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Moncheite and palladseite of unusual chemical composition from the Miessi

River, Inari, Northern Finland

18-Aug-14

Kari K. KOJONEN, Geological Survey of FinlandAndrew M. McDONALD , Dept. of Earth Sciences, Laurentian University, CanadaChris J. STANLEY, Natural History Museum, UKBo JOHANSON, Geological Survey of Finland

05/01/202321st IMA General Meeting , Johannesburg, RSA, September 1-5, 2014

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Outline

• Introduction• General geology of the northern Lapland• Methods of study• Placer PGM in Miessi River • Optical and SEM images + EDS results• EPMA results• VHN and R measurement results• XRD results• Conclusions

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Geological map of northern Finland and the distribution of PGM bearing layered intrusions and placer deposits

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General geology

• The bedrock in Lemmenjoki River tributary is granulite, including felsic granulites and granite gneisses and intrusive mafic enderbite-norites, quartz-, hematite, quartz-feldspar porphyry and pegmatite veins

• The ages obtained of zircons are 1.95Ga for the granulite

• Intrusive layered norites and enderbites give an age of 1.905Ga with zircon age determinations

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Digital bedrock map of Northernmost Finland in 2014

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Low altitude magnetic total intensity airborne map, height 31m

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Low altitude magnetic total intensity airborne map, height 31m, Lemmenjoki River area

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Kaarreoja River, contact between granulite and weathered gabbro MgO 9.9 wt.%

Layered gabbro, lower Miessi RiverMgO 6.8 wt.%

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Layered gabbro, lower Miessi River MgO 8.1 wt.%

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Methods of study• Heavy mineral sands were sieved in the laboratory with a

sieve set from 1 mm to 40 microns opening• Coarser grains were hand picked under stereo-

microscope/macroscope• Grains with size <250 microns were sieved to several

fractions and panned iin the laboratory, and finally run with the ”gold hound” spiral separator,

• Optical microscopy; macroscope, polarisation microscope, microphotography

• SEM/EDS • EMPA at the GSF• Reflectivity and Vickers hardness measurements at NHM• XRD at the Laurentian University, Sudbury

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Optical microscopy with a macroscope and polarizing ore microscope at GTK

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Jeol variable vacuum SEM/EDS with an automatic Oxford INCA Feature analysis program for PGM grain counting

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Cameca SX100 EMP at GSF

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Reflectivity and VHN measurement at NHM in London UK

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XRD studies using the Gandolfi camera at the Laurentian university, Sudbury Canada

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Gandolfi XRD method used atthe Laurentian University, Sudbury

• XRD analysis flowsheet

Gandolfi XRD camera (two axes of rotation; improved I data).

BaFEu Image Plate (IP, shown in cross-section). Eu2+↔ Eu3+

Advantages over film:Reusable, flexible, no dark room, greater sensitivity (record weak and strong reflections), increased dynamic range, affordable.

Conversion of image to diffractograms in seconds.Can be run under vacuum.Much smaller grains (~ 20 um) can now be run.S/M and Rietveld analyses are now possible.

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Optical macroscope studies of the grains monted with double sided tape on a 30 mm diameter

brass plates as monolayer grain mount

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Optical macroscope studies of the grains monted with double sided tape on a 30 mm diameter

brass plates as monolayer grain mount

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SEM EDS studies of the grains with low vacuum mode and automatic feature analysis of the grains

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Results of the automatic SEM EDS feature analysis of the 1019 grains

Class Rank Features % total features Feature area (sq. µm) % total areaSperrylite 1 380 37.29 29100000.00 49.41Mertieite 1 10 0.98 908000.00 1.54Vysotskite 1 1 0.10 82100.00 0.14Cooperite 1 11 1.08 1050000.00 1.78Braggite 1 5 0.49 384000.00 0.65Pt-oxide 1 2 0.20 150000.00 0.25Pt Te 1 21 2.06 141000.00 0.24PdSb 1 1 0.10 842.00 0.00Au 2 14 1.37 325000.00 0.55Electrum 2 0 0.00 0.00 0.00Cassiterite 2 66 6.48 4380000.00 7.44Native Bi 2 44 4.32 2510000.00 4.26Nb-Ta-minerals 3 14 1.37 1210000.00 2.05W-minerals 3 1 0.10 382.00 0.00Fe-sulfides 3 21 2.06 65500.00 0.11Zircon 4 8 0.79 89800.00 0.15Monazite 4 27 2.65 1110000.00 1.88Th-U_oxide 4 199 19.53 12000000.00 20.37Pb-oxide, galena 4 64 6.28 2210000.00 3.75Chromite 5 17 1.67 1150000.00 1.95Titanomagnetite 5 4 0.39 3810.00 0.01Magnetite, Hematite 5 37 3.63 803000.00 1.36Ilmenite 5 6 0.59 69200.00 0.12Rutile 5 8 0.79 69900.00 0.12Arsenopyrite 5 39 3.83 38200.00 0.06Phosphates 5 19 1.86 1050000.00 1.78total 1019 100.00 58900734.00 100

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Results of the automatic SEM EDS feature analysis of 431 PGM grains

Class Features% total features

Feature area (sq. µm)

% total area

Sperrylite 380 88.17 29100000.00 91.46Mertieite 10 2.32 908000.00 2.85Vysotskite 1 0.23 82100.00 0.26Cooperite 11 2.55 1050000.00 3.30Braggite 5 1.16 384000.00 1.21Pt-oxide 2 0.46 150000.00 0.47Pt Te 21 4.87 141000.00 0.44PdSb 1 0.23 842.00 0.00total 431 100.00 31815942.00 99.98

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Results of the automatic SEM EDS feature analysis of 431 PGM grains

91.46

2.850.26

3.30 1.210.470.440.00

SperryliteMertieiteVysotskiteCooperiteBraggitePt-oxidePt TePdSb

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Manual checkup of unclassified grains with SEM/EDS

Grains in the BSE images: 1) Cu bearing isomertieite, 2) Cu and Te bearing palladseite, 3) tellurian palladseite, 4) UM2004-52 Pd10(As,Te)3 5) Se bearing braggite, 6) Se bearing moncheite. BEI by Kari Kojonen.

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Previously discovered new mineral from the same place Miessiite Pd11Te2Se2

Isotropic, color grayish white, 1 polarizer, Miessi River, optical image by K. Kojonen

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UD (undefined) minerals in polished section

Reflected light, 1 polarizer. Optical images by K. Kojonen

EDS PtSeTeEDS PdSeTe

EDS PdSeTe

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EPMA results of the UD minerals• Calculations for mineral: PdSeTe Run on date: 7-October-2013• CHEM (Formula Weight) calculations• Total Wt% = 99.06 average of 30• Formula Weight = 3048.49 • Constit Data Mole Symbol Number• Name Wt % Ratio of Atoms• Cu 1.14 0.0179 Cu 0.55• Se 35.14 0.4450 Se 13.70• Pd 57.13 0.5368 Pd 16.52• Os 0.39 0.0021 Os 0.06• Pt 0.96 0.0049 Pt 0.15• Au 0.24 0.0012 Au 0.04• Sb 0.08 0.0003 Sb 0.02• Te 4.02 0.0315 Te 0.97• Total weight % = 99.06• Total Atom Number = 32.00• Total Atomic Ratio = 1.0398• f-Factor = 30.7742• Formula Weight = 3048.49• Empirical formulae (Pd15.84Pt0.03Os0.12Cu1.17Au0.02)17.18(Se12.03S0.51Sb0.02Te1.81)14.37 According to

Louis Cabri the formulae of palladseite is Pd17Se15.

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EPMA results of the UD minerals• Calculations for mineral: PtSeTe • CHEM (Formula Weight) calculations• Total Wt% = 100.05• Formula Weight = 420.02• Constit Data Mole Symbol Number• Name Wt % Ratio of Atoms• Se 10.05 0.1273 Se 0.53• Pd 0.66 0.0062 Pd 0.03• Pt 43.92 0.2251 Pt 0.95• Ag 0.02 0.0002 Ag 0.00• Sb 0.02 0.0002 Sb 0.00• Te 45.38 0.3556 Te 1.49• • Total weight % = 100.05• Total Atom Number = 3.00• Total Atomic Ratio = 0.7146• f-Factor = 4.1981• Formula Weight = 420.02• empirical formulae (Pt0.95Pd0.03)0.98(Se0.53Te1.49)2.02, that corresponds the formulae of moncheite

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VHN values of the UD minerals

For 25g load:

PtSeTe mean (3) 101 range 83-116 sf-cv. Equivalent to Mohs ~ 3 (moncheite QDF 381 VHN 128-153)

PdTeSe 1 mean (5) 472 range 459-484 p-sf. Equivalent to Mohs ~ 5.

PdTeSe 2 mean (3) 478 range 462-489 p-sf. Equivalent to Mohs ~ 5. (palladseite QDF 409 VHN 390-437)

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Reflectance curves of the UD minerals

400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 70040

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Selenian moncheite and moncheite Ro data

selenian moncheiteQDF3.379QDF3.380QDF3.381

lambda nm

R%

CIE color values (illuminant C) x 0.312, y 0.318, Y% 59.4, λd 585, Pe% 1.5.

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Reflectance curves of the PdTeSe mineral

400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 70030

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R data for tellurian palladseite and palladseite

pal.Nm1pal.Nm2QDF3.409

lambda nm

R%

CIE color values (illuminant C) x 0.314, y 0.323, Y% 48.1, λd 572, Pe% 2.8

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XRD of the PtSeTe mineral

• The PtSeTe phase gave a moncheite pattern.• Moncheite crystallizes in the space group P33̄̄m1

and unit-cell refinement based on 18 reflections (20-125º2Θ) gives a 3.994(2) Å, c 5.233(3) Å, V 70.49 Å3, Z = 1, class 33̄̄ m, c:a=1.310, density 9.590 g/cm3(calc). The chemistry shows Te>Se, so the mineral is a Se-bearing moncheite.

• Se replacing Te in the moncheite lattice decreases the lattice size and the VHN values.

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XRD of the PdTeSe phase

• The PdTeSe has an XRD pattern of palladseite.• Palladseite crystallizes in the space group Pm3m.

The refined unit-cell edge for the calculated Te-bearing palladseite (based on 28 reflections for 32-123º2Θ) is: a 10.653(2) Å, V 1208.97 Å3, Z 2, space group Pm3m, class m3m, density 7.958 g/cm3(calc.).

• Te replacing Se in the lattice of palladseite increases the cell size, VHN and the reflectance values compared to the normal palladseite .

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Summary and conclusions

• The PGM paragenesis is rather poor on sulphur containing only a few grains of cooperite – braggite – vysotskite, which have crystallized with the early magmatic minerals. Thus, it is evident that Se and Te are replacing each other in the lattice of natural moncheite and palladseite in the Miessi River area. Both elements belong to the group 16 of the periodic system of the elements and have a similar charge and less than 15 % differing ionic radius ( Se2- 1.98Å and Te2- 2.21Å, Vaughan and Graig, 1978). Almost similar replacement of Te and Se have been recently reported (Kojonen et al. 2007) in the description of miessiite discovered nearby in the same area. Miessiite is isostructural with isomertieite that is quite common in the area. The PGM paragenesis contains both Te and Se and miessiite is an example of a Te-Se member of the isomertieite group minerals.

• The decrease in the unit cell of selenian moncheite can be explained by the increased substitution of Se for Te: in [6], radius Te2-=2.21 and Se2- = 1.98 Å, so the more Se there is, the smaller unit cell will be.

• In terms of the hardness in moncheite, an increase in Se would increase the hardness, too, as the bond distances would shorten (relatively).

• Te replacing Se in the lattice of palladseite increases the cell size, VHN and the reflectance values compared to the normal palladseite.