Nomenclature and Classification of the Spinel Supergroup

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Eur. J. Mineral. 2019, 31, 183–192 Published online 12 September 2018

Nomenclature and classiﬁcation of the spinel supergroup

Ferdinando BOSI1,*, Cristian BIAGIONI2 and Marco PASERO2

1Department of Earth Sciences, Sapienza University of Rome, P. Aldo Moro 5, 00185 Roma, Italy *Corresponding author, e-mail: [email protected] 2Dipartimento di Scienze della Terra, Universita` di Pisa, Via S. Maria 53, 56126 Pisa, Italy

Abstract: A new, IMA-approved classification scheme for the spinel-supergroup minerals is here reported. To belong to the spinel supergroup, a mineral must meet two criteria: (i) the ratio of cation to anion sites must be equal to 3:4, typically represented by the general formula AB2X4 where A and B represent cations (including vacancy) and X represents anions; (ii) its structure must comprise a heteropolyhedral framework of four-fold coordination polyhedra (TX4) isolated from each other and sharing corners with the neighboring six-fold coordination polyhedra (MX6), which, in turn, share six of their twelve X-X edges with nearest- neighbor MX6. Regardless of space group, the X anions form a cubic close-packing and each X anion is bonded to three M-cations and one T-cation. The fifty-six minerals of the spinel supergroup are divided into three groups on the basis of dominant X anion: O2– (oxyspinel), S2– (thiospinel), and Se2– (selenospinel). Each group is composed of subgroups identified according to the dominant valence and then the dominant constituent (or heterovalent pair of constituents) represented by the letter B in the formula AB2X4. 2þ 3þ The oxyspinel group (33 species) can be divided into the spinel subgroup 2-3 ðA B2 O4Þ and the ulvo¨spinel subgroup 4þ 2þ 1þ 3:5þ 4-2 ðA B2 O4Þ, the thiospinel group (20 species) into the carrollite subgroup 1-3.5 ðA B2 S4Þ and the linnaeite subgroup 2þ 3þ 2+ 3+ 2-3 ðA B2 S4Þ, finally, the selenospinel group (3 species) into the bornhardtite subgroup 2-3 (A B 2Se4) and the potential 1þ 3:5þ ‘‘tyrrellite subgroup’’ (A B2 S4, currently composed by only one species). Once the subgroup is established based on the valence of B, then the mineral species is identified by the combination of the dominant A- and B-cations. Moreover, the present nomenclature redefines the ideal formulae of titanomaghemite, cuprorhodsite, malanite, maghemite, filipstadite, tegengrenite, rhodostannite, toyohaite and xingzhongite as well as discredits ‘‘iwakiite’’, ‘‘hydrohetaerolite’’ and ‘‘ferrorhodsite’’. Key-words: spinel; oxyspinel; thiospinel; selenospinel; nomenclature; classification.

1. Introduction 2. Spinel basic aspects

The central role of spinel-type compounds, both in the Earth 2.1. Structure Sciences and in many other scientific and technological fields, is well known. Indeed, minerals with the spinel struc- The spinel-type structure is adopted by many compounds ture crystallize in a wide range of physical-chemical condi- with a cations-to-anions ratio of 3:4, typically represented tions, from the upper mantle to crust, as well as in by the general formula AB2X4 where the letters (not itali- extraterrestrial bodies, e.g., Moon, Mars, and meteorites cized) A and B indicate constituents (not crystallographic (e.g., Papike et al., 2005; Righter et al., 2006). Compounds sites) represented by a large number of different cations or 2– 2– 2– with spinel-type structures also play a prominent role in vacancy and X represents anions, such as O ,S ,Se , 3– 2– 1– 1– – solid-state materials science because of their physical N , Te Cl ,(CN) and F . The basic structure may be properties. described as a slightly distorted cubic close-packed array of As pointed out by Biagioni & Pasero (2014), there is a X-anions with A- and B-cations occupying 1/8 of the tetrahe- wide chemical variability of the spinel minerals. Thus, the drally and 1/2 of the octahedrally coordinated sites (respec- development of a comprehensive classification system is a tively, T and M, letters italicized). The structures of mineral necessary step to understand the processes that govern their species belonging to the spinel supergroup can be more con- diversity. The present recommended nomenclature and clas- veniently described in terms of a heteropolyhedral framework sification was approved by the Commission on New Miner- that possesses some basic features: it is formed by TX4 als, Nomenclature and Classification (CNMNC) of the tetrahedra isolated from each other and sharing corners with International Mineralogical Association (IMA), voting pro- the neighboring MX6 octahedra; the MX6 octahedron shares posal 17-H. six of its twelve X–X edges with nearest-neighbor MX6

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Fig. 1. Crystal structure of spinel (a); details of the local bonding environment around the T site (b) and the M sites (c). Brown = TX4 polyhedron; light blue = MX6 polyhedron; red = X anion. The X anions are not shown in (a), and TX4 is shown as ball-and-stick in (c).

octahedra. The X anion is in turn four-fold coordinated by Consequently, spinels can display extensive cation three M cations and one T cation (Fig. 1). This polyhedral disorder over the T and M sites; an accurate determination arrangement is consistent with several space groups ranging of site partitioning would require the combination of data from the high-symmetry space-group type Fd3m of cubic obtained by different methods, such as single-crystal X-ray aristotype spinel (Bragg, 1915; Nishikawa, 1915)toP4132, diffraction (SC-XRD) and electron microprobe analysis P41212, I41/amd, I41/a, P4m2andR3. (EMPA), possibly integrated with additional spectroscopic On the other hand, there are minerals with general information (e.g., Mo¨ssbauer and optical absorption spec- formula AB2O4 but with a structure quite distinct from that troscopy). Hatert & Burke (2008) noted the problem of of spinel, which therefore do not belong to the spinel uncertainty in the spinel cation distributions for nomencla- supergroup. For example, the orthorhombic structure of ture purposes, and suggested to keep the A and B con- xieite (FeCr2O4, the high-pressure polymorph of chromite) stituents separate, regardless of their site occupancies as comprises eight-fold coordinated Fe2+; in wadsleyite these are not imposed by valency considerations. Hence, (b-Mg2SiO4), tetrahedra share a corner to form the Si2O7 the present classification is based on chemical formulae as group; the structure of chrysoberyl (BeAl2O4) and forsterite resulting from chemical information only. (a-Mg2SiO4) is based on a hexagonal close-packed array of oxygen atoms, in which tetrahedra shares three edges with 2.3. Chemical variability neighboring octahedra. The spinel structure is remarkably flexible in a chemical sense, accommodating atoms of a wide range of size and 2.2. Cation distributions charge, as well as vacancies. Extensive substitution series Traditionally spinels are denoted as either ‘‘normal spinels’’, occur among various spinel end-members, particularly where for example the T sites are occupied by divalent among pairs with the same ordering type. For example, cations and the M sites are occupied by trivalent cations, magnesiochromite–chromite MgCr2O4–FeCr2O4 is a binary or ‘‘inverse spinels’’ whose T sites are occupied by trivalent system consisting of two normal spinels (Lenaz et al., cations and the M sites by one divalent plus one trivalent 2004), whereas qandilite–ulvo¨spinel TiMg2O4–TiFe2O4 is T M cation. Given the structural formula (A1–iBi) (AiB2–i)X4, a binary system consisting of two inverse spinels (Bosi the two extreme distributions correspond to i =0(normal) et al., 2014). and i = 1 (inverse). The value of i, the inversion parameter, The chemical variability is not limited only to cations, depends on several factors, e.g., temperature, oxygen fugac- but encompasses anions, too. Most naturally occurring ity, crystallization kinetics, covalency effects (such as sp3 spinels are oxides, but there are also rare sulfide and selenide hybridization) and stabilization energies of cations at the spinel-type minerals. If one considers synthetic compounds, M site (e.g., O’Neill & Navrotsky, 1984; Nell et al., 1989; this structural family also includes halides, pseudo- O’Neill et al., 1992; Della Giusta et al., 1996; Redfern halides (cyanides), tellurides and nitrides. There are many et al., 1999; Andreozzi et al., 2000; Andreozzi & studies focusing on the synthesis and characterization of Princivalle, 2002; Lucchesi et al., 2010; Papike et al., spinel compositions, and several of them have produced 2015; Bosi & Andreozzi, 2017). compositions that are not presently known to occur in

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nature: e.g., MgMn2O4 (Radhakrishnan & Biswas, 1975), Table 1. A classiﬁcation of the 56 species in the spinel supergroup. ZnAl2S4 (Steigmann, 1967), ZnCr2Se4 (Von Philipsborn, Oxyspinel group 1971), NiLi2F4 (Mu¨ller, 2007), ZnLi2Cl4 (Sassmannshausen Spinel subgroup (2-3) A2+ B3+ X 2þ 2 et al., 1996), ZnK2(CN)4 (Rohrer, 2001), Cr Cr2Te4 Chromite Fe Cr2 O4 (ICSD No. 43041) and SiTi2N4 (Ching et al., 2001). Spinels Cochromite Co Cr2 O4 1– 1– 1– 2– 3– with X = F ,Cl ,(CN) ,Te ,andN are only known as Coulsonite Fe V2 O4 synthetic phases, whereas spinels with X = O2–,S2– and Se2– Cuprospinel Cu Fe2 O4 Dellagiustaite V Al O occur in nature. In contrast to the extended substitution series 2 4 Deltalumite (Al0.67h0.33)Al2 O4 occurring among end-members comprising the same anion, Franklinite Zn Fe2 O4 but different cations, there are miscibility gaps among the Gahnite Zn Al2 O4 oxy-, thio- and seleno-species in natural samples. Galaxite Mn Al2 O4 Guite Co Co2 O4 Hausmannite Mn Mn2 O4 3. The supergroup-group-subgroup hierarchy Hercynite Fe Al2 O4 Hetaerolite Zn Mn2 O4 of the spinels JacobsiteÀÁ Mn Fe2 O4 3þ Maghemite Fe0:67Ã0:33 Fe2 O4 In line with the CNMNC guidelines (Mills et al., 2009), the Magnesiochromite Mg Cr2 O4 56 valid species of the spinel supergroup are divided here Magnesiocoulsonite Mg V2 O4 Magnesioferrite Mg Fe O into three groups on the basis of the dominant X species 2 4 Magnetite Fe Fe2 O4 (O,S,Se;seeTable 1): Manganochromite Mn Cr2 O4 (1) Oxyspinel group; Spinel Mg Al2 O4 (2) Thiospinel group; Thermaerogenite Cu Al2 O4 Titanomaghemite Ti4þÃ Fe O (3) Selenospinel group. ð 0:5 0:5Þ 2 4 Trevorite Ni Fe2 O4 Within each group, subdivision into subgroups is per- Vuorelainenite Mn V2 O4 formed according to the cation charge arrangement combi- Zincochromite Zn Cr2 O4 4+ 2+ nations, constrained by the (A + B) to X atomic ratio of Ulvo¨spinel subgroup (4-2) A B X 3:4 (see above). Various distinct combinations may occur Ahrensite Si Fe2 O4 Brunogeierite Ge Fe2 O4 in the spinel supergroup, although some of them have been 3þ 5þ Filipstadite ðFe0:5Sb0:5Þ Mn2 O4 observed only in synthetic compounds. A useful way to Qandilite Ti Mg2 O4 groupthecationchargearrangementsisbasedonthe Ringwoodite Si Mg2 O4 Tegengrenite Mn3þSb5þ Mg O weighted average of the sum of formal charges of the con- ð 0:5 0:5Þ 2 4 Ulvo¨spinel Ti Fe O stituents A and B; for example: 2 4 R1+ R7+ A + B 3þ 5þ – 1-3.5,(A) (B2) as in (Cu ) (Cr1:5Sb0:5)S4 Thiospinel group

(florensovite); 1+ 3.5+ R2+ R6+ A 2+ B 3+ Carrollite subgroup (1-3.5) A B X – 2-3,(A) (B2) as in (Mg ) (Al )2O4 (spinel); a R4+ R4+ A 4+ B Carrollite Cu Co2 S4 2þ 3+ 4+ – 4-2,(A) (B2) as in (Ti ) (Fe2 )O4 (ulvo¨spinel). Cuproiridsite Cu (Ir Ir )S4 3+ 4+ Note that the constituent B (and sometimes also A) can Cuprokalininite Cu (Cr Cr )S4 a represent a heterovalent pair of ions imposed by the Fletcherite Cu Ni2 S4 3þ 5þ Florensovite Cu ðCr1:5Sb0:5Þ S4 electroneutrality principle in the end-member compositions. 3+ 4+ Malanite Cu (Ir Pt )S4 In accord with the above-mentioned cation charge arrange- 2þ 4þ Rhodostannite Cu ðFe0:5Sn1:5Þ S4 2þ 4þ ments, the oxyspinel group can be divided into subgroups 2-3 Toyohaite Ag ðFe0:5Sn1:5Þ S4 and 4-2. In thethiospinel and selenospinelgroups, someuncer- Linnaeite subgroup (2-3) A2+ B3+ X tainties in the ion oxidation states (e.g., Vaughan & Craig, Cadmoindite Cd In2 S4 Cuprorhodsite Cuþ Fe3þ Rh S 1978; Barkov et al., 2000; Pattrick et al., 2008) make the clas- ð 0:5 0:5Þ 2 4 Daubreélite Fe Cr S sification more difficult and, likely, the assignment of formal 2 4 Greigite Fe Fe2 S4 charges in such compounds has very little meaning (e.g., Indite Fe In2 S4 Goodenough, 1969). However, on the basis of the current Joegoldsteinite Mn Cr2 S4 knowledge, they can be theoretically divided as follows: thios- Kalininite Zn Cr2 S4 pinel-subgroups 1-3.5 and 2-3; selenospinel-subgroups 1-3.5 Linnaeite Co Co2 S4 Polydymite Ni Ni2 S4 and 2-3 (Fig. 2). This subdivision scheme can easily be Siegenite Co Ni2 S4 extended, for example, to possible subgroups 2-1 and 6-1. Violarite Fe Ni2 S4 Moreover, new groups can be established if AB2X4 com- Xingzhongite Pb Ir2 S4 pounds with spinel structures are discovered as new minerals. Selenospinel group

‘‘Tyrrellite subgroup’’ (1-3.5) A1+ B3.5+ X 4. Spinel classiﬁcation by chemical formula a Tyrrellite Cu (Co,Ni)2 Se4 Bornhardtite subgroup (2-3) A2+ B3+ X

In order to assist the geoscience community in identifying Bornhardtite Co Co2 Se4 spinel species, and to simplify the nomenclature (Hatert & Tru¨stedtite Ni Ni2 Se4 Burke, 2008), cations at the T and M sites are grouped aSee discussion in the text.

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Alternatively, the ratio RR3+/RR2+ can be used to distinguish between the two subgroups. This ratio varies from 0 to 2/3 for spinel 4-2, and from 2/3 to 2 for spinel 2-3. The special condition RR3+/RR2+ =2/3(RR2+ =1.5apfu and RR3+ =1.0 apfu) corresponds to the boundary between spinels 4-2 and 4þ 3þ 2þ 2-3 (e.g., Ti0:5Fe1:0Fe1:5O4). In order to avoid unrealistic end-member formulae, when RR2+ >1.5apfu (subgroup 4-2), the B constituent is identi- R2+ Fig. 2. The supergroup-group-subgroup hierarchy and general fied with and the chargeÀÁ balance is achieved by coupling formulae of the spinels. concentrations such as B2þ R3:02þ with the charge arrange- ÀÁ1:51 R4þ 2þ R4:98þ 4þ 3þ ment A1:0 B0:49 : e.g., Ti1:0Fe2:0O4. On the other together in spinel, and each species is defined in a systematic hand, when RR3+ >1.0apfu (subgroup 2-3), B = R3+ and fashion by stoichiometry, that is, on the basis of the chem- ÀÁ 3þ R3:03þ ical formula AB2X4. concentrations such as B1:01 will be consistent with In fact, the use of the structural formula, i.e. the application R2þ 3þ R4:97þ 2þ 3þ the charge arrangement ðA1:0 B0:99Þ : e.g., Fe1:0Fe2:0O4. of the standard CNMNC guidelines (Nickel & Grice, 1998) Once the valence of B is established, the dominant to each single crystallographic site to identify a spinel spe- B-cation and then the dominant A-cation are identified cies, may lead to the creation of a surplus of end-members, according to the dominant-constituent rule. A possible diffi- which would be difficult to distinguish on a routine experi- culty is that as a result of charge balance requirements, some mental basis and with little geological or petrological spinel end-members have A or B constituted by heterovalent significance. For example, Nakatsuka et al. (2004) studied R4þ 3þ pairs of ions (or ion + vacancy). For example, A1:0 and synthetic spinels along the series Mg(Al2xFe )R2O4 and R2þ x AÀÁ1:0 may representÀÁ the cation arrangements reported for the composition x = 1.21 the structural formula: A 3þ 5þ R4þ A 3þ 5þ R4þ A 4þu R2+ T 3þ M 3þ Fe0:5Sb0:5 or Mn0:5Sb0:5 and (Ti0:5 0:5) (Mg Al : Fe ) (Mg Al : Fe ) O . Acco- A 3þ R2+ 0:38 0 09 0:53 R1.00 0:62 0 70 0:68 R2.00 4 or (Fe u0:33) , respectively. This mixed occupancy rding to Hawthorne (2002), the above formula shows that 0:67 3+ R5+ of A (or also of B) is consistent with the rule of the Fe is dominant at the T site and the pair (Mg + Al) valency-imposed double site-occupancy (Hatert & Burke, is dominant at the M site, thus leading to end-member 3+ 2008). (Fe )(MgAl)O4. The latter would represent a potential ‘‘intermediate’’ end-member between spinel and magnesioferrite (MgAl2O4–MgFe2O4). Similar arguments apply to 5. Issues in the classification of chalcospinels the magnetite–ulvo¨spinel series (FeFe2O4–TiFe2O4), for according to their cation charge arrangements 3+ 2þ which the ‘‘intermediate’’ end-member (Fe )(Fe1:5Ti0:5)O4 is expected. Moreover, a classification scheme based on The assignment of the oxidation states of oxyspinels can be structural formulae will generate new species differing in considered relatively straightforward. On the other hand, their inversion parameter: e.g., the normal spinel ringwood- the assignment of formal charges in chalcospinels (thiospi- T M ite (Si) (Mg)2O4 would deserve a new name when occur- nels and selenospinels) could have very little meaning T M ring as inverse spinel (Mg) (MgSi)O4 (Bindi et al., 2018). (e.g., Goodenough, 1969), owing to the effects of elec- All aforementioned examples encumber the development of tron delocalization and their possible non-stoichiometry consistent nomenclature system aimed at minimizing the (e.g., Vaughan & Craig, 1978; Charnock et al., 1990). In proliferation of mineral names and identifying mineral addition, several transition elements occurring in Cu- and species with relatively simple methods. S-bearing compounds have two or more possible formal The total variations in spinel composition can easily be charges that need to be independently determined. In fact, described by the ternary system A–B–X, where parameters they cannot be constrained by charge balance as the charge A and B show the greatest degree of variation. Once the X is of S can assume non-integer values between –2 and –1, due established, the system is reduced to a binary system A–B. to the presence of holes in the S 3p band (e.g., Riedel et al., The variation of these parameters is constrained by the elec- 1981; Pattrick et al., 2008). troneutrality principle, and therefore only one parameter is Actually, many thiospinels and selenospinels, character- actually needed to identify the subgroup. This parameter is ized by the occurrence of Cr, Mn, Fe, Co, Ni, Zn, Cd, and represented by the most abundant constituent B in the In, could be reasonably assigned to subgroup 2-3, i.e., formula AB X . From chemical analytical data and in 2þ 3þ 2þ 3þ 2 4 cadmoindite (Cd In2 S4), daubreélite (Fe Cr2 S4), keeping with the dominant-valency rule (Hatert & Burke, 2þ 3þ 2þ 3þ greigite (Fe Fe2 S4), indite (Fe In2 S4), joegoldsteinite 2008), the dominant valence can be determined for B by 2þ 3þ 2þ 3þ 2þ 2+ (Mn Cr2 S4), kalininite (Zn Cr2 S4), linnaeite (Co summing the ions for each valence, e.g., RR = 3þ 2þ 3þ 2þ 3þ (Mg2+ +Fe2+ +Mn2+)orRR3+ =(Cr3+ +Fe3+ +Al3+). Co2 S4), polydymite (Ni Co2 S4), siegenite (Co Ni2 2+ 2þ 3þ 2þ 3þ Oxyspinels of subgroup 4-2 are characterized by RR > S4), tru¨stedtite (Ni Ni2 Se4), violarite (Fe Ni2 S4)and 2þ 3þ 1.5 atoms per formula unit (apfu), whereas oxyspinels of the questionable mineral bornhardtite (Co Co2 Se4). Some subgroup 2-3 are characterized by RR3+ >1.0apfu. of these assignments are supported by spectroscopic investi- When RR2+ >1.5apfu, RR3+ <1apfu and vice versa. gations (e.g., Vaughan & Craig, 1985).

However, the classification of Cu-bearing chalcospinels is The following spinels were redefined or discredited from more problematic. In particular, the charge of ions in the original description for internal consistency among the carrollite (CuCo2S4), fletcherite (CuNi2S4) and tyrrellite spinel species. [Cu(Co,Ni)2Se4] is a matter of discussion. Indeed, Cu usually occurs as Cu1+ in sulfides, whereas Cu2+ is very rare. 6.1. Redefined spinel species This notwithstanding, carrollite has been traditionally con- À 4þu 2þ 3þ 2 6.1.1. Titanomaghemite, redefined as Ti0:5 0:5ÞFe2O4 sidereda2-3species,Cu Co2 ðS Þ4 (e.g., Charnock et al., 1990). Recently, Pattrick et al. (2008) and Buckley Titanomaghemite was reported as a questionable mineral in et al. (2009) investigated the formal charge of Co in carrol- the IMA List of Minerals with the simplified formula lite through spectroscopic techniques. Both studies con- Fe(Fe,Ti) O . The latter did not reflect the occurrence of 1+ 2 4 firmed Cu , but the charge of Co is still under question. cation vacancies. Collyer et al. (1988) provided a detailed Pattrick et al. (2008) suggested a Co charge between +2 crystal-chemical characterization (EMPA, Mo¨ssbauer and 1+ 2+ 3+ and +3 along with the charge-arrangement Cu (Co Co ) SC-XRD) of a natural titanomaghemite sample from Preto- 1.5– (S )4. In contrast, Buckley et al. (2009),onthebasisofa ria, South Africa. The sample was refined in the space group 3+ study by Wada et al. (2002),proposedalow-spinCo with P4332 and its empirical structural formula corresponds to T 3þ M(1) 2þ 3þ 4þ M(2) 2þ a charge for S only 12.5% less negative than –2, which leads ðFe u0:04Þ ðFe Fe Ti u0:27Þ ðFe 1þ 3þ 1:75 0:96 R1:00 0:03 0:14 0:06 R0:50 0:20 to the charge arrangement Cu Co ðS Þ . With regard 4 2 4 Fe3þ Ti4þ u Þ O ,whereM(1) and M(2) are two non- to fletcherite, there are only two studies (Craig & Carpenter, 0:85 0:36 0:10 R1:50 1977; Ostwald, 1985) that provide chemical data. To date, equivalent octahedrally coordinated sites. In accordance no information on the fletcherite structure and the formal with Hawthorne (2002), this formula leads to the end- charge of its constituents is available. With regard to tyrrel- member composition:ÀÁ T 3+ M(1) h M(2) 3þ 4þ lite, Yang et al. (2007) studied the crystal structure reporting (Fe ) ( )0.5 Fe1:0Ti0:5 O4 which in terms of A A u 4þ R2þ a bond-valence sum value of 3.77 valence units for the and B constituents can be rearranged as ð 0:5Ti0:5Þ B 3þ R6þ=2¼3þ 4þu 3þ B constituent (Co + Ni), which suggests that the Co or Ni ðFe2 Þ O4 ¼ðTi0:5 0:5ÞFe2 O4. Titanomaghemite ions would exist in a formal charge greater than 3+. can hence be considered as a member of the spinel subgroup ItwasalsoconcludedthatCuis+1intyrrellitebyanalogy 2-3 of the oxyspinel group. The IMA status of titanomaghe- 1+ 3+ 4+ with studies on Cu (Cr Cr )Se4 (e.g., Yang et al., 2007). mite is consequently changed from Q (= questionable) to Rd For classification purposes, the formal charges of the (= redefined). The specimen ‘‘BM1929,1687’’ kept in the constituent elements in carrollite, fletcherite and tyrrellite Natural History Museum (London, UK) and used by Collyer are not crucial for their validity as mineral species, but et al. (1988) for their structural investigation is considered as merely for their inclusion in a subgroup. We are aware that the neotype material for titanomaghemite. further studies are needed in order to better define the 6.1.2. Cuprorhodsite, redefined as ðCu1þFe3þÞRh3þS formula of carrollite, fletcherite and tyrrellite. Presently, 0:5 0:5 2 4 we assume that the oxidation states of anions (S and Se) The formula of cuprorhodsite was reported as CuRh2S4 in and Cu are –2 and +1 (respectively). Such an assumption the IMA List of Minerals. However, reconsidering the 3+ 4+ 3+ 4+ implies the occurrence of Co ,Co ,Ni and Ni which empirical formula (Cu0.51Fe0.41)(Rh1.66Ir0.23Pt0.15)S4 lead to B3.5+. As a result, carrollite, fletcherite and tyrrellite (Rudashevsky et al., 1985) and the experimental data for fall into subgroup 1-3.5. As the minimum requirement to synthetic thiospinels indicating the oxidation states Cu1+, have a mineral subgroup is that it contains at least two Fe3+ and Rh3+ (Plumier & Lotgering, 1970; Plumier et al., species (Mills et al., 2009), tyrrellite is considered as an 1992), the ideal formula of cuprorhodsite can be redefined 1þ 3þ 3þ isolated member of the potential ‘‘tyrrellite subgroup’’. as ðCu0:5Fe0:5ÞRh2 S4.

6. Output of the recommended nomenclature 1+ 3+ 4+ 6.1.3. Malanite, redefined as Cu (Ir Pt )S4 In order to belong to the spinel supergroup, a mineral spe- The troubled history of malanite was briefly summarized by cies has to have an atomic ratio between (A + B) cations Biagioni & Pasero (2014). On the basis of the study of Yu 3+ (including vacancy) and X anions equal to 3:4, and a (1996), malanite was defined as CuPt2S4 with Pt . 1+ heteropolyhedral framework composed by TX4 tetrahedra However, the usual occurrence of Cu in sulfides (e.g., isolated from each other and sharing corners with the Barkov et al., 2000) and the existence of the monoclinic 1þ 2þ 4þ neighboring MX6 octahedra. The latter, in turn, share six synthetic compound Cu2 ðPt Pt3 ÞS8 (Gross & Jansen, 3+ of their twelve X–X edges with nearest-neighbor MX6.This 1994) do not support the occurrence of Pt .Inaddition,nat- supergroup is divided into three groups according to the ural occurrences of malanite are always rich in PGE other dominant X species, i.e., oxyspinel group (X = O2–), thios- than Pt: for instance, the two chemical analyses reported pinel group (X = S2–), and selenospinel group (X = Se2–). in the type description (Yu, 1996) correspond to Within each group, many cation charge arrangement combi- (Cu0.93Fe0.06)R0.99(Pt1.03Ir0.66Rh0.04Pd0.03Co0.21Ni0.03)R2.00 nations are possible, constrained by the electroneutrality S4.03 and (Cu0.95Fe0.07)R1.02(Pt1.37Ir0.45Co0.11Rh0.08)R2.01 principle. Each of these combinations corresponds to a S3.97. On the basis of arguments mentioned above, the ideal 1+ 3+ 4+ subgroup (Table 1). formula of malanite is consistent with Cu (Ir Pt )S4.

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ÀÁ 3þ 5þ Of particular interest in this regard is the composition of which dominates over Mg and Zn, and A ¼ Fe0:5Sb0:5 . ‘‘dayingite’’ Cu1.19(Co1.08Pt0.86)R1.94S4.00 (Yu et al., The corresponding ideal formula of filipstadite is therefore 1974), described as a cobaltian malanite by Yu (1981), consistent with that proposed by Dunn et al. (1988): 1+ 3+ 4+ A 3þ 5þ B 2þ which would lead to the ideal formula Cu (Co Pt )S4. ðFe0:5Sb0:5Þ ðMn Þ2O4. Barkov et al. (1997) described a Ni-malanite, corresponding 1þ 2þ to the ideal formula Cu ðPt Ni ÞS .Althoughnostruc- 3þ 5þ 1:5 0:5 4 6.1.6. Tegengrenite, redefined as ðMn Sb ÞMg2O4 tural data are available for both Co- and Ni-malanite 0:5 0:5 varieties, these phases should be further investigated as Tegengrenite was reported in the IMA List of Minerals with potentially new species in the carrollite subgroup 1-3.5 of the simplified formula Mg2(Sb,Mn)O4. Charge balance the thiospinel group. requirements indicate that the ideal formula is 3þ 5þ ðMn0:5Sb0:5ÞMg2O4. 3þ Tegengrenite is a rare mineral considered to be a 6.1.4. Maghemite, redefined as ðFe u0:33ÞFe2O4 0:67 Mg–Mn3+ analogue of filipstadite having a rhombohedrally The IMA List of Minerals gave for maghemite the formula distorted spinel-type structure (Holtstam & Larsson, 2000). Fe2O3. However, it is convenient to rewrite it as Bonazzi & Bindi (2015) refined the crystal structure of a 3þ u 3þ ðFe0:67 0:33ÞFe2 O4, which better reflects its relation to sample from the Filipstad district (Va¨rmland, Sweden) in the spinel supergroup. The presence of cation vacancies is the trigonal space group R3 and showed that the tegengren- a result of the oxidation of magnetite, to which maghemite ite superstructure has a close structural relationship to spinel. is structurally related. Depending on the degree of vacancy Owingtocationordering,theM and T sites of the structure ordering, maghemite can crystallize in space group Fd3m are split into ten and six independent sites, respectively. (vacancies randomly distributed), P4132 (vacancies partially Similarly, to filipstadite, the site allocation of cations in the ordered), or P41212, in a tetragonal multiple cell with c =3a tegengrenite superstructure is also quite complex. According (vacancies completely ordered) (e.g., Greaves, 1983; to the two empirical formulae reported in literature, Shmakov et al., 1995; Jørgensen et al., 2007; Bowles 2þ 3þ 3þ 5þ ðMg1:22Mn0:82Zn0:07Mn0:23Fe0:02Al0:03Si0:06Ti0:04Sb0:50ÞO4 et al., 2011). 2þ (Holtstam & Larsson, 2000)andðMg1:26Mn0:85 Similar arguments apply to the Al-analogue of maghemite, Zn Mn3þ Al Si Ti Sb5þ ÞO (Bonazzi & Bindi, 0:04 0:19 0:01 0:12 0:03 0:50 4 deltalumite, whose space group is P4m2(Pekov et al., 2016) 2015), the divalent cations (Mg + Mn2+ +Zn=2.15apfu) h and whose formula can be written as (Al0.67 0.33)Al2O4. are dominant and indicate that tegengrenite belongs to subgroup 4-2. The constituent B is represented by Mg2+, 3 5þ þ 5 Æ¼4þ 6.1.5. Filipstadite, redefined as ðFe0:5Sb0:5ÞMn2O4 3þ þ whereas A is represented by ðMn0:5Sb0:50Þ for charge Filipstadite was reported in the IMA List of Minerals with balance reasons. Therefore, the ideal formula of the 2+ 5+ 3+ the simplified formula (Mn ,Mg) (Sb ,Fe )O ,which 3þ 5þ R¼4þ 2 4 tegengrenite is ðMn Sb Þ Mg O4. could be resolved into the two potential end-members: 0:5 0:50 2 5þ 3þ 2þ 5þ 3þ Mg2ðSb0:5Fe0:5ÞO4 and Mn2 ðSb0:5Fe0:5ÞO4.Thelatter 1þ 2þ 4þ was proposed by Dunn et al. (1988) as the ideal formula 6.1.7. Rhodostannite, redefined as Cu ðFe0:5Sn1:5ÞS4 for filipstadite. Rhodostannite is a sulfide mineral that was first described by This rare mineral was found in Långban, Sweden, and Springer (1968) with the formula Cu2FeSn3S8 (specimen crystallizes in the space group Fd3m with a spinel-type from Vila Apacheta, Bolivia). Rhodostannite was included superstructure: a =25.93Å,Z =216(Bonazzi et al., in the IMA List of Minerals with the simplified formula 2013). Due to cation ordering, the structure comprises 11 (Cu,Ag)2FeSn3S8, which is too generalized to indicate what distinct cation sites and 10 distinct anion sites. Due to the distinction is between rhodostannite and toyohaite. In the complex cation allocation at each independent site, fact, such a formula could be resolved into two end-member Bonazzi et al. (2013) grouped the T(1-5) and M(1-6) sites formulae, i.e., Cu FeSn S and Ag FeSn S (toyohaite). T 2þ 3þ 2 3 8 2 3 8 and provided the empirical formula: ðMn0:60Mg0:07Fe0:30 M Rhodostannite is included in the spinel supergroup because 2þ 3þ 5þ Zn0:02Si0:01ÞR1:00 ðMn0:56Mg0:76Fe0:16Al0:02Sb0:50ÞR2:00O4. of its spinel-related structure and typical cation-to-anion The latter would lead to the end-member formula ratio of 3:4. Jumas et al. (1979) determined by SC-XRD T 2þ 3þ M 5þ the crystal structure of rhodostannite in the space group ðMn0:5Fe0:5Þ ðMg1:5Sb0:5ÞO4, which, however, does not reflect the empirical composition as it underestimates the I41/a. Copper is monovalent and occupies the T site, whereas 2+ Fe and Sn are randomly distributed over the equivalent M actual Mn content (always > 1.1 apfu) and overestimates 57 119 the Mg content. By considering only the chemical informa- sites. Garg et al. (2001) determined by Fe and Sn Mo¨ssbauer spectroscopy the valence states of iron (Fe2+)andtin tion, the populations of the T and M sites may be merged 4+ and the chemical formula expressed as: (Sn ). T+M 2þ 3þ 5þ 4þ In accordance with Jumas et al. (1979), the ideal formula of ðMn1:16Mg0:83Fe0:46Sb0:50Zn0:02Al0:02Si0:01ÞR3:00O4.This rhodostannite should be written as Cu(Fe0.5Sn1.5)S4 to better formula highlights the dominance of divalent cations, 1+ 1+ RR2+ (Mn2+ + Mg + Zn) = 2.01 apfu (> 1.5 apfu), indicat- show its relationship to the spinel supergroup. As Cu =A 2þ 4þ Æ¼7þ 3:5þ ing that filipstadite can be classified in the spinel subgroup and ðFe0:5Sn1:5Þ ¼ 2B , rhodostannite belongs to the 4-2 of the oxyspinel group: the constituent B = Mn2+, carrollite subgroup 1-3.5 of the thiospinel group.

1þ 2þ 4þ 6.1.8. Toyohaite, redefined as Ag ðFe0:5Sn1:5ÞS4 structure (e.g., Lenaz et al., 2008), the formula of hydrohetaerolite can be recalculated by taking into account cation Toyohaite is the Ag analogue of rhodostannite described by vacancies and (OH) groups. Thus, for example, the chemical Yajima et al. (1991) in the tetragonal space group I4 /a.The 1 formula HZnMn3þu O ,proposedbyMcAndrew (1956) ideal formula Ag FeSn S is revised as Ag(Fe Sn )S to 1:6 0:4 4 2 3 8 0.5 1.5 4 for a sample from Leadville (Colorado, USA), can be rear- better highlight its relation to the spinel supergroup. 3þu ranged as ZnMn1:6 0:4½O3ðOHÞ. Meisser & Perseil (1993) 6.1.9. Xingzhongite, redefined as Pb2þIr3þS reported three chemical analyses of hydrohetaerolite from 2 4 the Chez Larze Mine, Mont Chemin (Valais, Switzerland), Xingzhongite was reported in the IMA List of Minerals with containing 4-6.3 wt% H2O, resulting in the formula 2þ 3þ the simplified formula (Pb,Cu,Fe)Ir2S4 and questionable H1:27ðZn0:68Cu0:03ÞðMn1:68Si0:05ÞO4, which can be rear- 2þ u 3þ IMA status. Its empirical formula, (Pb0.37Cu0.35Fe0.17)R0.89 ranged as ðZn0:68Cu0:03Þ 0:56ðMn1:68Si0:05Þ½O2:73ðOHÞ1:27. (Ir1.33Rh0.41Pt0.29)R2.03S4 (Yu et al., 1974), leads to the ideal All these formulae are consistent with the end- 2þ 3þ one Pb Ir2 S4, although its status as a mineral species still member ZnMn2O4 because the dominant anion is always remains questionable. O2– (> OH) and Mn3+ >(Zn+Cu)2+ > h. Therefore, ‘‘hydrohetaerolite’’ is discredited as corresponding to (OH)-bearing 6.2. Discredited spinel species hetaerolite. 6.2.1. ‘‘Iwakiite’’, discredited as a polymorph of jacobsite 6.2.3. ‘‘Ferrorhodsite’’, discredited as corresponding Iwakiite was first described by Matsubara et al. (1979) to cuprorhodsite T 2+ with the approximate empirical formula (Mn )R1.0 The chemical formula of ferrorhodsite was reported as M(Fe3þMn3þ) O and tetragonal space group P4 /nnm, 1:3 0:7 R2.0 4 2 FeRh2S4 in the IMA List of Minerals. Ferrorhodsite was subsequently corrected as I41/amd by Jarosch (1987). described as the Fe analogue of cuprorhodsite, with empiri- Departure from the cubic symmetry is ascribed to the cal formulae (Fe0.57Cu0.42)R0.99(Rh1.72Ir0.23Pt0.05)R2.00S4 bond-length distortion of the octahedron, related to the and (Fe Cu Ni ) (Rh Pt Ir ) S M 3+ 0.52 0.48 0.03 R1.03 1.67 0.16 0.13 R1.96 4 Mn content. This notwithstanding, the empirical formula (Rudashevsky et al., 1998). However, these formulae lead to of iwakiite leads to an end-member formula identical to that the ideal formula of cuprorhodsite ðCu1þFe3þÞRh3þS . T 2+ M 3þ 0:5 0:5 4 of jacobsite: (Mn ) (Fe2 )O4. In accord with the defini- Therefore, ferrorhodsite is discredited as corresponding to tion of polymorphism of Nickel & Grice (1998),whichis cuprorhodsite, with a slightly different Fe/Cu atomic ratio. not restricted to substances with identical chemical compo- As cuprorhodsite was described earlier than ‘‘ferrorhodsite’’, sitions, but can be broadened somewhat to include relatively the former name is retained. minor chemical variations when the topology of the structure is retained, iwakiite is considered as the tetragonal dimorph of jacobsite. Accordingly, iwakiite and jacobsite 7. Applying the nomenclature of the spinel are regarded as polymorphs. Since jacobsite was discovered earlier than iwakiite, the former name is retained. Tetragonal supergroup: some examples ‘‘iwakiite’’ is hence referred as jacobsite-Q. Similar arguments apply to the natural (Zn,Mn)- For classification purposes, each spinel-supergroup species ferrite (Bailadores region, Me´rida, Venezuela), refined by is identified by the chemical formula AB2X4. Once the Marcano et al. (1996) in the space group I4 /amd.Its chemical composition of a mineral has been established, it 1 can be placed in the oxy-, thio- or seleno-spinel group. chemical composition (Zn0.75Mn0.75Fe1.50O4)canbe T 2þ The dominant primary parameter B defines the subgroup converted to the structural formula (Zn0.75Mn0:25)R1.00 M 2þ 3þ and, along with the secondary parameter A, is used to iden- (Mn0:50Fe1:50)R2.00O4, which leads to the end-member 3þ tify the species. In detail, the following systematic procedure ZnFe2 O4. Because of the tetragonal symmetry due to the Jahn-Teller effect of MMn3+, this mineral can be considered for naming spinel-supergroup species is suggested: as a tetragonal dimorph of franklinite, and thus termed as 1) Identify the group by the composition of X. franklinite-Q. 2) Identify the most abundant constituent B by summing the ions for each valence: e.g., RR2+ =(Mg2+ +Fe2+ +Mn2+)>1.5apfu leads to B2+,whereasRR3+ = 6.2.2. ‘‘Hydrohetaerolite’’, discredited as corresponding (Cr3+ +Fe3+ +Al3+)>1.0apfu leads to B3+. to hetaerolite 3) Identify the subgroup by the constituent B: e.g., for B3+ Hydrohetaerolite was listed as a valid species in the IMA (RR3+ >1.0apfu) the species belongs to subgroup 2-3 3þ 2þ 3þ 2+ 2+ List of Minerals with the formula HZnMn1:7O4.Itwasfirst ðA B2 O4Þ;forB (RR >1.5apfu) the species 4þ 2þ reported by Palache (1928) as a hydrous counterpart of belongs to the subgroup 4-2 ðA B2 O4Þ. Alterna- 3þ 3+ 2+ hetaerolite, ZnMn2 O4. McAndrew (1956) showed that tively, the RR /RR ratio can be used to identify the the X-ray powder pattern of hydrohetaerolite is consistent subgroups: it varies from 0 to 2/3 for spinel 4-2, and with the tetragonal space group I41/amd (a =5.73Å,c = from 2/3 to 2 for spinel 2-3. 9.00 Å, Z = 4), and pointed out the presence of cation vacan- 4) Once the valence of B is established, the dominant cies. No structural data have been presented for hydro- B-cation and subsequently the dominant A-cation are hetaerolite. As water should occur as (OH)1– in the spinel identified according to the dominant-constituent rule,

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in which heterovalent pairs of ions (or ion and vacancy) dominant R3+ cation is Cr (= B) and the dominant R2+ cation have to be considered for classification purposes. is Mg2+ (= A). Some examples referring to the oxyspinel group are given Although the subdivision of a mineral composition into below, including synthetic spinels because they provide end-member proportions is an important step for application instructive examples. of thermodynamics to mineral systems, the two spinel com- Example 1. Bosi et al. (2009) reported a synthetic spinel positions mentioned in Example 4 show that identifying a 2þ 3þ (sample FeTi30A) with chemical formula ðFe1:46Fe1:08 mineral species on the proportion of end-member compo- 4þ Ti0:46ÞR3:00O4. Such a composition is very close to the nents may be ambiguous and should not be applied to min- 3þ boundary between magnetite and ulvo¨spinel ðTi0:5Fe1:0 eral nomenclature (Bosi, 2018). 2þ 3+ 3+ 2+ Fe1:5O4Þ and has RR =Fe >1.0apfu,andRR = 2+ 3+ 2+ Fe <1.5apfu (RR /RR = 0.74). Thus, B is 3+ and Acknowledgements: We are grateful to the IMA- 2þ 3þ the species belongs to the subgroup 2-3: ðA B2 O4Þ.As 3+ CNMNC members for their suggestions on the nomencla- the dominant B-cation is Fe and the dominant A-cation ture proposal, and to the CNMNC vice-chairman (F. Hatert) 2+ 2þ 3þ 4þ is Fe , the composition ðFe1:46Fe1:08Ti0:46Þ O4 leads to A 2+ B 3+ R3:00 for handling the whole procedure. We thank M.E. Ciriotti, the ideal formula (Fe ) (Fe )2O4, i.e., magnetite. for his help in finding mineral species related to spinel, Example 2. Similarly, sample FeTi50Bb (Bosi et al., 2009) 2+ 3+ 3+ A.R. Chakhmouradian and an anonymous reviewer for their ischaracterizedby1.59Fe ,0.80Fe ,0.03Al and 0.59 helpful comments. Ti4+ apfu, with RR3+ (= Fe3++Al3+) = 0.83 apfu (<1 apfu) and RR2+ =Fe2+ >1.5apfu (RR3+/RR2+ =0.52).Thus,B is 2+ and the species belongs to the subgroup 4-2: References 4þ 2þ 2+ A B2 O4. As the dominant B-cation is Fe and the dom- 4+ inant A-cation is Ti , the empirical composition Andreozzi, G.B. & Princivalle, F. (2002): Kinetics of cation 2þ 3þ 3þ 4þ ðFe1:59Fe0:80Al0:03Ti0:59ÞR3:00O4 leads to the ideal formula ordering in synthetic MgAl2O4 spinel. Am. Mineral., 87, 838–844. A 4+ B 2+ 2 4 (Ti ) (Fe ) O , i.e.,ulvo¨spinel. Andreozzi, G.B., Princivalle, F., Skogby, H., Della Giusta, A. Example 3. Lenaz et al. (2011) reported a natural (2000): Cation ordering and structural variations with temper- spinel from the Rum Layered Suite, NW Scotland ature in MgAl2O4 spinel: An X-ray single crystal study. Am. (sample RumBAn2), with chemical formula Mineral., 85, 1164–1171. T+M(Fe2þMg Ni Mn Fe3þ Cr Al V Ti ÞO . Barkov, A.Y., Halkoaho, T.A.A., Laajoki, K.V.O., Alapieti, T.T., 0:9 0:14 0:01 0:01 0:76 0:71 0:38 0:02 0:07 4 Peura, R.A. (1997): Ruthenian pyrite and nickeloan malanite The sum of trivalent cations RR3+ (= Fe + Cr + Al + V) is 2+ from the Imandra layered complex, northwestern Russia. Can. 1.87 apfu, and the sum of divalent cations RR (= Fe + Mineral., 35, 887–897. 3+ Mg + Ni + Mn) is 1.07 apfu.AsRR >1.0apfu,and Barkov, A.Y., Martin, R.F., Halkoaho, T.A.A., Poirier, G. (2000): RR2+ <1.5apfu (RR3+/RR2+ = 1.75), the sample belongs The mechanism of charge compensation in Cu-Fe-PGE 3+ 3+ thiospinels from the Penikat layered intrusion, Finland. Am. to subgroup 2-3. Because2+ the dominant R cation is Fe Mineral., 85, 694–697. (= B) and the dominant R cation is Fe2+ (= A), the species Biagioni, C. & Pasero, M. (2014): The systematics of the spinel-type is identified as magnetite. minerals: an overview. Am. Mineral., 99, 1254–1264. Example 4. Oktyabrsky et al. (1992) reported chemical Bindi, L., Griffin, W.L., Panero, W.R., Sirotkina, E., Bobrov, A., Irifune, T. (2018): Synthesis of inverse ringwoodite sheds light analyses of spinels rich in Mg, Fe, and Ti from Kondyor, on the subduction history of Tibetan ophiolites. Sci. Rep., 8, Russia. Their analysis #3 corresponds to the following 5457. 2þ 2þ 3þ chemical formula: ðFe0:04Mg1:37Mn0:07Fe0:88Al0:17Ti0:47ÞO4. Bonazzi, P. & Bindi, L. (2015): Determination of the tegengrenite 3+ As the sum of trivalent cations RR3+ (= Fe + Al) is 1.05 superstructure: another case of tetrahedral Mn in spinel-type 2+ minerals? Mineral. Mag., 79, 425–436. apfu (>1.0 apfu) and the sum of divalent cations3+ RR 2+ Bonazzi, P., Chelazzi, L., Bindi, L. (2013): Superstructure, crystal (= Fe + Mg + Mn) is 1.47 apfu (<1.5 apfu)(RR /RR = chemistry and cation distribution in filipstadite, a Sb5+-bearing, 0.72), this sample3+ belongs to subgroup 2-3. The dominant spinel-related mineral. Am. Mineral., 98, 361–366. 3+ 2+ R 2+cation is Fe (= B) and the dominant R cation is Bosi, F. (2018): On the mineral nomenclatures: the dominant- Mg (= A), thus the species is identified as magnesioferrite, valency rule, Abstract to XXII meeting of the IMA, Melbourne, Australia, 354. although the proportion of end-member components indi- 2 4 3þ Bosi, F. & Andreozzi, G.B. (2017): Chromium influence on Mg-Al cate that TiMg O (45.5%) > MgFe2 O4 (44.0%). intracrystalline exchange in spinels and geothermometric Similarly, Mitchell (1995) reported representative composi- implications. Am. Mineral., 102, 333–340. tions of spinels from the Besterskraal, Sover North, and Bosi, F., Hålenius, U., Skogby, H. (2009): Crystal chemistry of the Pniel orangeites. Based on the dominant-end-member magnetite-ulvo¨spinel series. Am. Mineral., 94, 181–189. approach, analysis #1 (Table 2.27 of Mitchell, 1995)was —, —, — (2014): Crystal chemistry of the ulvo¨spinel-qandilite 2 4 2 4 series. Am. Mineral., 99, 847–851. identified as chromite: FeCr O (42.6%) > MgCr O Bowles, J.F.W., Vaughan, D.J., Howie, R.A., Zussman, J. (2011): (39.4%) > MgAl2O4 (6.6%) > FeFe2O4 (5.7%) > Mg2TiO4 2 4 Rock-Forming Minerals. Vol. 5A, Non-silicates: Oxides, (3.9%) > MnCr O (1.9%). However, the corresponding Hydroxides and Sulphides, The Geological Society, London. 2þ 2þ 3þ chemical formula, ðFe0:48Mg0:53Mn0:02Fe0:08Al0:13Cr1:73 Bragg, W.H. (1915): The structure of the spinel group of crystals. Ti0:03ÞO4, is consistent with magnesiochromite as the Philos. Mag., 30, 305–315.

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