The crystal structure of apatite, as seen along c . The unit cell is outlined. The M1-centred polyhedra are represented as six-fold coordinated metaprisms (the bonded O3 ligands are not included in this polyhedral representation). Two out of the seven bonds to M2 overlap in this projection, therefore only five are visible in the figure. 

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... potentially eligible as individual mineral species. The recently approved nomenclature scheme of Burke (2008) could logically be extended to the renaming of other apatite group minerals, changing, e.g. , pyromorphite into apa- tite-(PbCl), or alforsite into apatite-(BaCl). And one could be tempted to include also the various tetrahedral cations (P, As, or V), into the extended suffix [ e.g. , apatite-(PbAsCl) instead of mimetite, apatite-(CaAsOH) instead of johnbaumite]. The result would be mineral names that are more similar to chemical formulae. The limit of such an approach would be to adopt chemical formulae throughout, instead of mineral names. However, a nomenclature based extensively upon modified Levinson-style suffixes is likely to be rejected by the mineralogical community, since many would argue that multiple suffixes are difficult to be read, almost impossible to be spo- ken, not immediately self-explanatory, and unpleasant to the eye. Furthermore, such naming replaces many traditional names given to honour worthy individuals. A specific example of a nomenclature problem resulting from ordering in the apatite structure is presented by the recently approved Sr end-member of apatite lato sensu , Sr 5 (PO 4 ) 3 F (IMA 2008-009). While ‘‘strontioapatite’’ would have been an appropriate name for this species, a mineral with essentially the same name already exists, strontium-apatite [which has just been renamed apatite- (SrOH) after Burke (2008)]. Furthermore, the latter mineral has a cation-ordered structure, and it is actually F-dominant, with the ideal formula SrCaSr 3 (PO 4 ) 3 F (Efimov et al. , 1962; Klevtsova, 1964; Pushcharovsky et al. , 1987); therefore, its renaming as apatite-(SrOH) is incorrect. To make the issue even more complicated, there is also another cation-ordered member along the join Ca 5 (PO 4 ) 3 F , Sr 5 (PO 4 ) 3 F, namely fluorcaphite, SrCaCa 3 (PO 4 ) 3 F. For the above reasons the former chairman of the IMA CNMNC, E.A.J. Burke, asked us (MP & ARK) to convene a subcommittee to re-evaluate the nomenclature of minerals belonging to the ‘‘apatite group’’, to propose a new consistent nomenclature and to rationalize all existing mineral names in this group. The ‘‘apatite group’’ traditionally includes phosphate, arsenate and vanadate minerals. Other minerals belonging to different chemical classes, namely silicates ( e.g. , britholite), silicate-sulphates ( e.g. , ellestadite), and sulphates ( e.g. , cesanite) display the structural topology of apatite. In accordance with the newly approved standardisation of mineral group hierarchies (Mills et al ., 2009), all of these minerals can be included in the broader apatite supergroup. Because the same nomenclature questions are relevant for all members of the supergroup, they will all be considered in this report. All valid species within the apatite supergroup are listed in Table 1. Their ideal chemical formulae are also given, as they should appear in the official IMA List of Minerals. The minerals have here been divided into five groups on the basis of their crystallographic and/or chemical similarities. This report has been approved by the IMA Commission on New Minerals, Nomenclature and Classification. The archetype structure of apatite is hexagonal with space group P 6 3 / m and unit-cell parameters a 1⁄4 9.3 – 9.6, c 1⁄4 6.7 – 6.9 A ̊ . The generic crystal-chemical formula may be also written in its doubled form, which corresponds to the unit cell content, as follows: IX M1 M2 ( IV TO ) X ( Z 1⁄4 1), where the left superscripts indicate the ideal coordination numbers. In this report we will generally use the reduced formula with Z 1⁄4 2, which is commonly adopted in the mineralogical literature. Despite the rather simple formula, with only four key sites (M1, M2, T, and X) besides those (O1, O2, and O3) which are known to be occupied by O 2 À only, the number of distinct species based on cationic and anionic substitutions is quite large. This number increases further, because in some cases the M1 sites are split into pairs of non-equivalent sites with corresponding lowering of the space group symmetry. Concerning the coordination numbers, M1 has nine-fold (6 þ 3) coordination with the innermost six ligands forming a polyhedron that is often referred to as a metaprism (White & Dong, 2003; Dong & White, 2004a and b; Mercier et al. , 2005). When the three more distant ligands are included, the M1 coordination polyhedron is often described as a tri-capped trigonal prism. The M2 site is considered to be seven-fold coordinated whenever Ca is the central cation; such a polyhedron can be described as a distorted pentagonal bipyramid (Dolivo-Dobrovolsky, 2006); in other cases, e.g. , when the site is occupied by Pb and/or the X site is occupied by Cl, the coordination of M2 sites may be more irregular and the central cation may be considered to be eight- or nine-fold coordinated. A drawing of the apatite structure-type is shown in Fig. 2. The relationships among ionic sites and multiplicity and Wyckoff positions in all known space groups of apatite supergroup minerals are shown in Table 2. Species-forming M and T cations thus far known among minerals are: M 1⁄4 Ca 2 þ , Pb 2 þ , Ba 2 þ , Sr 2 þ , Mn 2 þ , Na þ , Ce , La , Y , Bi ; T P , As , V , Si , S , B 3 þ . Species-forming substitutions at the X anionic site are limited to the monovalent anions F À , Cl À , and (OH) À . This implies – for all minerals known thus far – a total of 50 negative charges per unit cell throughout [ i.e. , 24 O 2 À þ 2 (F, Cl, OH) À ]. In addition, many studies of synthetic compounds with the apatite structure have demonstrated that the X site can be occupied by O 2 À (which would increase the total negative charges) as well as vacancies and H 2 O molecules (which would decrease the total negative charges), that the M site can be occupied by Cd, Co, K, and almost all REE and that the T site can be occupied by Be, Cr, Ge, and Mn 5 þ . Even though in the present report we limit ourselves to natural compounds, the presence of M cations with charge 1 þ , 2 þ , and 3 þ , and of T cations with charge 4 þ , 5 þ , and 6 þ , implies a great number of possible combinations of different atoms, and thus of potentially different species. The valid, IMA-accepted mineral species within the apatite supergroup can be divided into five groups. Although we are aware that different groupings could be chosen, e.g. based upon purely chemical grounds, we prefer for our purposes a subdivision based on a combination of crystallographic and chemical criteria. Our five groups are as follows: (1) Apatite group : hexagonal and pseudo-hexagonal phosphates, arsenates, and vanadates containing the same prevailing (species-defining) cation at both the M1 and M2 sites. (2) Hedyphane group : hexagonal and pseudo-hexagonal phosphates, arsenates and sulphates containing different prevailing (species-defining) cations at the M1 and M2 sites. (3) Belovite group : hexagonal and trigonal phosphates with the M1 site split into the M1 and M1 0 sites containing different prevailing (species-defining) cations. (4) Britholite group : hexagonal and pseudo-hexagonal silicates, typically with partially ordered M1 and M2 cations. (5) Ellestadite group : hexagonal and pseudo-hexagonal sulphato-silicates with the ideal ratio (SiO ) 4 À : (SO ) 2 À 1⁄4 1: 1. These three minerals have ideal formulas Ca 5 (PO 4 ) 3 F, Ca 5 (PO 4 ) 3 OH, and Ca 5 (PO 4 ) 3 Cl, respectively. They were formerly known as fluorapatite, hydroxylapatite, and chlorapatite and were recently renamed as apatite-(CaF), apatite-(CaOH), apatite-(CaCl) (Burke, 2008). The birth of the three distinct names to denote the F-, OH-, and Cl- dominant variants, and their distinction with respect to the original ‘‘apatite’’ sensu lato is uncertain, but is generally ascribed to Damour (1856) for ‘‘hydroxylapatite’’ and Rammelsberg (1860) for ‘‘fluorapatite’’ and ‘‘chlorapa- tite’’. The crystal structure of ‘‘apatite’’ was first solved by Mehmel (1930) and N ́ray-Szab ́ (1930) in the space group P 6 3 / m , and typically all apatites crystallize in that space group. However, crystal structure refinement in lower symmetry space groups have been carried out on synthetic Ca 5 (PO 4 ) 3 Cl (Mackie et al. , 1972), Ca 5 (PO 4 ) 3 OH (Elliott et al. , 1973) and natural apatite-(CaCl) (Hughes et al. , 1990). This latter mineral, a monoclinic chlorapatite from Jackson Peak, Gunkock, Washington Co., Utah, USA is structurally identical (space group P 2 1 / b , with a doubled b -axis, and g % 120 ;) to apatite-(CaOH)- M (see below), of which it represents the Cl-dominant analogue. The mineral was referred to as clinohydroxylapatite by Chakhmouradian & Medici (2006), and subsequently renamed apatite-(CaOH)- M (Burke, 2008). Its monoclinic symmetry (non standard space group P 2 1 / b , evidently chosen so as to maintain the typical axial setting of apatites) probably results from orientational ordering of (OH) À anions within [00 z ] anionic columns, with consequent doubling of the periodicity along [010]. The following unit-cell parameters are given: a 9.445(2), b 18.853(4) c 6.8783(6) A ̊ , g 120.00(2) . All chemical analyses point to the ideal formula Ca 5 (PO 4 ) 3 (OH). A coupled substitution of Ca 2 þ by Na þ and of (PO 4 ) 3 À by (SO 4 ) 2 À has been reported. The formula of the most Na- and S-rich apatite- (CaOH)- M is ca. (Ca Na )(PO ) (SO ) (OH). Svabite, Ca 5 (AsO 4 ) 3 F, was first described by Sj ̈gren (1892) from the Hartsigen mine, V ̈rmland, Sweden. It is the arsenate analogue of apatite-(CaF). In the mineralogical literature svabite is commonly reported as hexagonal, P 6 3 / m . However, a crystal structure refinement of natural svabite is lacking. The structure of the synthetic analogue of svabite has been recently refined in the triclinic space group P " 1 (Baikie et al. , 2007). The hydroxyl analogue of svabite was observed as early ...