline rocks; corundum (Al₂O₃; 20 occurrences), observed most commonly in Al-rich syenites; and cassiterite (SnO₂; 21 occurrences) and uraninite (UO₂; 9 occurrences), usually from granites and their complex pegmatites.

By far the most frequently encountered oxides are members of the spinel group, which occur in more than half of the igneous rocks we tabulated. This binary oxide group, with the general formula [(Mg,Fe²⁺)(Al,Fe³⁺,Cr³⁺,Ti)₂O₄], encompasses a complex range of solid solutions, spanning eight major end-members: chromite F e 2 + C r 2 3 + O 4 , hercynite (Fe²⁺Al₂O₄), magnesiochromite (MgAl₂O₄), magnesioferrite M g F e 2 3 + O 4 magnetite F e 2 + F e 2 3 + O 4 , qandilite (Mg₂Ti⁴⁺O₄), spinel (MgAl₂O₄), and ulvöspinel F e 2 2 + T i 4 + O 4 , with some specimens incorporating significant amounts of other cations, including Ca, Mn²⁺, Zn, V³⁺, Ti³⁺, and/or Si (Bowles et al. 2011). Because most of these examples are opaque in thin section, petrographers often do not differentiate the several spinel species. Alternatively, generic names such as “pleonaste”, “chrome-spinel”, and “picotite” are employed (Table 2).

In this study, we recognize four natural kinds of oxide spinels. Magnetite is by far the most frequently reported oxide, occurring in 1161 of the 1850 igneous rocks we compiled (63%), and appearing as a major phase (>5 vol%) in 292 of those rocks. Included in our tabulations of primary magnetite are minerals identified as “iron ores,” “martite” (hematite pseudomorphs after magnetite), and “Ti-magnetite” or “titanomagnetite.” In addition, when explicitly reported, we distinguish chromite (31 occurrences; including “chrome-spinel”) and hercynite (8; also cited as “picotite”) from mostly mafic/ultramafic rocks, and spinel (53; often reported as “pleonaste”) from mafic/ultramafic rocks and carbonatites.

Ilmenite (FeTiO₃) is another common binary oxide, which occurs in 330 (18%) of the rocks we compiled. Like magnetite, ilmenite is an opaque Fe-rich phase that occurs across the spectrum of igneous rocks.

Perovskite (CaTiO₃) was reported from 137 basic and alkaline rocks and carbonatites; it is a major phase in 27 of those lithologies.

Complex oxides are important phases in many highly differentiated rocks, notably rare-element granitic and agpaitic pegmatites. We lump six IMA-CNMNC-approved species of the columbite-tantalite series [(Fe,Mg)(Ta,Nb)₂O₆] into columbite, which appears as a minor phase in 62 rare-element pegmatites. Similarly, 7 species of the euxenite group [(Y,Ca,Ce,U,Th)(Nb,Ta,Ti)₂O₆], reported from 8 highly differentiated rocks, are lumped into euxenite; 7 species from the fergusonite group [(Y,REE)(Nb,Ta)O₄] from 7 rocks are lumped into fergusonite; 4 species from the zirconolite group [(Ca,Y)Zr(Ti,Mg,Al)₂O₇] from 9 agpaitic rocks are lumped into zirconolite; and 8 species from the aeschynite group [(Y,REE,Ca,Th) (Ti,Nb,Ta)₂(O,OH)₆] from 10 rocks are lumped into aeschynite.

The pyrochlore oxide supergroup [(Ca,Na,REE,U,o)₂(Nb,Ta,Sb,Ti,W)₂O₆ (O,OH,F,H₂O)] presents significant challenges in species identification for the petrographer. The IMA-CNMNC has recognized five mineral groups (pyrochlore, microlite, roméite, betafite, and elsmoreite, with Nb, Ta, Sb, Ti, and W dominant, respectively), which are further divided into at least 38 species (Atencio et al. 2010; Christy and Atencio 2013). The petrographic literature only records the supergroup or group name and does not distinguish among these numerous species. Accordingly, in Online Materials^[1] Tables OM2, OM3, and OM6 we lump 7 species into the Nb-dominant pyrochlore, which is recorded from 172 igneous rocks (mostly complex pegmatites, carbonatites, and alkaline rocks), and 10 species into Tadominant microlite, reported from 27 rocks of similar lithologies.

Halides

At least 20 halide minerals have been reported as primary phases in igneous rocks (Online Materials^[1] Table 2). However, fluorite (CaF₂) is the only common igneous halide (Chang et al. 1996). We record 213 fluorite-bearing igneous rocks, mostly granites, alkaline rocks, and carbonatites.

Carbonates

Over the past century, carbonate minerals have been increasingly recognized as primary igneous minerals (Tilley 1921; Chang et al. 1996), with more than 60 recorded species (Online Materials^[1] Table OM1), most notably in carbonatites (Woolley 1987, 2001, 2019; Kogarko et al. 1995). The majority of occurrences of igneous carbonate minerals feature divalent cations with a general formula [(Ca,Mg,Fe)₂(CO₃)₂]— a compositional span that encompasses five igneous species approved by the IMA-CNMNC: calcite (CaCO₃), magnesite (MgCO₃), siderite (FeCO₃), dolomite [CaMg(CO₃)₂], and ankerite [CaFe(CO₃)₂]. An important mineralogical distinction occurs between carbonates in which all divalent cation sites have the same average composition (calcite, magnesite, and siderite), vs. those that display cation ordering in alternating sites (dolomite, ankerite).

Petrographers employ these same five carbonate mineral names; however, the nomenclature in the petrographic literature is at times confused. Calcite is correctly named in 347 igneous rocks, the great majority of which are Ca-dominant carbonatites known as “sovites” or “alvikites.” Dolomite, reported from 99 rocks in our compilation, in most instances occurs as the major mineral in Ca-Mg-bearing carbonatites, sometimes recorded as “beforsites” or “rauhaugites.” However, in these instances “dolomite” may represent either the ordered Ca-Mg carbonate or disordered Mg-bearing calcite.

The two iron-bearing carbonatites reported in the carbonatite literature, ankerite (50 occurrences) and siderite (11 occurrences), often do not strictly conform to the IMA species definitions. In many instances, “ankerite” in the petrographic literature denotes an Fe-bearing dolomite with significant Fe substituting for Mg, though Mg > Fe in most examples. Similarly, “siderite” may correspond to calcite with significant Fe substituting for Ca, even if Ca > Fe. In Online Materials^[1] Table OM3 we record the mineral name included in the original literature (in particular as recorded by Woolley 1987, 2001, 2019; Kogarko et al. 1995).

In addition to these most common carbonate minerals, a few rocks incorporate other divalent-cation carbonates that occur as primary igneous phases: magnesite (MgCO₃; 3 occurrences), rhodochrosite (MnCO₃; 2), strontianite (SrCO₃; 26), norsethite [BaMg(CO₃)₂; 2], barytocalcite [BaCa(CO₃)₂; 2], and alstonite [BaCa(CO₃)₂; 2].

Several rare alkali carbonates, including burbankite [(Na,Ca)₃(Sr,Ba,Ce)₃ (CO₃)₅, fairchildite [K₂Ca(CO₃)₂], gregoryite [Na₂(CO₃)], and nyererite [Na₂Ca(CO₃)₂], occur as abundant primary phases in rare alkali carbonatites, such as those erupted at Oldoinyo Lengai in Tanzania. These minerals may be relatively common in young carbonatites, but they are ephemeral and easily lost in the geological record (Zaitsev and Keller 2006). As a result, alkali carbonatites may be significantly under-represented in the modern sample (Zaitsev et al. 2013). Consequently, our catalog records fewer than five primary igneous localities for each alkali carbonate mineral. Nevertheless, we include gregoryite and nyererite in our list of the 115 most common primary igneous minerals, because they are the two dominant carbonate minerals in alkali carbonatites.

Of special note are four groups of rare-earth element carbonates that occur as accessory minerals in some carbonatites. We lump two species of the ancylite group [(La,Ce)Sr(CO₃)₂(OH)·H₂O] into ancylite, which is recorded from 17 carbonatites and complex pegmatites. Similarly, we lump 7 species of the bastnaesite group into bastnaesite [(Y,REE)(CO₃)(F,OH); 45 occurrences]; 2 species of the parisite group into parisite [Ca(REE)₂(CO₃)₃F₂; 22 occurrences]; and 4 species of the synchesite group into synchesite [(Ca,Ba)(Y,REE)(CO₃)₂F; 20 occurrences].

Sulfates

A few sulfate minerals with divalent cations Ca, Sr, and Ba have been reported as primary igneous minerals. Baryte (BaSO₄; often reported as “barite”) is the most common example, occurring in 79 in igneous rocks, primarily carbonatites.

Phosphates

Primary igneous phosphates are common accessory phases, with more than 100 recorded species, most of which occur in trace amounts (Online Materials^[1] Table OM1). “Apatite” [Ca₅(PO₄)₃(F,OH,Cl)] is the most common primary igneous mineral, reported in 1234 of 1850 (67%) of rocks in our compilation (Online Materials^[1] Table OM3). The great majority of these occurrences correspond to the F-dominant species, fluorapatite, which is common across the full range of igneous rocks as the most common P- and F-bearing phase. However, a small fraction of apatite occurrences may be hydroxylapatite or chlorapatite, which are occasionally encountered as igneous minerals (Chang et al. 1996).

Two types of rare-earth element phosphates, the monazite and xenotime groups, occur frequently as accessory minerals in granites and their pegmatites, carbonatites, and other igneous lithologies (Chang et al. 1996). We lump 4 species of the monazite group [(REE)PO₄] into monazite, which we record from 94 rocks. Three species of the xenotime group [(Y,REE)(P,As)O₄] are combined into xenotime, with 23 occurrences.

We also lump three species of Li-bearing phosphates into amblygonite [Li(Al,Fe³⁺)PO₄(F,OH)], which we record from 16 rare-element granite pegmatites, typically in association with beryl, lepidolite, and spodumene.

Silicates

Silicates are by far the most common primary igneous phases, both volumetrically and in terms of diversity. In Online Materials^[1] Table OM2 we record 375 different silicate mineral kinds, of which 69 are frequently encountered major phases and/or common accessory minerals. In the following sections we review these more common silicates.

Nesosilicates or Orthosilicates

Three members of the olivine group [(Mg,Fe,Ca)₂SiO₄] are important primary igneous minerals (Deer et al. 1982). Olivine with Mg/Fe >1 (i.e., forsterite) is reported in 412 of the rocks we surveyed, notably in mafic and ultramafic rocks, as well as alkaline rocks. Fayalite (with Mg/Fe < 1; ideally Fe₂SiO₄) is much less common, occurring in 30 rocks, mostly granites, syenites, and their extrusive equivalents. Monticellite (CaMgSiO₄), though more familiar as a skarn mineral in the context of contact metamorphism, also occurs occasionally in carbonatites, alkaline rocks, and mafic lithologies (Deer et al. 1982). Here, we record monticellite as a primary igneous mineral in 24 rocks.

Four members of the garnet group—almandine F e 3 2 + A l 2 S i O 4 3 , andradite C a 3 F e 2 3 + S i O 4 3 , pyrope [Mg₃Al₂(SiO₄)₃], and spessartine M n 3 2 + A l 2 S i O 4 3 - are reported as primary minerals in igneous rocks (Deer et al. 1982). Note, however, that almost all igneous garnets are solid solutions of three or more end-members (Chiama et al. 2020, 2022). Andradite, also cited as “melanite” or “schorlomite,” is the only common igneous garnet, recorded in 199 of the rocks in our compilation, including granitic, intermediate, basic, and alkaline rocks. In addition, Almandine is reported from 7 granite pegmatites, whereas spessartine is found in 7 granite or syenite pegmatites.

Clinohumite [Mg₉(SiO₄)₄(F,OH)₂], an orthosilicate most commonly associated with metamorphosed carbonate sediments, is recorded as a likely primary phase from 6 carbonatites and ultramafic rocks in our inventory.

Among the most common silicate accessory igneous minerals are the orthosilicates zircon (ZrSiO₄), listed in 400 of 1850 rocks in our tabulation (22%); its isomorph thorite (ThSiO₄) identified in 30 rocks; and titanite (CaTiSiO₅; commonly reported as “sphene”), which occurs in 518 rocks (28%). Both zircon and sphene occur in a wide range of igneous lithologies, whereas thorite is found primarily in alkaline rocks, carbonatites, and complex granite pegmatites.

Several orthosilicates are most frequently found in complex granite pegmatites. Topaz (Al₂SiO₄F₂), phenakite (Be₂SiO₄), and eucryptite (LiAlSiO₄), recorded from 26, 9, and 5 rocks in Online Materials₁ Table OM3, respectively, are commonly associated with beryl, lepidolite, muscovite, and tourmaline. Note that topaz is usually a pneumatolytic phase, and thus one of the last primary igneous minerals to crystallize (Deer et al. 1982). Johannsen (1932, p. 21) notes that “It is sometimes rather difficult to draw the line between contact metamorphosed and true igneous quartz-topaz rocks. In each case the minerals are pneumatolytic; but in the former a pre-existing rock was saturated and changed by the mineralizers; while in the latter the mineralizer-rich solutions crystallized as quartz and topaz.”

The britholite group [(Y,REE,Ca)₅(SiO₄)₃(OH,F)], also known as “beckelite,” encompasses several silicate apatites. We lump 5 species of this group into britholite, which we record in 14 carbonatites, granites, and alkaline rocks.

Sorosilicates or disilicates

Melilite is the group name for the solid solution between åkermanite [Ca₂(Al₂SiO₇)] and gehlenite [Ca₂(MgSi₂O₇)] (Deer et al. 1986). Melilite is a common major mineral in basic alkaline magmas, being represented in 83 rocks of our compilation, with 63 examples containing more than 5 modal percent.

Several sorosilicates occur as occasional accessory phases in igneous rocks. For example, the beryllium silicate bertrandite [Be₄Si₂O₇(OH)₂] is recorded from 6 complex granite pegmatites in Online Materials₁ Table OM3.

Five other important examples of accessory igneous sorosilicates come from mineral groups that incorporate rare elements. Most frequently encountered are species of the allanite subgroup (also called “orthite”), which are Y- and REE-rich members of the diverse epidote group (Armbruster et al. 2006) with a range of compositions: [(Ca,Mn)(Y,REE)(Al,Fe³⁺,Fe²⁺)(Si₂O₇)(SiO₄)O(OH)]. We lump 9 species of this subgroup into allanite, which appears in 95 of the igneous rocks in our compilation, most of which are in felsic lithologies from granite to syenite. Allanite is present in major amounts in only one example, a carbonatite from Ahaggar, Algeria (Ouzegane et al. 1988). Note that several other epidote group minerals, including zoisite, clinozoisite, epidote, and piemontite (all hydrous Ca-bearing aluminosilicates), are sometimes reported from igneous rocks, but we ascribe these phases to secondary processes, such as metamorphism or hydrous alteration (e.g., saussuritization; Deer et al. 1982, 2001).

Members of the chevkinite group ( R E E ) 4 T i 4 + , F e 2 + , F e 3 + , Z r , M n , C r , W , ◻ 5 O 8 (Si₂O₇)₂], including 6 species that we lump into chevkinite, occur as scarce accessory minerals in alkaline rocks (Macdonald et al. 2019). We list 14 occurrences in Online Materials^[1] Table OM3.

The lamprophyllite group (Rastsvetaeva et al. 2016), with 8 species lumped into lamprophyllite [corresponding to (Sr,Ba)(Na,K)Ti^@@@4+Na₃Ti₄₊(Si₂O₇)₂O₂(OH)₂], has a similar paragenesis, with 13 occurrences in alkaline rocks.

The seidozerite supergroup includes a suite of complex and diverse minerals (Sokolova and Cámara 2017), with at least 48 approved species (https://rruff.info/ima, accessed 20 January 2022). We lump 12 of these species from the rinkite group [(Na,Ca,Mn,Y,REE)₄(H₂O,□)₂(Ti⁴⁺,Zr,Nb)(Si₂O₇)₂(OH,F)_4–x(H₂O)x] into rinkite. Our tabulations record 39 occurrences of rinkite as an accessory mineral in nepheline syenites and their pegmatites.

We lump five monoclinic members of the wöhlerite group (Merlino and Perchiazzi 1988) with compositions corresponding to [(Na,Ca)(Ca,Mn²⁺,Fe²⁺)₂ (Ti⁴⁺,Nb⁵⁺,Zr⁴⁺)(Si₂O₇)(O,F)₂] into wöhlerite. The 35 occurrences recorded in Online Materials^[1] Table OM3 are all from alkaline rocks.

Cyclosilicates

Members of the beryl group [(Na,Cs,□)Be²(Be,Li,B)(Al,Mg,Fe³⁺)₂Si₆O₁₈] are by far the most abundant Be-bearing minerals (Deer et al. 1986). We lump 6 species into beryl, and record occurrences in 41 igneous rocks, most notably in granitic pegmatites, where it often occurs as a major phase in association with albite, elbaite, lepidolite, and various scarce lithium and beryllium minerals.

The diverse minerals of the tourmaline supergroup, including more than 30 IMA-approved species, are the most common boron-bearing phases in igneous rocks (Deer et al. 1986; Henry et al. 2011). We divide the igneous tourmalines into two kinds. The more common tourmaline, embracing 20 different IMA-CNMNC-approved species and sometimes reported in the petrographic literature as “dravite” (with Mg > Fe²⁺) or “schorl” (with Fe²⁺ > Mg), has a broad compositional range of [(Na,Ca,□)(Mg,Al,Fe²⁺,Fe³⁺,Ti⁴⁺)₃(Al,Fe³⁺,Mg)₆(Si₆O₁₈)(BO₃)₃(OH)₃(O,F)]. Tourmaline occurs widely in igneous rocks, with 54 occurrences in our tabulation. We also distinguish 5 Li-bearing tourmaline species as elbaite (Henry et al. 2011), which we record from 24 complex granite pegmatites.

We lump the two species of the solid solution between catapleiite [Na₂Zr(Si₃O₉)·2H₂O] and calciocatapleiite [CaZr(Si₃O₉)·2H₂O] into catapleiite. We report 13 occurrences in alkaline rocks, in only one of which is catapleiite a major phase—an agpaitic nepheline syenite from the Norra Kärr alkaline complex, Sweden (Sjöqvist et al. 2017).

The eudialyte group boasts at least 30 approved species corresponding to end-member compositions in the general formula [(Na,Ca,REE)(Fe,Mn)(Zr,Ti) (Si₃O₉)₂(OH,F)·n_H2O] (Johnsen and Grice 1999; Rastsvetaeva and Chukanov 2012). While petrographers rarely identify the exact species, they often distinguish between Na-dominant “eudialyte” and more calcic “eucolite.” Here we lump all 30 species of the group into eudialyte, as detailed in Hazen et al. (2022). We report 69 alkaline rocks with eudialyte, 14 of which incorporate >5 mode percent in eudialyte-bearing agpaitic pegmatites.

Inosilicates

Pyroxene Group: Chain silicates, notably pyroxenes and amphiboles, are found in the majority of igneous rocks (Deer et al. 1997a, 1997b); they occur in 79% of the modes recorded in Online Materials^[1] Table OM3. We recognize six different kinds of clinopyroxenes based on their major element compositional ranges in the system [(Na,Li,Mg,Fe²⁺,Ca,Al,Fe³⁺)₂Si₂O₆], noting that continuous solid solutions occur among several of these pyroxenes. Aegirine includes Na- and Fe³⁺-bearing clinopyroxenes that range from the IMA-approved species aegirine (NaFe³⁺Si₂O₆) to aegirine-augite [(Ca,Na)(Fe³⁺,Mg,Fe²⁺)Si₂O₆]. Aegirine is an abundant phase that we record from 521 rocks in Online Materials^[1] Table OM3 (28%), usually in alkaline lithologies.

Augite, one of the few IMA-approved species that is not defined by a single end-member composition, represents clinopyroxenes in the [(Ca,Mg,Fe²⁺)₂Si₂O₆] system with Ca occupying between ~25 and ~45 at% of the Ca-Mg-Fe sites. We adopt the IMA definition for augite, which we report from 504 rocks that span a range from granitic to ultramafic lithologies. Pigeonite, similarly, is defined for clinopyroxenes with ~5 to ~15 at% Ca in the Ca-Mg-Fe sites. We recognize pigeonite as a relatively uncommon primary igneous phase in terrestrial rocks. We record only 16 instances in 1850 rocks compiled—a sharp contrast to meteoritic clinopyroxenes that often display pigeonite lamellae exsolved from augite (Deer et al. 1997a; Morrison and Hazen 2021).

We recognize two kinds of Ca-rich clinopyroxene, diopside (ideally CaMgSi₂O₆) and hedenbergite (ideally CaFe²⁺Si₂O₆). Both of these minerals display compositional plasticity, with significant Mg-Fe²⁺ solid solution in both end-members. Additionally, diopside often incorporates significant amounts of Al, Fe³⁺, Ti, and Cr in variants sometimes named “chrome-diopside,” “diallage,” “fassaite,” “malacolite,” or “salite.” We record 228 occurrences of diopside in a wide variety of igneous rocks. The Fe²⁺-dominant calcic pyroxene, hedenbergite, is much less common, restricted to 15 occurrences of granitic or alkaline rocks in our compilation.

The clinopyroxene spodumene (LiAlSi₂O₆), an important lithium ore mineral, is restricted to 24 complex granite pegmatites in our study.

We document orthorhombic pyroxenes, with end-members enstatite (MgSiO₃) and ferrosilite (Fe²⁺SiO₃), in 135 rocks, primarily in intermediate and mafic igneous rocks. Many citations refer to intermediate Mg-Fe²⁺-bearing compositions “bronzite” or “hypersthene;” we call all of these occurrences orthoenstatite—a name that underscores the Mg-dominant composition of most examples, and that distinguishes the orthorhombic pyroxene from clinoenstatite.

Pyroxenoid Group: Wollastonite (CaSiO₃), most commonly associated with metamorphosed limestone, occurs as a primary phase in a few igneous rocks, as well. We list 15 occurrences in alkaline rocks.

Amphibole Group: With at least 110 IMA-CNMNC-approved species, the amphibole supergroup of double-chain silicates is the most diverse of all mineral structure types (Hawthorne et al. 2011). Amphiboles occur in 775 (42%) of the 1850 rocks in Online Materials^[1] Table OM3. Identifying natural kinds of amphiboles through the construction of comprehensive data resources and application of cluster analysis (e.g., Ewing 1976; Gregory et al. 2019; Boujibar et al. 2021; Hystad et al. 2021) remains a challenging ambition. Here we provisionally lump these varied amphibole species into 6 groups of primary igneous minerals.

The hornblende group of calcic amphiboles, with a general formula [(o,Na,K) Ca₂(Mg,Fe²⁺,Al,Fe³⁺)₅(Si,Al)₈O₂₂(OH,F,Cl)₂], is the most common igneous double-chain silicate (Deer et al. 1997b; Hawthorne et al. 2011). A wide range of solid solutions, including Na-K in alkali sites, Mg-Fe-Al in octahedral sites, Al-Si in tetrahedral sites, and OH-F-Cl (see Online Materials^[1] Table OM6 for a list of end-member compositions) have been documented (Deer et al. 1997b). We lump 26 species into hornblende, which we record from 487 granitic, intermediate, and mafic rocks (Online Materials^[1] Table OM3).

Kaersutite [NaCa₂(Mg₃Fe³⁺Ti⁴⁺)(Si₆Al₂)O₂₂O₂], encompassing 2 IMA-approved species, is an anhydrous Na-bearing calcic amphibole that occurs primarily in alkalic, mafic, and ultramafic rocks. We record 19 examples in Online Materials^[1] Table OM3.

Richterite encompasses 15 species of calcic alkali amphiboles with the general formula [(Na,K)(Na,Ca)₂(Mg,Fe²⁺,Al,Fe³⁺)₅(Si₆Al₂)O₂₂(OH)₂], which likely represents a continuous range of solid solutions (Deer et al. 1997b). We list 47 examples from various alkaline rocks.

We lump 10 species of alkali amphiboles spanning the compositional range [(Na,K)Na₂(Mg,Fe²⁺,Al,Fe³⁺)₅Si₇(Si,Al)O₂₂(OH,F)₂] into arfvedsonite. We list 229 occurrences, primarily from alkaline rocks. We distinguish riebeckite, a group of 3 alkali amphibole species with vacancies, corresponding to [□Na₂(Mg,Fe²⁺)₃ Fe^@@@@3+Si⁸O²²(OH,F)²]. Online Materials^[1] Table OM3 includes 68 occurrences, primarily in alkali granites and other alkaline rocks.

Actinolite, a calcic amphibole with vacancies [□Ca₂(Mg,Fe²⁺)₅Si₈O₂₂(OH,F)₂], is an intermediate member of the tremolite-actinolite (Mg-Fe²⁺) series. We lump all 10 occurrences into actinolite, which is most often associated with metamorphic rocks and only occurs rarely as a primary igneous mineral in carbonatites and other Ca-rich lithologies.

The pectolite group of single-chain silicates, with 4 species lumped into pectolite [(Na,Li)(Ca,Mn²⁺)₂Si₃O₈(OH)], is a late-stage mineral that forms near the transition from magmatic to hydrothermal crystallization in some mafic and alkaline rocks (Deer et al. 1997a). We list 12 occurrences of pectolite, in most of which it is associated with nepheline and aegirine.

Two additional inosilicate groups, both branched-chain silicates, are associated with alkaline rocks and their pegmatites. We lump 12 members of the astrophyllite group [(K,Na,Li,Cs,□,Ca)₃(Fe²⁺,Mn²⁺,Mg,Na)₇(Ti⁴⁺,Zr,Nb)₂(Si₄O₁₂)₂O₂ (OH)₄(OH,O,F)(H₂O)n] into astrophyllite, which occurs in 37 agpaitic rocks listed in Online Materials₁ Table OM3. We also include aenigmatite N a 4 F e 10 2 + T i 2 O 4 S i 12 O 36 , recorded in 53 varied alkaline rocks.

Phyllosilicates

Mica Group: Mica group minerals occur abundantly as primary igneous minerals (Speer 1984; Fleet 2003), being found in 1107 of 1850 rocks (60%) compiled in this survey (Online Materials^[1] Table OM3). In 646 instances (35%), one or more kinds of mica is a major phase. At least 34 mica species have been approved by the IMA-CNMNC; however, from a petrographic perspective we recognize only 4 mica groups as primary igneous phases. By far the most common mica group is biotite K F e 2 2 + F e 2 + , M g , M n 2 + S i , A l , F e 3 + 2 S i 2 O 10 ( O H , F , C l ) 2 which encompasses a range of Fe-bearing, dark-colored trioctahedral micas that have been reported in 866 of 1850 rock modes (47%), and as a major phase in 466 of those rocks (25%). Biotite lumps 6 species of micas [Hazen et al. (2022); Online Materials^[1] Table OM3; see Online Materials^[1] Table OM6 for a list of end-member compositions].

Phlogopite includes various Mg-dominant, light-colored trioctahedral micas [KMg₂(Mg,Fe²⁺,Mn²⁺,Fe³⁺,Ti⁴⁺)(Si,Al,Fe³⁺)₂Si₂O₁₀(OH,F)₂], representing 6 IMA-CNMNC-approved species (see Online Materials^[1] Table OM6 for a list of end-member compositions). It was reported in 176 rocks, primarily carbonatites and alkaline rocks, in which it is a major phase (>5 vol%) in 117 examples.

The dioctahedral aluminous mica, muscovite [KAl₂(Si₃Al)O₁₀(OH)₂], occurs in 91 rocks, in 22 of which it is a major phase. Muscovite is a common primary mineral in granitic rocks and their pegmatites. However, it can also be a secondary mineral in igneous rocks—occurrences that can usually be distinguished by their textures (Speer 1984; Roycroft 1991; Fleet 2003).

In addition, at least 11 species of lithium-bearing micas have been approved; we lump all of these into lepidolite, which is recorded in 42 complex pegmatites in our survey.

An important task for future research is a compilation of data on numerous igneous mica compositions and their distinctive lithologies, with the objective of defining mica natural kinds through cluster analysis. It is very likely that such widely occurring igneous minerals as biotite and muscovite should be split into multiple natural kinds.

In addition to the mica group minerals, petalite (LiAlSi₄O₁₀) is a relatively uncommon sheet silicate that we record in 19 complex granite pegmatites (Deer et al. 2009). It is almost always associated with beryl, lepidolite, and tourmaline and/or elbaite.

Tectosilicates

Silica Group: Quartz, tridymite, and cristobalite, all polymorphs of SiO₂, occur as primary igneous phases (Deer et al. 2004). However, only quartz is frequently encountered as a major igneous mineral. We record quartz from 531 rocks, most of which are varieties of granite and their pegmatites.

Tridymite is a high-temperature form of SiO₂ that must cool rapidly; therefore it occurs most frequently in rhyolites and other acidic extrusive rocks. We list 5 instances in our compilation.

The volcanic glass, obsidian, though not an approved mineral, is an important silica-rich phase in rhyolites. Obsidian, which typically contains > >70 wt% SiO₂, and usually with ~90 wt% (SiO₂ + Al₂O₃), may represent >99 vol% of rapidly cooled, silica-rich extrusives. We record 5 examples in Online Materials^[1] Table OM3, though that number underestimates the frequency of obsidian occurrence because the modes of extrusive rocks are rarely reported.

Feldspar Group: Minerals of the feldspar group are the most common phases in igneous rocks, appearing in 1375 of 1850 diverse rocks in our study (74%), while forming an estimated 50 vol% of Earth’s crust (Deer et al. 2001; Rudnick and Gao 2003). The only major rock-forming feldspars occur with compositions close to two binary systems: alkali feldspars (NaAlSi₃O₈ to KAlSi₃O₈) and plagioclase feldspars (NaAlSi₃O₈ to CaAl₂Si₂O₈). Each of these series has complexities related to petrographic nomenclature.

The IMA-CNMNC recognizes the two end-members of the plagioclase series—albite and anorthite—as valid species. Petrographers, on the other hand, have traditionally split plagioclase into six compositional types: albite, oligoclase, andesine, bytownite, labradorite, and anorthite (with compositional boundaries of 0 to 10, 10 to 30, 30 to 50, 50 to 70, 70 to 90, and 90 to 100 at% anorthite content, respectively). Complexities arise both because many igneous plagioclase crystals are zoned across two or three of these types, and because specimens often display fine-scaled exsolution, termed peristerite, Bøggild intergrowths, and Huttenlocher intergrowths (e.g., Deer et al. 2001). In our survey we adopt a compromise nomenclature, with three compositional ranges. Na-rich albite and oligoclase are lumped into albite (543 occurrences; Online Materials^[1] Table OM3); intermediate andesine and bytownite are lumped into plagioclase (210 occurrences), and Ca-rich labradorite and anorthite are lumped into anorthite (245 occurrences).

The alkali feldspars present their own complexities in nomenclature. In the petrographic literature, sodium-rich varieties include albite and K-bearing “anorthoclase,” as well as “oligoclase,” all of which we record as albite. Intermediate alkali feldspar compositions commonly display exsolution, either perthite with Na-rich exsolution lamellae in a K-rich feldspar, or antiperthite with K-rich exsolution lamellae in a Na-rich feldspar. We classify all of these alkali feldspars with prominent exsolution textures as perthite, which occurs in 304 rocks of Online Materials^[1] Table OM3. Feldspars near the potassium-rich end-member occur in several structure types, identified as sanidine, orthoclase, and microcline depending primarily on the ordered state of Al and Si (which is in turn a function of cooling history). In the petrographic literature they are often referred to collectively as “K-spar” without further identification. Sanidine is the high-temperature form with disordered Al-Si; it typically occurs in rapidly cooled magmas from near-surface environments and is reported from 82 extrusive rocks in our survey. Orthoclase (447 occurrences) and microcline (310 occurrences) incorporate ordered Al-Si arrangements, indicative of more slowly cooled environments. In some reports, petrographers are able to distinguish microcline based primarily on its distinctive pattern of fine-scale cross-hatched twinning. Other potassic feldspars are identified as orthoclase or simply K-spar. With acknowledged uncertainties, we identify sanidine and microcline when those minerals are explicitly named; otherwise we lump orthoclase and K-spar into orthoclase.

Feldspathoid Group: Feldspathoid group minerals, defined as alkali- or Cabearing phases that are chemically similar to feldspar but incorporating less silica, are major primary igneous phases in a range of silica-undersaturated, alkaline rocks (Deer et al. 2004). They appear in 659 of 1850 rocks (35%) tabulated in Online Materials^[1] Table OM3, though that number likely exaggerates the volumetric importance of feldspathoids because of our inclusion of 795 modes of alkaline rocks and carbonatites from Woolley’s comprehensive review (Woolley 1987, 2001, 2019; Kogarko et al. 1995). Nevertheless, in many subsilicic rocks feldspathoid minerals, notably nepheline, take the place of feldspar as the volumetrically dominant rock-forming phase.

Nepheline [Na₆(K,Cao)₂(Al₈Si₈O₃₂)] is the most frequently encountered feldspathoid in our tabulations with 567 occurrences. It is the dominant mineral in nepheline syenites, in which it is often associated with alkali feldspars (Deer et al. 2004). Nepheline also occurs with plagioclase in various mafic lithologies, for example nepheline gabbros.

Other common feldspathoids include leucite [K(Si₂Al)O₆], recorded in 103 rocks, at times as “pseudoleucite,” in which nepheline, K-feldspar, and other phases replace primary leucite. Sodalite [Na₄Si₃Al₃O₁₂Cl], listed in the modes of 158 alkaline rocks, and the structurally related hauyne [Na₃(Ca,Na,K)(Si₃Al₃) O₁₂(SO₄)·(H₂O)], found in 63 rocks, commonly occur in association with nepheline or leucite. Kalsilite [(K,Na)AlSiO₄], the K-dominant isomorph of nepheline, is much less abundant, reported from only 6 alkaline rocks in our compilation.

The cancrinite group, with 22 species lumped into cancrinite [(Na,K,Cao)₈ (Al₆Si₆)O₂₄(CO₃,SO₄,Cl,OH)₂·n_H2O], presents an added complication. Cancrinite is a relatively common feldspathoid, appearing in the modes of 119 alkaline igneous rocks. However, because cancrinite paragenesis requires a fluid rich in carbonate or sulfate ions, it can form both as a late-stage primary igneous phase and as a secondary mineral, for example through alteration of nepheline by reaction with carbonate-rich fluids. We have excluded cancrinite from Online Materials^[1] Table OM3 when it is explicitly described as a secondary phase. However, in many other instances the mode of cancrinite formation is unknown. In those instances, we record cancrinite as a primary phase.

Scapolite, with 3 lumped species [(Na,Ca)₄(Al,Si)₁₂O₂₄(CO₃,SO₄,Cl)], usually occurs as a metamorphic or metasomatic phase. In Online Materials₁ Table OM3 we list 12 occurrences that may represent formation during initial magma cooling of alkaline or mafic lithologies, though uncertainties remain.

Zeolite Group: The great majority of zeolite group framework aluminosilicates (Deer et al. 2004), as well as closely associated minerals such as prehnite and apophyllite (Deer et al. 2009), occur as secondary/hydrothermal phases deposited by post-magmatic fluid interaction/alteration of vesicular basalt and other igneous rocks. However, we list 3 kinds of zeolite that also may occur as late-stage primary phases in igneous rocks.

Analcime [NaAlSi₂O₆·H₂O] is the most abundant of these phases, with 151 occurrences in our list (in 88 of which it is reported as a major phase). Analcime is most commonly associated with nepheline in a wide range of intrusive and extrusive lithologies, typically alkaline and silica-undersaturated rocks (Johannsen 1938; Deer et al. 2004).

Natrolite [(Na,Ca)₂(Si₃Al₂)O₁₀·2H₂O] is reported as a primary zeolite in the modes of 17 rocks in Online Materials^[1] Table OM3, notably as a late-stage major phase in some agpaitic pegmatites, in which it may form from auto-metasomatism during cooling. Natrolite usually occurs with nepheline and aegirine in alkaline rocks.

Pollucite [Cs(Si₂Al)O₆·n_H2O] is a common late-stage (T < 500 °C) zeolite phase (London 1986, 2008; Teertstra and Černý 1995), though it also occurs as a secondary zeolite (Deer et al. 2004). We report 22 occurrences, exclusively from complex rare-element granite pegmatites (Online Materials^[1] Table OM3), in which it is almost always associated with beryl, lepidolite, and one or more tourmaline group minerals. In spite of the scarcity of Cs, representing <5 ppm of atoms in Earth’s crust (Rudnick and Gao 2005), pollucite can be present as a major phase in the terminal crystallization of a pegmatite, with crystal pods in the Tanco pegmatite, Manitoba, Canada, exceeding 10 m in length (London 2008).

Silicate glass: Primary igneous glass with a mixture of rock-forming elements [e.g., (Si,Al,Ca,Mg,Fe,O)] is an important primary extrusive igneous phase, most notably as glass of basaltic composition but also in a range of intermediate to mafic extrusive rocks. We provisionally employ silicate glass as a catch-all term for such amorphous igneous phases that are less Si-rich than obsidian (i.e., <70 wt% SiO₂), as recorded in 48 modes in Online Materials^[1] Table OM3. Future research on the tabulation of igneous glass compositions and cluster analysis of their distributions will be required to determine if silicate glass and obsidian represent a continuum, or if there are multiple natural kinds of Si-bearing glass in extrusive igneous rocks.

Rarer Primary Igneous Minerals: More than 800 other minerals (in addition to the 115 kinds outlined above) occur rarely as trace phases in igneous rocks, as documented by reports in numerous primary sources and compilations, notably Anthony et al. (1990–2003) and references cited in https://mindat.org and https://rruff.info/ima (both accessed 20 January 2022). Most of these scarce minerals, which are listed in Online Materials^[1] Table OM2, were not recorded from any of the 1850 igneous rock modes in Online Materials^[1] Table OM3.

However, a few of the less common minerals that are designated “2” (i.e., uncommon accessory phases) in Online Materials^[1] Table OM2, were also recorded in one or more igneous rock modes. Among these minor minerals, listed alphabetically, are agrellite (1 occurrence), armalcolite (2), baotite (2), calzirtite (2), canasite (1), carbocernaite (2), cerianite (1), cerite (3), charoite (1), chrysoberyl (3), chrichtonite group (3), dalyite group (2), daqingshanite (1), elpidite (3), fersmanite (1), florencite (2), gadolinite group (1), gittinsite (2), hellandite (1), helvine group (2), hogbomite group (2), hollandite group (3), isokite (1), joaquinite group (1), karnasurite (1), labuntsovite (3), lanthanite group (1), lazurite (1), lievrite (2), lorenzenite (2), lovozerite group (1), lueshite (1), murmanite (2), narsarsukite (2), natroniobite (1), neotocite (1), neptunite group (2), pachnolite (1), rhabdophane group (1), rhönite (4), sahamalite (1), samarskite group (1), scheelite (2), steenstrupine (2), thorianite (3), tinaksite (1), uraninite (1), vlasovite (1), weberite (1), wolframite (1), wulfenite (3), and zirkelite (4).