Search

Author(s): Barkov, AY; Bindi, L; Tamura, N; Martin, RF; Ma, C; Winkler, B; Shvedov, GI; Morgenroth, W | Abstract: Fleetite, Cu2RhIrSb2, a new species of platinum-group mineral (PGM), was discovered intergrown with an Os–Ir–Ru alloy in the Miass Placer Zone (Au–PGE), southern Urals, Russia. A single grain 50 lm across was found. Osmium, ruthenium, and iridium are the main associated minerals; also present are Pt–Fe alloys, laurite, Sb-rich irarsite, Rh-rich tolovkite, kashinite, anduoite, ferronickelplatinum, heazlewoodite, PGE-bearing pentlandite and digenite, as well as micrometric inclusions of forsterite (Fo93.7), chromite–magnesiochromite, and Mg-rich edenite. In reflected light, fleetite is light gray; it is opaque, isotropic, non-pleochroic, and non-bireflectant. We report reflectance values measured in air. A mean of seven point-analyses (wavelength-dispersive spectrometry) gave Cu 13.93, Ni 8.60, Fe 0.10, Ir 28.07, Rh 7.91, Ru 1.96, Sb 39.28, total 99.85 wt.%, corresponding to (Cu1.41Ni0.58Fe0.01)R2.00(Rh0.49Ni0.36Ru0.12)R0.97Ir0.95Sb2.08 on the basis of six atoms per formula unit, taking into account the structural results. The calculated density is 10.83 g/cm3. Single-crystal X-ray studies show that fleetite is cubic, space group Fd3m (#227), a = 11.6682(8) A, V = 1588.59(19) A3, and Z = 16. A least-squares refinement of X-ray powder-diffraction data gave a = 11.6575(5) A and V = 1584.22(19) A3. The strongest five reflections in the powder pattern [d in A(I)(hkl)] are: 6.70(75)(111), 4.13(100)(220), 3.52(30)(311), 2.380(50)(422), 2.064(40)(440). Results of synchrotron micro-Laue diffraction experiments are consistent [a ¼ 11.66(2) A]. The crystal structure of fleetite was solved and refined to R ¼ 0.0340 based upon 153 reflections with Fo . 4r(Fo). It is isotypic with Pd11Bi2Se2 and best described as intermetallic, with all metal atoms in 12-fold coordination. Fleetite and other late exotic phases were formed by reaction of the associated alloy phases with a fluid phase enriched in Sb, As, and S in circulation in the cooling ophiolite source-rock. The mineral is named after Michael E. Fleet (1938–2017) in recognition of his significant contributions to the Earth Sciences.

The major and trace element geochemistry of mafic silicate minerals from alkaline pegmatites within the 10 ring sections of the Larvik plutonic complex, Oslo rift, southern Norway, have been studied to shed light on the complex evolutionary history of the pegmatites. Pyroxene compositions plot within the aegirine-diopside-hedenbergite ternary diagram, trending from the 50-50% Di-Hd tie line towards the 100% Ae endmember. Primary magmatic diopside and hedenbergite with minor aegirine occur primarily within miaskitic to low agpaitic pegmatites, with compositions ranging from Ae 3 Di 45 Hd 51 to Ae 89 Di 5 Hd 6 , whereas aegirine and aegirine-augite are the primary magmatic clinopyroxenes in agpaitic pegmatites, with compositions ranging from Ae 31 Di 27 Hd 42 to Ae 90 Di 6 Hd 4 . Secondary clinopyroxene, after amphibole or as a hydrothermal phase within fractures and vugs, is dominantly aegirine, with compositions ranging from Ae 59 Di 16 Hd 25 to Ae 99 Di 1 Hd 0 . Primary amphibole compositions are dominantly calcic (edenite, ferro-edenite, hastingsite, magnesio-hastingsite, pargasite, and ferro-pargasite), with less common late-stage sodic-calcic amphiboles (katophorite, magnesio-katophorite, ferro-richterite, and taramite); sodic amphiboles (ferro-ferri-nyboite, ferri-nyboite) are only observed in agpaitic pegmatites. All early pegmatite mafic minerals in low agpaitic pegmatites have negative Eu anomalies (Eu/Eu* = 0.16–0.26), positive Ce anomalies, and low Nb/Ta N and Y/Ho N ratios. Sodic mafic silicates in highly agpaitic pegmatites are HREE-enriched, with Ce/Yb N = 0.22–0.58 compared to Ce/Yb N > 1 for all other primary mafic phases, are strongly depleted in MREE, and have only slightly negative Eu anomalies (Eu/Eu* = 0.66–0.75). On the basis of major and trace element geochemistry, it can be concluded that miaskitic and alkaline pegmatites in Ring Sections 1–8 within the Larvik complex have been derived from the same parental melt, whereas pegmatites in Ring Sections 9 and 10 have a distinct geochemical signature, suggesting a separate source.

Epidote-group minerals (EGM), including some of unusual chemical compositions, occur with pyrite and pyrrhotite in the Lower Paleozoic carbonaceous amphibole schist of the meta-ophiolitic Pernek Group at Rybnicek, near Pezinok, southwestern Slovakia. Euhedral to anhedral dissakisite-(La), mukhinite, and clinozoisite are associated with V- and Cr-rich garnet (goldmanite – grossular – uvarovite solid-solution), V- and Cr-rich muscovite, amphibole-group minerals (magnesiohornblende, tremolite, actinolite, and edenite), diopside, titanite, albite, quartz, siderite and sulfide minerals (pyrite, pyrrhotite, rarely chalcopyrite and sphalerite). Strong compositional zoning and three stages of EGM formation were found. The core of EGM grains consists of V- and Cr-rich dissakissite-(La) (V ≤ 0.33 apfu , Cr ≤ 0.44 apfu ); it grades into REE-rich mukhinite of varying Cr contents (REE ≤ 0.46 apfu , 0.13 to 0.43 apfu Cr). Clinozoisite I, which is V- and Cr-rich (V ≤ 0.40 apfu , Cr ≤ 0.42 apfu ), occurs as an overgrowth upon the dissakisite–mukhinite cores. A second generation of V-, Cr- and REE-poor clinozoisite (clinozoisite II) has replaced mukhinite and clinozoisite I at the grain rim, and it is also found in veins that cut the grains. Many substitutions can change the compositions of the EGM: V 3+ and Al 3+ cause the VAl −1 substitution; CrAl −1 and CrV −1 substitutions may be extensive, but both are of limited extent, and Cr 3+ is partly independent of the contents of substituents; Mg 2+ substitution reflects the REEMgCa −1 Al −1 , REEMgCa −1 V −1 , and REEMgCa −1 Cr −1 exchange-vectors. Most REE patterns show enrichment in the light REE. Chondrite-normalized REE patterns show a negative Ce anomaly and a slightly positive Eu anomaly in clinozoisite I and dissakisite-(La), but mukhinite lacks this Eu anomaly. Dissakisite-(La) most likely originated at or near the peak conditions of contact thermal metamorphism induced by the Hercynian-age intrusion of the Modra granitic massif, whereas mukhinite and clinozoisite I formed during retrogression. Clinozoisite II may reflect the younger (Alpine?) low-grade metamorphic overprint.

Minor dikes 8–70 cm thick of ankaramitic composition were emplaced late in the crystallization of the Greenhills Complex, a layered dunite – wehrlite – hornblende gabbronorite intrusion of Permian age in the volcanic arc-derived Brook Street Terrane of southern New Zealand. Mg contents range from 10 to 28 wt% MgO. The interior of a hornblende ankaramite dike at Steep Head contains phenocrysts of olivine Fo 93–82 , in some cases skeletal, and scattered phenocrysts of diopside, set in a groundmass of magnesiohornblende. The chilled margins of this dike consist almost wholly of amphibole (91%) and anorthite, An 99–94 (9%). In other dikes, crystallization of amphibole followed that of olivine, diopside, enstatite, and overlapped with plagioclase. Olivine in these dikes varies in composition from Fo 90 to Fo 72 . Diopside contains oriented blebs of magnesiohornblende and edenite. We believe that the components of these amphibole inclusions were in part introduced and in part derived from the host pyroxene by a process of reactive exsolution. Spinel varies from chromite containing 53 wt% Cr 2 O 3 through compositions at first increasingly Al-rich, and then through chromian magnetite to virtually Cr-free magnetite. Amphiboles range from magnesiohornblende to edenite, pargasite, and tschermakite, with minor late-stage high-Si magnesiohornblende. The aphyric margin of the Steep Head dike contains 17.4 wt% MgO on a volatile-free basis. Some of the melts may have exceeded this MgO content. Dike centers are more Mg-rich than margins, reaching 28.3% in one case, in part an effect of concentration of phenocrysts by flow differentiation, and in part a result of changing composition of the melt. The presence of abundant magmatic amphibole, especially in the most magnesian rocks, indicates a relatively hydrous magma. Various geothermometers suggest extensive low-temperature re-equilibration. Even where the dikes consist of almost monomineralic amphibole, chemically they are arc-ankaramites with CaO/Al 2 O 3 >1, REE contents less than twice primitive mantle, low abundances of high-field-strength elements and of incompatible elements, highly calcic plagioclase, and Cr-rich spinels with Ti, Mg, and Cr contents typical of low-K, high-Mg ankaramitic arc-tholeiitic suites. They have some chemical similarities to komatiites, but have lower Ni/MgO, Cr/MgO, and Ti/V values. The most highly magnesian dike-rocks have olivine in excess of diopside, whereas in dikes with lower Mg contents, the proportion of diopside phenocrysts exceeds that of olivine. Progressive fractionation of these two minerals lowered the bulk CaO/Al 2 O 3 ratio. Below about 10% MgO, CaO/Al 2 O 3 was no longer greater than one, the ankaramitic nature was lost, and evolution continued along an arc-tholeiitic trend. The dike compositions support the hypothesis that the Greenhills Complex is the remnant of an upper-crust magma chamber from which arc-ankaramites, tholeiitic basalts, and more evolved effusive volcanic products in the Brook Street Terrane were erupted. The highly magnesian ankaramitic dikes and their effusive analogues provide another example of an association increasingly recognized in Phanerozoic island-arc systems.

The rare Sc mineral thofieitite, (Sc,Y12Si2O7, occurs as pm- to mm-sized crystals in fluorite-bearing granitic pegmatites and the host melagabbro within the Crystal lrtoirn6in fluorite deposit ltavalti County, Montana. Thortveitite is found as colorless and clear to smok! and translucent, suLhedral to euhedral prisms up to 3 mm in length in the massive fluorite, as mm-sized anhedra to subhedra in diopside and edenite, and as pm-sized droplet-like crystals in actinolite. Micrometric textures suggest that some thorweitite exsoived from actinolite, which contains between 1.2 and 2.9 tut.Vo Sc2O3. Associated fenoan diopside contains as much as 4.8, edenite as much as 2.1, actinolite as much as 2.0, allanite as 4gch as 0.5 and titanite as much as 0.4wt.Vo Sc2O3' respectivd. The source of the Sc is believed to be magmatic, rather than from assimilation and extraction of Sc from adjacent anr,phibolite xenoliths. Indices of refraction, determined using spindle-stage techniques , arct a7.752, p 1.780, 1 1.804' all fl.m3' T-cl = 0.052. The mineral is biaxial negative, 2Vr*. = JQo, 2V"*. = 84". The empirical formula derived on the basis of 7 atoms of o*ygen from electron-microprobJand tasei-iep analysej is (Sc1.83Y0.05Fe3*o.orRq.mLz.oo(Sir.g4 .d>r.go lvhere R represeits tvtn, Ca Mg, Ti, Na Ii, Nb, Sn, Ce, Pr, Nd, Snr, Eu, Gd, Tb, Dy, {o, Er' rm' Yb, Lu, Th, u,Ht'7'r, P' Li' Be' B' v' Cr, Co, Ni, Cu, Ba Ga Ge, As, Rb, Sr, and In. Du = 3.50, Dc = 3.50 g cm-3. The mineral contains *EE = 4.63 wt.7o and is enriched in the heaw rare-eartli elements. Cell dimensions are: a 6.5304(4), b 8.5208(4)' c 4.6806(5) A' p 102.630(7)" 'V 254.1(l) N,Z=Z,spa"egroop C2lm.'tlrcwit-cellvolumeissmallerthanthatofthorweititefrommostotherlocalitiesbecauseofitsSc enrichment. Simitartj, Oe metal--oxygen distances (2.088 to 2.199 A) are smaller than those for most natural specimens of thorweitite. Keryords: thortveitite, scandium silicate, rare-earth mineral, fluorite, actinolite, diopside, granitic pegmatite, X-ray dat& crystal-structure refinement, Crystal Mountain mine, Ravalli County' Montana