Mineral species should be identified by an end-member formula and by using the dominant-valency rule as recommended by the IMA–CNMNC. However, the dominant-end-member approach has also been used in the literature. These two approaches generally converge, but for some intermediate compositions, significant differences between the dominant-valency rule and the dominant end-member approach can be observed. As demonstrated for garnet-supergroup minerals, for example, the end-member approach is ambiguous, as end-member proportions strongly depend on the calculation sequence. For this reason, the IMA–CNMNC strongly recommends the use of the dominant-valency rule for mineral nomenclature, because it alone may lead to unambiguous mineral identification. Although the simple application of the dominant-valency rule is successful for the identification of many mineral compositions, sometimes it leads to unbalanced end-member formulae, due to the occurrence of a coupled heterovalent substitution at two sites along with a heterovalent substitution at a single site. In these cases, it may be useful to use the site-total-charge approach to identify the dominant root-charge arrangement on which to apply the dominant-constituent rule. The dominant-valency rule and the site-total-charge approach may be considered two procedures complementary to each other for mineral identification. Their critical point is to find the most appropriate root-charge and atomic arrangements consistent with the overriding condition dictated by the end-member formula. These procedures were approved by the IMA−CNMNC in May 2019.

Raree arth carbonatehydroxides,R E(CO 3 )OH, werehydrothermallys ynthesized from formica cida ndthe hydroxide gels ofN d, Sm,E u,G d, Tb, Dy,H o,E r,T m, Yb, andY .Ano rthorhombicphasewith akozoite-type structurewaso btained for RE ¼ Nd andS m. Another orthorhombicmodification ofthekozoite-typestructure waso btained for RE ¼ Eu,G d, Tb, Dy,H o,E r,T m,a nd Y.Thelatterp hasehasalowers ymmetry( space group: P 2 1 2 1 2 1 )i ncomparison to thetruekozoite-typestructure ( Pnma ). Anewt etragonalp hase(space group: P 4 2Thec rystals tructureso fR E(CO 3 )OH wererefined for Pnma (RE ¼ Nd andS m), P 2 1 2 1 2 1 (RE ¼ Eu,G d, Tb, Dy, Ho,Er,TmandY),and P 4 2 / nmc phases(RE ¼ TmandYb) usingsingle-crystalX-raydiffraction intensity data.The distinctfeatureso fthed ifferencesamongthethree structuresarethec oordination numbers oftheR E 3 þ ions:10, 9,a nd8for the Pnma , P 2 1 2 1 2 1 ,a nd P 4 2 / nmc phases,respectively.Asystematicc omparison ofthetwo orthorhombicstructuresr evealed ad ynamicvariation in thec oordination environment oftheR E 3 þ ions accompanied by variationsi nt heir ionicradii.Althought heinteratomic RE--Od istancest endto decreasewith thelanthanide contraction,e xceptions wereobserved for two oftheR E--O distances. Themutualcloseproximity ofC O 3 2 À anions caused byt helanthanide contraction led to repulsion betweent heC O 3 2 À anions,whichdecreased thesymmetry ofthec onfiguration ofC O 3 2 À anions aroundthec entral RE 3 þ ion,a ndsomeoxygenatoms ofC O 3 2 À moved away from thef irst coordinations hell oftheR E 3 þ ions dueto thee longation oftheR E-Od istances.Thec rystals tructureso fthetetragonalRE(CO 3 )OH are distinctfrom thoseoftheorthorhombicphases. Theyconsist ofladders ofR E 3 þ ions 8-coordinated byt heC O 3 2 À andO H À anions arrangedin thef orm ofadoublec ross. ThepowderXRD patternso fthetetragonalRE(CO 3 )OH areidenticalt ot hato fasyntheticmaterialp reviously reported asTm 6 (OH) 4 (CO 3 ) 7 .

is a new member of the perrierite-chevkinite group found in the jades from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P2 1 /a, a = 13.97(1), b = 5.675 (7), c = 11.98(1) A Ê , b = 114.26 (8)8, V = 866 A Ê 3 and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.12 (s) (40-3), 3.05 (vvs) (31-3), 2.99 (vs) (311), 2.84 (s) (020), 2.74 (s) (004), 2.20 (s) (31-5). Electron microprobe analysis gave SiO 2 22.58, TiO 2 29.88, ZrO 2 9.49, Nb 2 O 5 0.24, Ta 2 O 5 0.07, Al 2 O 3 0.20, FeO 0.10, CaO 0.43, SrO 34.32, BaO 0.13, La 2 O 3 0.00, Ce 2 O 3 0.38, Pr 2 O 3 0.10, Nd 2 O 3 0.29, Sm 2 O 3 0.04, total 98.25 wt.%, corresponding to on the basis of O = 22.The unitcell parameters and chemical composition imply that rengeite is the Sr and Zr-analogue of perrierite or high-b analogue of strontiochevkinite. It is transparent, dark brown with adamantine lustre. Its streak is pale greenish brown, and no cleavage was observed. The hardness is VHN 100 606 698 kg mm 2 (Mohs 5 5.5). The calculated density is 4.12 g cm 3 . It is strongly pleochroic from pale green to pale greenish brown where the REE contents are <1 wt.% and pale violet to greenish brown where the REE contents are between 3 and 10 wt.%. It occurs as anhedral grains in close association with titanite, zircon and tausonite in a pebble of blue titanian omphacite-jadeite rock from the seashore of Oyashirazu, Ohmi Town, in a boulder of lavender-coloured Ti-bearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, and in a boulder of green jade from the bed of the Hime-kawa river, Itoigawa City, Niigata Prefecture, central Japan. Rengeite is considered to have crystallized by interaction between pre-existing minerals (rutile, anatase, titanite and zircon) and Sr-rich metamorphic fluid during later stage activity of high-P/T metamorphism. The name is for Mt. Renge near the locality and the Renge metamorphic belt where jadeitite deposits are found.

The newly defined gadolinite supergroup approved by the IMA CNMNC (vote 16-A) includes mineral species that have the general chemical formula A 2 MQ 2 T 2 O 8 ' 2 and belong to silicates, phosphates and arsenates. Each site is occupied by: A À Ca, REE (Y and lanthanoids), actinoids, Pb, Mn 2þ , Bi; M À Fe, □ (vacancy), Mg, Mn, Zn, Cu, Al; Q À B, Be, Li; T À Si, P, As, B, Be, S; and ' À O, OH, F. The classification of the gadolinite supergroup is based on the occupancy of A, M, Q, T and ' sites and application of the dominant-valency and dominant-constituent rules. The gadolinite supergroup is divided into two groups defined by prevailing charge occupancy at the T site À Si 4þ in gadolinite group and P 5þ or As 5þ in herderite group. The gadolinite group is divided into the gadolinite and datolite subgroups. The A site is dominantly occupied by divalent cations in the datolite subgroup and by trivalent cations in the gadolinite subgroup. Accordingly, the Q site is dominantly occupied by B 3þ in the datolite subgroup and by Be 2þ in the gadolinite subgroup. The herderite group is divided into two subgroups. The herderite subgroup is defined by the dominant divalent cation (usually Ca 2þ ) in the A site and Be 2þ in the Q site, while the M site is vacant. The drugmanite subgroup is defined by the dominance of divalent cations (usually Pb 2þ ) in the A site, vacancy in the Q site and the occupation of the M site. Moreover, "bakerite" is discredited as mineral species because it does not meet the conditions of the dominant-constituent rule.

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