Distinct Mineral Species Research Articles

Revisiting the Chemistry and Crystal Structure of Sveite

Abstract Sveite, KAl7(NO3)4(OH)16Cl2·8H2O, is a rare, water-soluble mineral with an undetermined atomic structure. New major, minor, and trace element data for sveite from three localities (Cerro Autana Cave, Venezuela; San Joaquin Valley, USA; Ponta Grossa, Brazil) are reported using modern analytical techniques (inductively coupled plasma-optical emission spectrometry, inductively coupled plasma-mass spectrometry, and ion chromatography). Powder X-ray diffraction, micro-X-ray diffraction, and scanning electron microscopy-energy dispersive X-ray spectroscopy are used to characterize the identity of residues following digestions of different strengths, i.e., ultrapure H2O, 2%, 20%, and concentrated HCl acid. Sveite from Venezuela has approximately 3 wt.% more K and 5 wt.% less Al than reported by Martini (1980), and that from California has 20 wt.% less Al than was reported by Graham et al. (1988). Sveite from California has higher N (∼8 wt.%) and lower K (4.5 wt.%), Al (∼12 wt.%), and Cl (∼0.5 wt.%) contents compared to that from Venezuela (∼6 wt.% N, ∼7 wt.% K, ∼15 wt.% Al, and ∼10 wt.% Cl contents). Sveite from Brazil has extremely low K (<0.1 wt.%) and high N (∼24 wt.%) contents with respect to those from the other localities. To inhibit formation of aluminum chloride on dissolution, which can subsequently impact results from geochemical analyses, it is recommended that ultrapure H2O be used to digest sveite. Geochemical and spectroscopic evidence suggests that sveite from different localities may represent distinct mineral species within a potentially larger group of sveite-type minerals. A revised chemical formula for sveite from Venezuela is K2.73Al7.97(NO3)6O8.27Cl4.1·35H2O, Z = 2, leading to a more general formula of (K3−x□x) Al8(NO3)6O8Cl4·nH2O, Z = 2. The lack of K and Cl in material from other localities that shows similar powder diffraction patterns would require a more general formula.

A brief comment on Hawthorne (2023): “On the definition of distinct mineral species: A critique of current IMA-CNMNC procedures”

AbstractIn this communication we present a brief response to Hawthorne (2023) who, in a paper in volume 87,doi.org/10.1180/mgm.2023.8(this journal), claims evidence for violations of the electroneutrality principle in mineral formulae derived through IMA–CNMNC procedures: i.e. the dominant-constituent rule, the valency-imposed double site-occupancy, the dominant-valency rule, and the site-total-charge approach (STC).His statement is not correct as the STC method is based on the end-member definition; thus, it cannot violate the requirements of an end-member, particularly the laws of conservation of electric charge. The STC was developed to address the shortcomings in the previous IMA–CNMNC procedures.The real question is: which method to use to define an end-member formula? Currently, there are two approaches: (1) STC, which first identifies the dominantend-membercharge arrangement and then leads to the dominantend-member composition; (2) the dominant end-member approach.

On the definition of distinct mineral species: A critique of current IMA–CNMNC procedures

AbstractThe criteria for the definition of a new mineral species currently used by the Commission on New Minerals Nomenclature and Classification (CNMNC) of the International Mineralogical Association are critically examined. In particular, the rule of the dominant constituent can violate the laws of conservation of electric charge. A series of additional rules: (1) valency-imposed double site-occupancy; (2) the dominant-valency rule; and (3) the site-total-charge approach, have been developed in an attempt to correct this error. However, none of these rules can overcome the fundamental flaw introduced by the rule of the dominant constituent, and the chemical formulae resulting from application of these rules can violate the requirements of an end-member, particularly that of electroneutrality. As a result, the IMA–CNMNC rules cannot derive end-member formulae for some groups of minerals, giving rise to many ad hoc decisions in defining distinct mineral species.

Nomenclature of wöhlerite-group minerals

AbstractA nomenclature and classification scheme for wöhlerite-group minerals has been established. The general formula of minerals belonging to this group is given by X8(Si2O7)2W4, where X = Na+, Ca2+, Mn2+, Ti4+, Zr4+ and Nb5+; and W = F– and O2–. In addition, they may incorporate significant amounts of Mg2+, Fe2+, Y3+ and REE3+, where REE are the lanthanides. The main structural feature of these minerals is the four-columns-wide octahedral walls, which are interconnected through corner sharing and via the disilicate groups. The wöhlerite-group minerals crystallise in different unit-cell settings and symmetries, depending on the cationic ordering in the octahedral walls and the relative position of the disilicate groups. Different combinations of X and W constituents should be regarded as separate mineral species. In the case of coupled heterovalent substitutions at different crystallographic sites, it is advised to use the site-total charge approach to determine the correct end-member composition. Due to their structural and chemical features, wöhlerite-group minerals can easily form crystals with several micro domains, showing different crystal structures and chemical compositions. In addition, the crystallisation of polytypes is relatively common, although they should not be regarded as distinct mineral species. To date, ten minerals belonging to the wöhlerite group are considered as valid species: baghdadite, burpalite, cuspidine, hiortdahlite, janhaugite, låvenite, moxuanxueite, niocalite, normandite and wöhlerite. Låvenite and normandite are isostructural and are respectively the Zr and Ti end-members of a solid-solution series. Marianoite is discredited, as it is corresponds to wöhlerite. The ideal formula of hiortdahlite is revised as Na2Ca4(Ca0.5Zr0.5)Zr(Si2O7)2OF3, with one cationic site characterised by a valency-imposed double site-occupancy. These changes have been approved by the IMA–CNMNC (Proposal 20–D).

Copper in Coloradoite, HgTe: Actual State or Analytical Artifact?

Chemical and structural data of two Cu-rich coloradoite crystals from Kalgoorlie, Australia, are reported. Two grains with simple stoichiometries, i.e., Hg1 – xCuxTe with x ~0.30, and ~0.22, were characterized by single-crystal X-ray diffraction and electron-probe microanalysis. In the absence of single-crystal data it might be tempting to hypothesize that the presence of large amounts of other elements can induce some ordering leading to distinct mineral species. However, structural analyses show that all of the phases are cubic and crystallize in the space group $$F\overline 4 3m$$ . On the basis of information gained from the chemical and structural characterization it can be concluded that there is a complete solid solution between coloradoite and a possible, still unknown, Cu-coloradoite in nature.

The crystal structure of Ni-rich gordaite–thérèsemagnanite from Cap Garonne, France

AbstractThe crystal structure of Ni-rich gordaite–thérèsemagnanite has been determined from a sample collected at pillar 80 in the North mine, Cap Garonne, Var, France. The structure was refined toR1= 0.0693 for 935 reflections withI> 2σ(I). The mineral is isostructural with gordaite, forming a layered structure with an extensive hydrogen-bonding network. The possible polytypic relationship between gordaite, thérèsemagnanite and guarinoite is also discussed. The guarinoite formula (Zn,Co,Ni)6(SO4)(OH,Cl)10·5H2O is also likely to be incorrect and is more likely to be Na(Zn,Co)4(SO4)(OH)6Cl·5–6H2O, meaning that guarinoite is equivalent to Co-rich gordaite-2Hand would not be a distinct mineral species.

Heptasartorite, Tl7Pb22As55 S108, enneasartorite, Tl6 Pb32 As70 S140 and hendekasartorite, Tl2Pb48As82 S172, three members of the anion-omission series of 'sartorites' from the Lengenbach quarry at Binntal, Wallis, Switzerland

Detailed electron-microprobe investigations and crystal-structure determinations established that ‘sartorite’ represents a group of distinct mineral species, each with unique chemistry and crystal structure. These manifest themselves as the 7-, 9- and 11-fold P 2 1 / c superstructures of the basic 4.2 A substructure. Heptasartorite is Tl 7 Pb 22 As 55 S 108 [based on 192 atoms per formula unit ( apfu ), 84Me + 108S] with a = 29.269(2), b = 7.8768(5), c = 20.128(2) A, β = 102.065(2)° and unit-cell volume V = 4537.8 A 3 ; enneasartorite is Tl 6 Pb 32 As 70 S 140 (based on 248 apfu , 108Me + 140S) with a = 37.612(6), b = 7.8777(12), c = 20.071(3) A, β = 101.930(2)° and V = 5818.6(15) A 3 ; hendekasartorite is Tl 2 Pb 48 As 82 S 172 (based on 304 apfu , 132Me + 172S) (empirical ΣMe = 132.48) with a = 31.806(5), b = 7.889(12), c = 28.556(4) A and β = 99.034(2)° with V = 7076.4(15) A 3 . Physical and optical properties (grey with metallic lustre, in polished section white with visible bireflectance, red internal reflections; reflectance curves span 28.7–42.5%; Mohs hardness 3–3½) of these phases are very similar so that chemical analysis and/or single-crystal X-ray diffraction is needed to distinguish them. A brief description of complicated As m S n crank-shaft chains in the walls of double-ribbons which form the As-based slabs of these structures is given. The three new mineral species differ in their structures by 4.2 A modular increments, not just by cation substitutions. They represent anion-omission derivatives of the ‘ideal’ PbAs 2 S 4 composition with an important role for thallium in charge compensation. The described minerals belong to the late sulfide phases in the Pb–Tl–Ag–As deposit of Lengenbach, Wallis, Switzerland.

Focused Ion Beam and Advanced Electron Microscopy for Minerals: Insights and Outlook from Bismuth Sulphosalts

This paper comprises a review of the rapidly expanding application of nanoscale mineral characterization methodology to the study of ore deposits. Utilising bismuth sulphosalt minerals from a reaction front in a skarn assemblage as an example, we illustrate how a complex problem in ore petrology, can be approached at scales down to that of single atoms. We demonstrate the interpretive opportunities that can be realised by doing this for other minerals within their petrogenetic contexts. From an area defined as Au-rich within a sulphosalt-sulphide assemblage, and using samples prepared on a Focused Ion Beam–Scanning Electron Microscopy (SEM) platform, we identify mineral species and trace the evolution of their intergrowths down to the atomic scale. Our approach progresses from a petrographic and trace element study of a larger polished block, to high-resolution Transmission Electron Microscopy (TEM) and High Angle Annular Dark Field (HAADF) Scanning-TEM (STEM) studies. Lattice-scale heterogeneity imaged in HAADF STEM mode is expressed by changes in composition of unit cell slabs followed by nanoparticle formation and their growth into “veins”. We report a progressive transition from sulphosalt species which host lattice-bound Au (neyite, lillianite homologues; Pb-Bi-sulphosalts), to those that cannot accept Au (aikinite). This transition acts as a crystal structural barrier for Au. Fine particles of native gold track this progression over the scale of several hundred microns, leading to Au enrichment at the reaction front defined by an increase in the Cu gradient (several wt %), and abrupt changes in sulphosalt speciation from Pb-Bi-sulphosalts to aikinite. Atom-scale resolution imaging in HAADF STEM mode allows for the direct visualisation of the three component slabs in the neyite crystal structure, one of the largest and complex sulphosalts of boxwork-type. We show for the first time the presence of aikinite nanoparticles a few nanometres in size, occurring on distinct (111)PbS slabs in the neyite. This directly explains the non-stoichiometry of this phase, particularly with respect to Cu. Such non-stoichiometry is discussed elsewhere as defining distinct mineral species. The interplay between modular crystal structures and trace element behaviour, as discussed here for Au and Cu, has applications for other mineral systems. These include the incorporation and release of critical metals in sulphides, heavy elements (U, Pb, W) in iron oxides, the distribution of rare earth elements (REE), Y, and chalcophile elements (Mo, As) in calcic garnets, and the identification of nanometre-sized particles containing daughter products of radioactive decay in ores, concentrates, and tailings.

Chemographic Exploration of the Milarite-Type Structure

Abstract The milarite structure-type has an uncommonly large number of distinct mineral species (23 at present). Here we explore this structure type from the point of view of possible root-charge arrangements and endmember compositions. Enumeration shows that there are 34 distinct root-charge arrangements with Si = 12 apfu and 39 distinct root-charge arrangements with Si = 8–11 apfu. A priori bond-valence calculations for all root-charge arrangements allows evaluation of lattice-induced strain as a function of chemical composition for all arrangements for which the detailed atomic arrangement has been refined. Analysis of localized strain indicates that the B site has the highest amount of strain in the structure, and in accord with this finding, milarite-group minerals with vacancies at the B site are more common than milarite-group minerals with fully occupied B sites. The a priori bond-valence calculations suggest that many other compositions are possible for the milarite structure-type. Examination of synthesis results reveals 20 synthetic compounds with the milarite-type structure that have distinct (dominant) endmember compositions. Examination of ∼350 chemical analyses from the literature reveals 29 distinct endmember compositions, six of which deserve to be described as new mineral species. Two additional analyses could lead to new minerals, but require confirmation of site populations by structure refinement.

A re-examination of petrogenesis and 40Ar/39Ar systematics in the Chain of Ponds K-feldspar: “diffusion domain” archetype versus polyphase hygrochronology

K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, upon which thermochronological modeling of Ar diffusion in discrete “domains” was founded. We re-examine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. 40Ar/39Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that post-date magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest “diffusion domains.” Heterochemical phases also play an important role in producing nonlinear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick’s Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the “diffusion domain” model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution among different coexisting phases that gives rise to the staircase-shaped age spectrum. Microbeam imaging of textures is as essential for 40Ar/39Ar hygrochronology as it is for U–Pb geochronology.

The crystal structure of parnauite: a copper arsenate?sulphate with translational disorder of structural rods

New data for parnauite from the type locality, Majuba Hill, Nevada, USA (MH; type specimen), and also from Cap Garonne, Var, France (CG), and the Clara Mine, Baden-Wurttemberg, Germany, are presented. The average chemical composition of MH material is (Cu 8.82 Al 0.16 Fe 0.02 )∑ 9.00 (As 1.78 Al 0.07 Si 0.08 S 0.07 )∑ 2.00 O 8 (SO 4 )(OH) 10 ·7H 2 O and that of CG parnauite, (Cu 8.42 Al 0.21 Zn 0.10 )∑ 8.73 (AsO 4 ) 2 [(S 0.97 As 0.10 )∑ 1.07 O 4 ](OH 9.23 Cl 0.77 )∑ 10.00 · 7H 2 O. Both of these formulae confirm the 9:2:1 Cu:As:S ratio obtained from earlier descriptions of parnauite. Raman spectra for parnauite from both localities are very similar. Bands are assigned, but show no evidence of the presence of CO 3 , in contrast to previous studies, and no distinct Cu–Cl stretching mode. It appears that neither the minor CO 3 and PO 4 previously reported nor Cl are essential constituents of parnauite. Single-crystal XRD analysis indicates a primitive orthorhombic unit cell with dimensions 6 × 14 × 15 A, similar to previous studies, but h = odd reflections were heavily streaked and diffuse, preventing full refinement. A 3 A substructure was refined, with space group Pmn 2 1 , to R 1 ( F ) = 0.0750 (MH). For a MH crystal, the subcell had a = 3.0113(4), b = 14.259(3), c = 14.932(2) A, V = 641.13(16) A 3 and Z = 1. The structure is of a new type, and contains Cu in 6 distinct sites, forming two three-polyhedron wide ribbons of edge-sharing Cu-O polyhedra extended parallel to the a - axis. The two ribbons lie back-to-back and are bridged by two AsO 4 tetrahedra. The collection of 6Cu + 2As cations plus ligands forms a rod-like moiety extended || a . These rods link through polyhedral corners to form complex, corrugated (010) layers. The interlayer space is occupied by H 2 O molecules. Thus, the disorder observed by XRD is of an unusual type, in which the shape of the unit mesh within layers is variable, rather than the stacking of the layers. Disorder arises because each AsO 4 tetrahedron shares a face with a Cu(O,OH,H 2 O) 5–6 polyhedron in the substructure, necessitating partial occupancy of both As and Cu sites. The S atoms were not located in the refinement, but four electron-density maxima in the interlayer region were interpreted as H 2 O molecules. Hence, the simplified structural formula derived from the substructure is (Cu 10 □ 2 )(As 2 □ 2 )O 8 (OH) 14 ·8H 2 O, deviating from that obtained in chemical analyses. The discrepancy presumably arises due to strong delocalisation of the sulphur and the apical oxygen of the SO 4 tetrahedron in the substructure. Short-range order of Cu–As and Cu–S || a can occur independently in the relevant structural rods, which accounts for the observed long-range disorder. Cell parameters and substructures obtained from CG and Clara material are similar to those from the MH crystal. Site splitting of OH positions in the CG refinement indicates that Cl is distributed over several sites in the 3 A substructure, making the mineral a Cl-rich variety of parnauite rather than a distinct mineral species.

The solid solution between platinum and palladium in nature

AbstractChemical and structural data are reported for platinum–palladium intermediates from two nuggets found at Córrego Bom Sucesso, Minas Gerais, Brazil. Three grains with simple stoichiometries (i.e. PtxPd1−x with x ∼0.67, ∼0.5 and ∼0.33, which correspond to Pt2Pd, PtPd and PtPd2, respectively) were characterized by single-crystal X-ray diffraction and electron-probe microanalysis. In the absence of single-crystal data it might be tempting to hypothesize that such simple stoichiometries represent distinct mineral species, however structural analyses show that all of the phases are cubic and crystallize in space group Fmm. They are, therefore, natural intermediates in the palladium–platinum solid solution. Reflectance and micro-hardness values are reported for the samples and a comparison with the pure metallic elements made. On the basis of information gained from the chemical and structural characterization it can be concluded that there is a complete solid solution between Pt and Pd in nature. These findings corroborate results from experiments on synthetic compounds.

The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites

Stichtite, ideally Mg6Cr2CO3(OH)16∙4H2O, from Stichtite Hill, Tasmania, Australia, and barbertonite, also ideally Mg6Cr2CO3(OH)16∙4H2O, from the Kaapsehoop asbestos mine, South Africa, have been studied by powder X-ray diffraction and their structures have been refined using the Rietveld method. Stichtite from Stichtite Hill crystallizes in the rhombohedral space group R3̄m, with unitcell parameters a = 3.09575(3) and c = 23.5069(6) Å, V = 195.099(6) Å3, with Z = 3/8. Barbertonite from the Kaapsehoop asbestos mine crystallizes in the hexagonal space group P63/mmc. The co-type specimens of barbertonite were found to be intergrown mixtures consisting of barbertonite and stichtite. Unit-cell parameters of barbertonite from the co-type specimens were a = 3.09689(6), c = 15.6193(8) Å, and V = 129.731(8) Å3 and a = 3.09646(6), c = 15.627(1) Å V = 129.76(1) Å3, and Z = ¼. Rietveld refinements of both stichtite and barbertonite show that they are polytypes rather than polymorphs and do not represent distinct mineral species. Several possible nomenclature systems are discussed for the naming of hydrotalcite minerals and groups. Raman band assignments are also presented for stichtite from Stichtite Hill.

Nomenclature of the apatite supergroup minerals

The apatite supergroup includes minerals with a generic chemical formula IX M12 VII M23( IV TO4)3 X( Z ¼ 2); chemically they can be phosphates, arsenates, vanadates, silicates, and sulphates. The maximum space group symmetry is P63/m, but several members of the supergroup have a lower symmetry due to cation ordering and deviations from the ideal topology, which may result in an increase of the number of the independent sites. The apatite supergroup can be formally divided into five groups, based on crystal-chemical arguments: apatite group, hedyphane group, belovite group, britholite group, and ellestadite group. The abundance of distinct ions which may be hosted at the key-sites (M ¼ Ca 2þ , Pb 2þ , Ba 2þ , Sr 2þ , Mn 2þ , Na þ , Ce 3þ , La 3þ ,Y 3þ , Bi 3þ ;T ¼ P 5þ , As 5þ ,V 5þ , Si 4þ ,S 6þ ,B 3þ ;X ¼ F � , (OH) � , Cl � ) result in a large number of compositions which may have the status of distinct mineral species. Naming of apatite supergroup minerals in the past has resulted in nomenclature inconsistencies and problems. Therefore, an ad hoc IMA-CNMNC Subcommittee was established with the aim of rationalizing the nomenclature within the apatite supergroup and making some order among existing and potentially new mineral species. In addition to general recommendations for the handling of chemical (EPMA) data and for the allocation of ions within the various sites, the main recommendations of this subcommittee are the following: 1. Nomenclature changes to existing minerals. The use of adjectival prefixes for anions is to be preferred instead of modified Levinson suffixes; accordingly, six minerals should be renamed as follows: apatite-(CaF) to fluorapatite, apatite-(CaOH) to hydroxylapatite, apatite-(CaCl) to chlorapatite, ellestadite-(F) to fluorellestadite, ellestadite-(OH) to hydroxylellestadite, phospho- hedyphane-(F) to fluorphosphohedyphane. For the apatite group species these changes return the names that have been used in thousands of scientific paper, treatises and museum catalogues over the last 150 years. The new mineral IMA 2008-009, approved without a name, is here named stronadelphite. Apatite-(SrOH) is renamed fluorstrophite. Deloneite-(Ce) is renamed deloneite. The new mineral IMA 2009-005 is approved with the name fluorbritholite-(Y).

MOSANDRITE: STRUCTURAL AND CRYSTAL-CHEMICAL RELATIONSHIPS WITH RINKITE

Mosandrite, Ti(□,Ca,Na) 3 (Ca,REE) 4 (Si 2 O 7 ) 2 [H 2 O,OH,F] 4 ·~1 H 2 O, is a distinct mineral species in the rinkite group. Chemical (EPM data), TG–DSC studies and structural investigations indicate that mosandrite presents a particular chemical composition (low Ca, Na and F contents, high amount of H 2 O), unit-cell parameters ( a 7.398, b 5.595, c 18.662 A, β 101.37°, V 757.29 A 3 ; space-group symmetry P 2 1 / c ; Z = 2), and a rinkite-type structure characterized by a low occupancy of the M (2) and M (3) sites.

Pressure-induced hydration in zeolite tetranatrolite

The tetranatrolite-paranatrolite transformation has remained a key problem in understanding the paragenesis of zeolites in the natrolite family. It is accepted that when paranatrolite, approximate formula Na 16-x Ca x Al 16+x Si 24-x O 80 · 24 H 2 O, is removed from an aqueous environment and exposed to the atmosphere, it loses water and transforms to tetranatrolite, Na 16-x Ca x Al 16+x Si 24-x O 80 ·nH 2 O (n ≤ 24). Here we show that this transformation is not only reversible, but that tetranatrolite exhibits two sequential pressure-induced hydrations leading first to paranatrolite and then to a superhydrated tetranatrolite above 0.2 and 3.0 Gpa, respectively. We have previously reported similar behavior for the corresponding system with an ordered Si/Al distribution, i.e., natrolite itself, however the ordered version of paranatrolite exists over a much smaller pressure range. The pressure-induced transformations of natrolite and tetranatrolite thus further supports the supposition that paranatrolite is a distinct mineral species, with a pressure-stability field dependent upon composition.

ON THE CLASSIFICATION OF AMPHIBOLES

Major issues involved in the classifi cation of the amphiboles are examined: (1) the role of (OH), Li and Fe 3+ , (2) the formal defi nition of a root name, (3) irreducible charge-arrangements and distinct species, (4) the use of prefi xes, (5) the principal chemical variables used in a classifi cation procedure, and (6) the use of the dominant-constituent principle. The current IMAapproved classifi cation scheme is based on the A, B and T groups of cations in the amphibole formula: AB2C5T8O22W2. We argue here that classifi cation should be based on the A, B and C groups of cations as (i) it is in these groups of cations that the maximum variation in chemical composition occurs, and (ii) as a result of (i), the scheme is more in accord with the IMA-sanctioned dominant-constituent principle, which governs the recognition (and approval) of distinct mineral species. Two new classifi cations are presented here; one is based on the A, B and C groups of cations, and another on the dominant-constituent principle. These two schemes were produced to illustrate (i) the problems inherent in the classifi cation of a group of minerals as complicated as the amphiboles, and (ii) the sometimes disparate needs of crystallographer, mineralogist, petrologist and geochemist. Scheme 1 conserves current formulae and names as much as possible, whereas scheme 2 minimizes the number of formulae and names as much as possible. The differences between the current classifi cation and the two schemes presented here are discussed, and we highlight the problems associated with each scheme.

Clinoptilolite-Heulandite Nomenclature

Heulandite was first named in 1822 for the English mineral collector, J. H. Heuland (see Dana 1914, p. 574–576). Although it is typically found as macroscopic (commonly 0.2–2 cm in size) crystals in cavities in mafic igneous rocks and is volumetrically minor on the Earth’s surface, heulandite also occurs in larger amounts in some sedimentary deposits, often in association with clinoptilolite. Due to the large size of typical heulandite crystals, its chemical composition and crystallographic and optical properties were easily characterized. Mineralogists recognized early that heulandite is monoclinic and that heating changes its optical properties in a predictable manner (Slawson 1925). Unlike heulandite, the mineral known today as clinoptilolite typically occurs as microscopic crystals, commonly 2–20 μm in size and commonly intimately admixed with other fine-grained minerals. Although clinoptilolite is known today as the most common natural zeolite, occurring in large amounts (millions of tons) in altered volcanic tuffs and saline, alkaline-lake deposits, it is generally mentioned only in passing in beginning mineralogy texts. Clinoptilolite was not described as a distinct mineral species until 1932 (Schaller 1932). Prior to that time, the platy material described by Pirsson (1890) in amygdules in weathered basalt from the Hoodoo Mountains, Wyoming, had been classified as “mordenite,” based primarily on its chemical similarity with mordenite from Nova Scotia. In spite of the chemical similarities, Pirsson did recognize the crystallographic similarity between the Wyoming “mordenite” and heulandite. Dana’s sixth edition (1914, p. 572–573) lists both ptilolite and mordenite as members of the mordenite group. Ptilolite (which we now recognize as mordenite) is listed by Dana as commonly occurring in aggregates of needles having parallel extinction, but “mordenite” is reported by Dana to be monoclinic, having a form closely approximating that of heulandite. Schaller (1932) proposed the name clino ptilolite based on the chemical …

RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS, ITALY. III. WITTITE AND CANNIZZARITE

Wittite and cannizzarite, two rare and structurally related nonstoichiometric sulfosalts belonging to the system Pb‐Bi‐S‐Se, were found together around high-temperature fumaroles at La Fossa crater, Vulcano island, Italy. This occurrence makes a comparative study by different methods possible (scanning electron microscope, electron microprobe, and X-ray and electron diffraction). We discuss once more the question whether or not they are distinct mineral species. Cannizzarite was found in samples collected from 1990 to 1995 in assemblages with bismuthinite and galenobismutite (mostly), in some cases with lillianit e, heyrovskiite, Se-bearing galena, kirkiite, and the new species mozgovaite. Wittite was revealed only in samples collected in 19 95 around the fumarole vent F11 in association with bismuthinite. Both minerals form aggregates of very tiny sheaves of slightly divergent, bladed crystals (cannizzarite up to 0.5 mm in length and 0.07 mm in width; wittite up to 2 mm in length). The electr onmicroprobe data obtained (141 analyses) show significant variations in proportions of the main elements. In terms of Bi/(Bi + P b), the range of composition is 3.14 at.%; the Se content varies from less than 1 to 14.65 wt%. The composition field of the minera ls under consideration may be described by the general empirical chemical formula Pb 3+xBi4‐x(S9‐ySe y)9‐x/2 or Pb3(Bi4‐xPbx)4 (S9‐ySey)9‐x/2, with 0.04 ≤ x ≤ 0.28, and 0.5 ≤ y ≤ 3.5. Electron-diffraction data and X-ray investigations of a sample of wittite containing ~8.5 wt% Se confirm that cannizzarite and wittite have the same structure. Measured unit-cell parameters show that volumes of the H and Q subcells both vary in direct proportion to Se content. No chemical discontinuity between cannizzarite an d wittite series has been observed up to 40 at.% Se; consequently, wittite may be validated or discredited as a distinct mineral species only when a definitive crystallographic work will prove that Se exceeds S in the H layers.

Nepal Gem Tourmalines

Gem quality tourmaline crystals from pockets in the pegmatites at Hyakule and Phakuwa in the Sankhuwa Sabha District, Kosi Zone, eastern Nepal, have been mined for over 50 years with a production of at least a ton of gem crystals. Mining continues, despite land-slide problems, in sporadic operations at both mines. Minerals associated with the elbaite tourmalines include quartz, muscovite biotite, lepidolite, beryl (occasionally of gem aquamarine), both spessartine and hessonite garnets, danburite, hambergite, apatite, and both micro-cline and cleavelandite feldspar The gem-bearing pegmatites are associated with schist and carbonate rocks in the Khitya Khola formation as defined by E. Andrews in an article in this journal in 1985. The origin of these intrusive pegmatites is still controversial as no body of source granite is exposed in the area. Additional tourmaline-bearing pegmatites are reported in the Sankhuwa Sabha, Taplejung Jajarkot, and Rasuwa districts. Five distinct mineral species of the tourmaline group occur in Nepal.