Mindat Mineralogy Messageboard - Mineralogical Classification

Re: Vaterite news.

Uwe Kolitsch — Mon, 29 Apr 2013 17:12:50 +0000

Vaterite page updated.

Re: Vaterite news.

Timothy Greenland — Mon, 29 Apr 2013 12:57:20 +0000

Thank you Marco - I should have added the original reference. Sorry.

Tim

Re: Vaterite news.

Marco E. Ciriotti — Mon, 29 Apr 2013 11:33:57 +0000

Reference:
▪ Kabalah-Amitai, L., Mayzel, B., Kauffmann, Y., Fitch, A.N., Bloch, L., Gilbert, P.U.P.A., Pokroy, B. (2013): Vaterite Crystals Contain Two Interspersed Crystal Structures. Science, 340, 454-457.

Abstract:
Calcite, aragonite, and vaterite are the three anhydrous polymorphs of calcium carbonate, in order of decreasing thermodynamic stability. Although vaterite is not commonly found in geological settings, it is an important precursor in several carbonate-forming systems and can be found in biological settings. Because of difficulties in obtaining large, pure, single crystals, the crystal structure of vaterite has been elusive for almost a century. Using aberration-corrected high-resolution transmission electron microscopy, we found that vaterite is actually composed of at least two different crystallographic structures that coexist within a pseudo–single crystal. The major structure exhibits hexagonal symmetry; the minor structure, existing as nanodomains within the major matrix, is still unknown.

The crystal structure of creaseyite

Marco E. Ciriotti — Sun, 28 Apr 2013 18:53:27 +0000

Reference:
▪ Malcherek, T., Köslin, B., Schlüter, J. (2013): The crystal structure of creaseyite: a disordered, nanoporous lead iron copper alumosilicate. Zeitschrift für Kristallographie, 228, 134-139.

Abstract:
The crystal structure and composition of the mineral creaseyite from Iquique, Chile has been studied using X-ray single crystal diffraction and electron microprobe analysis. The needle like, pale green crystals are elongated parallel [001]. An idealized composition Pb2Cu2Fe23+(Si4.667Al0.333)O15.333(OH)3·H2O has been obtained by crystal structure analysis. Creaseyite is orthorhombic, space group Cmcm, a = 12.502(8), b = 21.402(11), c = 7.303(6) Å. It exhibits substantial disorder of Si, Al and their oxygen ligands along [001]. Pb2+ occurs in a strongly asymmetric coordination, imposed by its lone pair electrons. Creaseyite is nanoporous, forming open channels of elliptical cross section along [001]. In the untreated crystals the channels are occupied by water molecules in an off-centred position. The lone pair electrons of Pb2+ extend into the channels. It is expected that the lone pair electrons are accessible to small acceptor molecules or polarizable atoms substituting for water as the extra framework constituent.

Vaterite news.

Timothy Greenland — Fri, 26 Apr 2013 08:17:41 +0000

I found this intriguing reference today. An article in Science magazine (of the AAAS) suggests that vaterite is orgganised at two distinct interpenetrating crystal structures... I am not sure where to put such news, so have posted in 'General', but if someone thinks it should be shifted - OK

[www.sciencedaily.com]

Cheers

Tim

Re: Gray Sulphuret of Copper

Ralph Bottrill — Sat, 20 Apr 2013 23:51:20 +0000

I wouldn't say I had a reverence for the term, but you see it in some old reports from the 1800's, So I added it to the glossary. It's hard to know where to put old terms like grey copper ore, if at all.

Re: Gray Sulphuret of Copper

Paul Brandes — Sat, 20 Apr 2013 19:31:25 +0000

Rock and others,

The term is listed in the Dictionary of Mining, Minerals, and Related Terms but not under the British spelling "Sulphuret", rather, it is listed under "Sulfuret" as:

Quote
Dictionary of Mining, Minerals, and Related Terms
Pac. The undecomposed metallic ores, usually sulfides. Chiefly applied to
auriferous pyrites. Concentrate and sulfide are preferable. An old syn.
for sulfide. Obsolete.

This term Sulfuret has already been added to the Glossary, and I just added that it is also known as Sulphuret to avoid confusion.

Re: Gray Sulphuret of Copper

Rock Currier — Sat, 20 Apr 2013 17:22:11 +0000

The term is not in our glossary. Ralph, why don't you enter it into our Glossary. If you have a reverence for the term, which by the way, is not listed in the Dictctionary of Mining, Mineral and Related Terms and you have a reference for it, check our index of glossary references: and you can use that reference and if it is not there, then you can add yours to our index.

Re: Gray Sulphuret of Copper

David Von Bargen — Sat, 20 Apr 2013 11:33:56 +0000

It can also refer to tetrahedrite/tennantite. (especially if it is in dodecahedral crystals)

Re: Gray Sulphuret of Copper

Ralph Bottrill — Fri, 19 Apr 2013 23:04:52 +0000

This is a general name for grey copper sulphides, generally chalcocite, digenite, djurleite and others, often a mixture.

Gray Sulphuret of Copper

Ronnie Van Dommelen (2) — Fri, 19 Apr 2013 22:23:51 +0000

I'm doing some reasearch on minerals from my area. I have an 1855 reference to 'gray sulphuret of copper'. This was found with magnetite in veins in basalt. Is this chalcocite? Could someone provide me with a reference that clearly identifies what the mineral is? I checked the Mindat database and there are similar entries, but not this specific one.

Thanks in advance.

Crystal structure of mimetite-2M

Marco E. Ciriotti — Fri, 19 Apr 2013 10:47:05 +0000

Reference:
▪ Yang, Z., Ding, K., de Fourestier, J., Hi, He (2013): The crystal structure of mimetite-2M, a new polymorph of mimetite from Xianghualing tin-polymetallic orefield, Hunan Province, P. R. China. Neues Jahrbuch für Mineralogie - Abhandlungen, 190, 229-235.

Abstract:
Mimetite-2M, ideally Pb5(AsO4)3Cl, is a new polymorph of mimetite in the apatite group occurring as a skarn mineral in the Xianghualing Sn-polymetallic ore field, Linwu County, Hunan Province, P. R. China. The associated minerals include fluorite, calcite, cassiterite, pyrite, sphalerite, pyrrhotite, galena, spinel, zinnwaldite and phlogopite. Mimetite-2M is pale yellow and forms pseudo-hexagonal columnar crystals of about 1 mm to 5 mm in size. The empirical formula is Pb5.04As2.97O11.95Cl1.05.
The super-structure reflections of mimetite were observed and require doubling of the a-axes of mimetite-H. The crystal structure was refined in space group P21; with a = 20.422(4), b = 7.4379(15), c = 20.435(4) Å., β = 119.95(3)°, V = 2689.5(9) Å3, Z = 8, R1 = 0.067, wR2 = 0.180.
The largest differences in positions of analogous atoms between the hexagonal phase mimetite-H and monoclinic mimetite-2M occur in the positions relative to the O(3) and Cl atoms in the hexagonal phase. The actual positions of the atoms O(3)1 to O(3)24 and Cl in mimetite-2M results in the reduction of symmetry to monoclinic with both doubled a-axes of the hexagonal phase.

High resolution X-ray diffraction data of pirssonite

Marco E. Ciriotti — Fri, 19 Apr 2013 10:45:52 +0000

Reference:
▪ Callegari, A.M. & Boiocchi, M. (2013): High resolution X-ray diffraction data of pirssonite from Searles Lake, San Bernardino County, California. Neues Jahrbuch für Mineralogie - Abhandlungen, 190, 221-227.

Abstract:
The crystal chemistry of pirssonite: CaNa2(CO3)2·2H2O; has been re-evaluated on the basis of high resolution X-ray diffraction data collected on a single crystal from the type locality. Pirssonite is orthorhombic with a = 11.3285(3), b = 20.0579(4), c = 6.0313(1) Å; V = 1370.47(5) Å3, non-centrosymmetric space group Fdd2, Z = 8.
The original crystal-structure determination of pirssonite is confirmed and the crystallographic results obtained on a natural sample have been compared with the available data of literature. The new structure refinement converges to a final Robs value of 0.0246 for 3768 reflections with I/σ(I) ≥ 3 and allows an accurate description of the crystal structure.

Re: New slag phases, in part related to minerals

Rob Woodside — Wed, 17 Apr 2013 23:53:15 +0000

THANKS!!!

Re: New slag phases, in part related to minerals

Jim Ferraiolo — Wed, 17 Apr 2013 20:35:16 +0000

Nice!

Re: New slag phases, in part related to minerals

Uwe Kolitsch — Wed, 17 Apr 2013 17:30:33 +0000

Finally finished uploading the numerous photos made by us (presently 255 are on Mindat):
[www.mindat.org]

AMPH2012 - a new and implemented release

Roberta Oberti — Mon, 08 Apr 2013 10:51:29 +0000

For those who are interested in amphiboles.

A new and implemented version of the program AMPH 2012, which gives the correct name to amphibole compositions after the IMA report (Hawthorne et al. American Mineralogist, 97, 2031-2048,l 2012) can now be downloaded at [www_crystal.unipv.it].

This 2.0 version also covers orthorhombic amphiboles and acknowledges the recent recognition of the new rootnames ghoseite (for rootname 11) and suenoite (for the orthorhombic counterpart of rootname 3).

Mistakes in the use of the magnesio- prefix have been corrected.

Multiple entries from a file are now allowed.

Should you need any help or further explanation, or should you find any mistake in the procedure, please do not hesitate to contact me. Your feedback has proven to be very useful.

All the best

Roberta Oberti

Re: The crystal structure of cualstibite-1M

Uwe Kolitsch — Mon, 08 Apr 2013 09:23:12 +0000

PDF available for anyone interested.

Crystal structure of ilinskite

Marco E. Ciriotti — Sat, 06 Apr 2013 12:59:38 +0000

Reference:
▪ Krivovichev, S.V., Filatov, S.K., Vergasova, L.P. (2013): The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, and review of mixed-ligand CuOmCln coordination geometries in minerals and inorganic compounds. Mineralogy and Petrology, 107, 235-242.

Abstract:
The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, a rare copper selenite chloride from volcanic fumaroles of the Great fissure Tolbachik eruption (Kamchatka peninsula, Russia), has been solved by direct methods and refined to R 1 = 0.044 on the basis of 2720 unique observed reflections. The mineral is orthorhombic, Pnma, a = 17.769(7), b = 6.448(3), c = 10.522(4) Å, V = 1205.6(8) Å3, Z = 4. The CuOmCln coordination polyhedra share edges to form tetramers that have 'additional' O1 and O2 atoms as centers. The O1Cu4 and O2Cu4 tetrahedra share common Cu atoms to form [O2Cu5]6+ sheets. The SeO3 groups and Cl atoms are adjacent to the [O2Cu5]6+ sheets to form complex layers parallel to (100). The Na+ cations are located in between the layers. A review of mixed-ligand CuOmCln coordination polyhedra in minerals and inorganic compounds is given. There are in total 26 stereochemically different mixed-ligand Cu-O-Cl coordinations.

The crystal structure of cualstibite-1M

Marco E. Ciriotti — Sat, 06 Apr 2013 12:58:25 +0000

Reference:
▪ Kolitsch, U., Giester, G., Pippinger, T. (2013): The crystal structure of cualstibite-1M (formerly cyanophyllite), its revised chemical formula and its relation to cualstibite-1T. Mineralogy and Petrology, 107, 171-178.

act:
The crystal structure of the rare secondary mineral cualstibite-1M (formerly cyanophyllite), originally reported to have the chemical formula 10CuO·2Al2O3·3Sb2O3·25H2O and orthorhombic symmetry, was solved from single-crystal intensity data (Mo-Kα X-radiation, CCD area detector, 293 K, 2θmax = 80) collected on a twinned crystal containing very minor Mg. The mineral is monoclinic, P21/c (no. 14), with a = 9.938(1), b = 8.890(1), c = 5.493(1) Å, β = 102.90(1)°, V = 473.05(11) Å3; R1(F) = 0.0326. All crystals investigated turned out to be non-merohedric twins. The atomic arrangement has a distinctly layered character. Brucite-like sheets composed of two [4 + 2]-coordinated (Cu,Al,Mg) sites are linked by weak hydrogen-bonding (O···O ~ 2.80 Å) to isolated regular Sb(OH)6 octahedra ( = 1.975 Å). The layered, pseudotrigonal character explains the perfect cleavage and the proneness to twinning. The Sb site is fully occupied and the two (Cu,Al,Mg) sites have occupancies of Cu0.79Al0.17Mg0.04 and Cu0.72Al0.23Mg0.05. The Cu-richer site shows a slightly stronger Jahn-Teller-distortion. The resulting empirical formula, which necessitates a H2O-for-OH substitution to obtain charge balance, is (Cu2.23Al0.63Mg0.14)(OH)5.63(H2O)0.37[Sb5+(OH)6]. The ideal chemical formula is (Cu,Al)3(OH)6[Sb5+(OH)6], with Cu:Al = 2:1. The structure is closely related to those of trigonal cualstibite-1T [Cu2AlSb(OH)12, P-3, with ordered Cu-Al distribution in the brucite sheets], and its Zn analogue zincalstibite-1T [Zn2AlSb(OH)12]. Cualstibite-1M and cualstibite-1T are polytypes and, together with zincalstibite-1T, zincalstibite-9R and omsite, belong to the cualstibite group within the hydrotalcite supergroup, which comprises all natural members of the large family of layered double hydroxides (LDH).

The crystal structure of galgenbergite-(Ce)

Marco E. Ciriotti — Sat, 06 Apr 2013 12:56:59 +0000

Reference:
▪ Walter, F., Bojar, H.-P., Hollerer, C.E., Mereiter, K. (2013): The crystal structure of galgenbergite-(Ce), CaCe2(CO3)4∙H2O. Mineralogy and Petrology, 107, 189-199.

Abstract:
Galgenbergite-(Ce) from the type locality, the railroad tunnel Galgenberg between Leoben and St. Michael, Styria, Austria, was investigated. There it occurs in small fissures of an albite-chlorite schist as very thin tabular crystals building rosette-shaped aggregates associated with siderite, ancylite-(Ce), pyrite and calcite. Electron microprobe analyses gave CaO 9.49, Ce2O3 28.95, La2O3 11.70, Nd2O3 11.86, Pr2O3 3.48, CO2 30.00, H2O 3.07, total 98.55 wt.%. CO2 and H2O calculated by stoichiometry. The empirical formula (based on Ca + REE ∑3.0) is Ca1.00(Ce1.04La0.42Nd0.42Pr0.12)2.00(CO3)4⋅H2O, and the simplified formula is CaCe2(CO3)4⋅H2O. According to X-ray single crystal diffraction galgenbergite-(Ce) is triclinic, space group P1−, a = 6.3916(5) , b = 6.4005(4), c = 12.3898(9) Å, α = 100.884(4), β = 96.525(4), γ = 100.492(4)°, V = 483.64(6) Å3, Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [d calc in Å/(I)/hkl]: 5.052/(100)/011; 3.011/(70)/0-22; 3.006/(66)/004; 5.899/(59)/-101; 3.900/(51)/1-12; 3.125/(46)/-201; 2.526/(42)/022; 4.694/(38)/-102. The infrared absorption spectrum reveals H2O (OH-stretching mode at 3,489 cm−1, HOH bending mode at 1,607 cm−1) and indicates the presence of distinctly non-equivalent CO3-groups by double and quadruple peaks of their ν1, ν2, ν3 and ν4 modes. The crystal structure of galgenbergite-(Ce) was refined with X-ray single crystal data to R1 = 0.019 for 2,448 unique reflections (I > 2σ(I)) and 193 parameters. The three cation sites of the structure Ca(1), Ce(2) and Ce(3) have a modest mixed site occupation by Ca and small amount of REE (Ce, La, Pr, Nd) and vice versa. The structure is based on double layers parallel to (001), which are composed of Ca(1)Ce(2)(CO3)2 single layers with an ordered chessboard like arrangement of Ca and Ce, and with a roof tile-like stacking of the CO3 groups. Perpendicular to (001) the double layers are connected to a triclinic framework structure with good cleavage parallel to (001) by a differently organized and more open part of the structure formed by Ce(3)(CO3)2(H2O). Based on the topology of the CaCe(CO3)2 single layer in galgenbergite-(Ce), structural relationships to rutherfordine, to aragonite and ancylite type minerals, and to lanthanite are outlined.

Cu and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations

Marco E. Ciriotti — Sat, 06 Apr 2013 12:55:41 +0000

Reference:
▪ Ertl, A., Giester, G., Schüssler, U., Brätz, H., Okrusch, M., Tillmanns, E., Bank, H. (2013): Cu and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations. Mineralogy and Petrology, 107, 265-279.

Abstract:
Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.5 wt%). Structurally investigated samples show a pronounced positive correlation between the distances and the (Li + Mn2+ + Cu + Fe2+) content (apfu) at this site with R 2 = 0.90. An excellent negative correlation exists between the distances and the Al2O3 content (R 2 = 0.94). The samples at each locality generally show a strong negative correlation between the X-site vacancies and the (MnO + FeO) content. The Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between MnO and CuO we believe that the Cu content in tourmaline is essentially dependent on the availability of Cu and on stereochemical constraints.

Crystal chemistry of the voltaite group

Marco E. Ciriotti — Sat, 06 Apr 2013 12:53:59 +0000

Reference:
▪ Majzlan, J., Schlicht, H., Wierzbicka-Wieczorek, M., Giester, G., Pöllmann, H., Brömme, B., Doyle, S., Buth, G., Bender Koch, C. (2013): A contribution to the crystal chemistry of the voltaite group: solid solutions, Mössbauer and infrared spectra, and anomalous anisotropy. Mineralogy and Petrology, 107, 221-233.

Abstract:
Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K2Fe5 2+Fe3 3+Al(SO4)12·18H2O and the nominal symmetry is cubic, Fd3−c . The tetragonal (I41/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe2+ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH4 + instead of K+. The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M3+ and M2+ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M2+. In the Fe2+-Mg-Zn solid solutions, the Fe2+-Mg and Fe2+-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M2+-O bonds within the chains. In the Mg-Zn solid solution, some of the M2+-O bonds expand while none contract. In the other solid solutions, expansion of some M2+-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data.

Description and crystal structure of albrechtschraufite

Marco E. Ciriotti — Sat, 06 Apr 2013 12:52:18 +0000

Reference:
▪ Mereiter, K. (2013): Description and crystal structure of albrechtschraufite, MgCa4F2[UO2(CO3)3]2⋅17-18H2O. Mineralogy and Petrology, 107, 179-188.

Abstract:
Albrechtschraufite, MgCa4F2[UO2(CO3)3]2⋅17-18H2O, triclinic, space group Pī, a = 13.569(2), b = 13.419(2), c = 11.622(2) Å, α = 115.82(1), β = 107.61(1), γ = 92.84(1)° (structural unit cell, not reduced), V = 1774.6(5) Å3, Z = 2, D c = 2.69 g/cm3 (for 17.5 H2O), is a mineral that was found in small amounts with schröckingerite, NaCa3F[UO2(CO3)3](SO4)⋅10H2O, on a museum specimen of uranium ore from Joachimsthal (Jáchymov), Czech Republic. The mineral forms small grain-like subhedral crystals (≤ 0.2 mm) that resemble in appearance liebigite, Ca2[UO2(CO3)3]⋅~11H2O. Colour pale yellow-green, luster vitreous, transparent, pale bluish green fluorescence under ultraviolet light. Optical data: Biaxial negative, nX = 1.511(2), nY = 1.550(2), nZ = 1.566(2), 2V = 65(1)° (λ = 589 nm), r < v weak. After qualitative tests had shown the presence of Ca, U, Mg, CO2 and H2O, the chemical formula was determined by a crystal structure analysis based on X-ray four-circle diffractometer data. The structure was later on refined with data from a CCD diffractometer to R1 = 0.0206 and wR2 = 0.0429 for 9,236 independent observed reflections. The crystal structure contains two independent [UO2(CO3)3]4− anions of which one is bonded to two Mg and six Ca while the second is bonded to only one Mg and three Ca. Magnesium forms a MgF2(Ocarbonate)3(H2O) octahedron that is linked via the F atoms with three Ca atoms so as to provide each F atom with a flat pyramidal coordination by one Mg and two Ca. Calcium is 7- and 8-coordinate forming CaFO6, CaF2O2(H2O)4, CaFO3(H2O)4 and CaO2(H2O)6 coordination polyhedra. The crystal structure is built up from MgCa3F2[UO2(CO3)3]⋅8H2O layers parallel to (001) which are linked by Ca[UO2(CO3)3]⋅5H2O moieties into a framework of the composition MgCa4F2[UO2(CO3)3]⋅13H2O. Five additional water molecules are located in voids of the framework and show large displacement parameters. One of the water positions is partly vacant, leading to a total water content of 17-18H2O per formula unit. The MgCa3F2[UO2(CO3)3]⋅8H2O layers are pseudosymmetric according to plane group symmetry Cmm. The remaining constituents do not sustain this pseudosymmetry and make the entire structure truly triclinic. A characteristic paddle-wheel motif Ca[UO2(CO3)3]4Ca relates the structure of albrechtschraufite partly to that of andersonite and two synthetic alkali calcium uranyl tricarbonates.

Crystal structure and chemistry of Bi-rich vesuvianite

Marco E. Ciriotti — Tue, 02 Apr 2013 10:00:25 +0000

Reference:
▪ Hålenius, U., Bosi, F., Gatedal, K. (2013): Crystal structure and chemistry of skarn-associated bismuthian vesuvianite. American Mineralogist, 98, 566-573.

Abstract:
Due to its strong chalcophile character and the influence of its s2 lone-pair electrons on the crystal structure trivalent bismuth is extremely rare in silicate minerals, with Bi-contents in common silicates typically below 1 ppm. In the present paper, we report on an exceptionally Bi-rich variety of the rock-forming mineral vesuvianite with up to ca. 20 wt% Bi2O3, occasionally in combination with enhanced Pb contents up to ca. 5 wt% PbO. The mineral occurs as small (≤300 μm) idiomorphic, black crystals in a sulfide-free silicate skarn in the Långban Mn-Fe deposit, central Sweden. The major skarn minerals comprise Ba-rich potassium feldspar, albitic plagioclase, Pb-rich scapolite, and phlogopite, while Pb-rich epidote, vesuvianite, and calcic garnets are minor phases. The vesuvianite grains are intensely zoned displaying Bi-rich cores surrounded by thinner Bi-poor rims. Although generally high in bismuth, the crystal cores invariably show oscillatory zoning. In addition to high Bi- and Pb-contents, the crystals are occasionally enriched in copper, cerium, antimony, and arsenic, thus reflecting the complex chemistry and evolution of the Långban mineralization.
Chemical analyses demonstrate a strong negative correlation between Ca and Bi, hence confirming that Bi replaces Ca at X sites of the vesuvianite structure. Concentrations of Si and Al are lower, while Fe and Ti contents are somewhat enhanced in the Bi-rich cores. Maximum Bi and Pb contents analyzed in the present vesuvianite crystals correspond to 3.19 and 0.87 atoms per formula unit, respectively. This exceeds by far previous reports in the literature. X-ray single-crystal diffraction studies of a crystal splinter with intermediate Bi-content (1.08 apfu) show that the space group P4/nnc is the most appropriate to describe the crystal structure; the refinement converged to an R1 index of 0.0493. The recorded unit-cell parameters, a = 15.7018(6), c = 11.8648(6) Å, and V = 2925.2(2) Å3, are to our knowledge the largest ones observed so far for P4/nnc vesuvianite. Bismuth was demonstrated to order at the X3′(Bi) site that is only 0.46 Å distant from the nearest X3(Ca) site. Consequently, the X3 and X3′ sites cannot be simultaneously fully occupied.

Revision of the crystal structure and chemical formula of haiweeite

Marco E. Ciriotti — Tue, 02 Apr 2013 09:58:48 +0000

Reference:
▪ Plášil, J., Fejfarová, K., Čejka, J., Dušek, M., Škoda, R., Sejkora, J. (2013): Revision of the crystal structure and chemical formula of haiweeite, Ca(UO2)2(Si5O12)(OH)2·6H2O. American Mineralogist, 98, 718-723.

Abstract:
The previously published crystal structure study suggested that haiweeite is orthorhombic, Cmcm, with a = 7.125(1), b = 17.937(2), c = 18.342(2) Å, and V = 2344.3(7) Å3, and an ideal chemical formula Ca[(UO2)2Si5O12(OH)2]·3H2O, with Z = 4. Using single-crystal X-ray diffraction and electron microprobe analysis we re-examined haiweeite from the Teofilo Otoni, Minas Gerais, Brazil. Our diffraction experiment provided weak reflections responsible for doubling of the b cell parameter (for the current space-group settings), leading finally to the choice of a different space group. Haiweeite is thus orthorhombic, the space group Pbcn, with the unit-cell parameters a = 18.3000(5), b = 14.2331(3), c = 17.9192(5) Å, V = 4667.3(2) Å3, and an ideal formula Ca[(UO2)2(SiO3OH)2(Si3O6)]·6H2O (6.25 H2O inferred from the thermal analysis; 7.50 H2O from the structure model), with Z = 8. The structure refinement yielded R1 = 0.0512 for 2498 observed reflections [Iobs > 3σ(I)] and wR2 = 0.1286 for all 6117 unique reflections. Structure solution confirmed by subsequent refinement provided a structure model with full occupancies for U, Si, and Ca atoms, contrasting to previous average structure model. Although the general topology of our structure resembles that reported previously, all Si and O sites in our structure are fully occupied, in contrast to the previous structure determination.

Nomenclature of the garnet supergroup

Marco E. Ciriotti — Tue, 02 Apr 2013 09:57:32 +0000

Reference:
▪ Grew, E.S., Locock, A.J., Mills, S.J., Galuskina, I.O., Galuskin, E.V., Hålenius, U. (2013): Nomenclature of the garnet supergroup. American Mineralogist, 98, 785-811.

Abstract:
The garnet supergroup includes all minerals isostructural with garnet regardless of what elements occupy the four atomic sites, i.e., the supergroup includes several chemical classes. There are presently 32 approved species, with an additional 5 possible species needing further study to be approved. The general formula for the garnet supergroup minerals is {X3}[Y2](Z3)ϕ12, where X, Y, and Z refer to dodecahedral, octahedral, and tetrahedral sites, respectively, and ϕ is O, OH, or F. Most garnets are cubic, space group Ia3-d (no. 230), but two OH-bearing species (henritermierite and holtstamite) have tetragonal symmetry, space group, I41/acd (no. 142), and their X, Z, and ϕ sites are split into more symmetrically unique atomic positions. Total charge at the Z site and symmetry are criteria for distinguishing groups, whereas the dominant-constituent and dominant-valency rules are critical in identifying species. Twenty-nine species belong to one of five groups: the tetragonal henritermierite group and the isometric bitikleite, schorlomite, garnet, and berzeliite groups with a total charge at Z of 8 (silicate), 9 (oxide), 10 (silicate), 12 (silicate), and 15 (vanadate, arsenate), respectively. Three species are single representatives of potential groups in which Z is vacant or occupied by monovalent (halide, hydroxide) or divalent cations (oxide). We recommend that suffixes (other than Levinson modifiers) not be used in naming minerals in the garnet supergroup. Existing names with suffixes have been replaced with new root names where necessary: bitikleite-(SnAl) to bitikleite, bitikleite-(SnFe) to dzhuluite, bitikleite-(ZrFe) to usturite, and elbrusite-(Zr) to elbrusite. The name hibschite has been discredited in favor of grossular as Si is the dominant cation at the Z site. Twenty-one end-members have been reported as subordinate components in minerals of the garnet supergroup of which six have been reported in amounts up to 20 mol% or more, and, thus, there is potential for more species to be discovered in the garnet supergroup. The nomenclature outlined in this report has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (Voting Proposal 11-D).

Crystal structure of ramdohrite

Marco E. Ciriotti — Fri, 29 Mar 2013 15:47:17 +0000

Reference:
▪ Makovicky, E., Mumme, W.G., Gable, R.W. (2013): The crystal structure of ramdohrite, Pb5.9Fe0.1Mn0.1In0.1Cd0.2Ag2.8Sb10.8S24: a new refinement. American Mineralogist, 98, 773-779.

Abstract:
The crystal structure of ramdohrite, Pb5.9Fe0.1Mn0.1In0.1Cd0.2Ag2.8Sb10.8S24, from the Chocaya mine, Potosí, Bolivia, determined by Makovicky and Mumme from film data in 1983, was refined from single-crystal diffractometer data to the R value 0.060, based on 5230 reflections [I > 2σ(I)] from a twinned crystal. Lattice parameters are a = 8.7348(3), b = 13.0543(4), c = 19.3117(6) Å, and β = 90.179(2)°, space group P21/n.
Two bicapped trigonal prismatic sites of lead bridge and unite adjacent (311)PbS slabs. These slabs contain five distinct coordination pyramids of Sb with trapezoidal cross sections, a mixed and disordered Sb-Ag-Cd-(Pb) site, refined as 0.39 Sb + 0.61 Ag, a pure Ag site with a very open, irregular tetrahedral coordination, and an octahedral site occupied by Pb. The (311)PbS slabs contain large lone electron pair micelles formed by four distinct antimony sites in alternation with small such micelles formed by a single Sb site. The geometric arrangement of these slabs is not based on crankshaft chains of short, strong Me-S bonds but on a chess-board arrangement of (predominantly) Sb pairs that share two common S atoms via short bonds. Relationships to, and differenced from, fizelyite and uchucchacuaite are described and discussed.

layered single and double hydroxides

Stuart Mills — Tue, 05 Mar 2013 00:45:57 +0000

Ian G. Richardson (2013) The importance of proper crystal-chemical and geometrical reasoning demonstrated using layered single and double hydroxides. Acta Crystallographica, B69.

Abstract

Atomistic modelling techniques and Rietveld refinement of X-ray powder diffraction data are widely used but often result in crystal structures that are not realistic, presumably because the authors neglect to check the crystal-chemical plausibility of their structure. The purpose of this paper is to reinforce the importance and utility of proper crystal-chemical and geometrical reasoning in structural studies. It is achieved by using such reasoning to generate new yet fundamental information about layered double hydroxides (LDH), a large, much-studied family of compounds. LDH phases are derived from layered single hydroxides by the substitution of a fraction (x) of the divalent cations by trivalent. Equations are derived that enable calculation of x from the a parameter of the unit cell and vice versa, which can be expected to be of widespread utility as a sanity test for extant and future structure determinations and computer simulation studies. The phase at x = 0 is shown to be an form of divalent metal hydroxide rather than the polymorph. Crystal-chemically sensible model structures are provided for -Zn(OH)2 and Ni- and Mg-based carbonate LDH phases that have any trivalent cation and any value of x, including x = 0 [i.e. for -M(OH)2·mH2O phases].

PDF can be downloaded here: [journals.iucr.org]

Re: Amphibole group: programme for classifying microprobe or wet chemical analysis

EK Esawi — Mon, 04 Mar 2013 03:23:45 +0000

Hi Olav,

I have a copy of the program too and it works for the example with the program. However I have problems figuring out how it works with data files, the format of such files, etc.

I am planning on updating my previous program to accommodate IMA12. This is basically because I believe Excel is an overall better suited for such calculations and the fact that one still has to calculate the structural formula and readjust the ferric/ferrous ratio-a simple task but very clumsy to do via formulas in Excel, convert to text files, etc.
Thanks, EK Esawi