Triplite and Wolframite from a Greisen-Bordered Veinlet in Eurajoki, Sw Finland

Bull. Geol. Soc. Finland 41, 99—105 (1969)

TRIPLITE AND WOLFRAMITE FROM A GREISEN-BORDERED VEINLET IN EURAJOKI, SW FINLAND

ILMARI HAAPALA and PENTTI OJANPERÄ

Geological Survey of Finland, Otaniemi

ABSTRACT

Triplite and wolframite are described from a greisen-bordered veinlet associated with the local varieties of the Lai- tila rapakivi granite massif. The chemical analysis, unit cell dimensions and specific gravity are given for these minerals, and in addition the optical properties and the indexed powder diffraction data for triplite. Single-crystal studies of triplite gave a space group 12, I2/m or Im, and a pseudo space group I2/c or 1c. A comparison is made with other mineral occurrences of the same type. The hybnerite-ferberite ratio (0.29) of the wolframite is within the range generally observed for wolframites occurring in greisen veins and quartz veins.

Introduction

During the exploration work carried out by ramite, molybdenite, scheelite, gahnite, ilmenite, the Geological Survey in the summer of 1967 in rutile, arsenopyrite, pyrite, galena and stannite. the Eurajoki area, SW Finland, several sets of In connection with the diamond drillings carried greisen-bordered veins and some irregular out at a vein set between the villages of Lapijoki greisen bodies were found. The geology of the and Hankkila, also triplite was found in association area is described briefly by Kahma (1951). The with wolframite, in one greisen-bordered veinlet veins and greisen bodies are situated in the local (drill hole No. 8, depth 21.4 m). The purpose of varieties of the Laitila rapakivi granite complex. this paper is to give a mineralogical description of Most of the veins are in the so called »Tärkki this rare type of mineral occurrence. The chemi- granite» (an orthoclase-biotite-hornblende gra- cal analyses of triplite and wolframite were made nite) and some in the »Väkkärä granite» (a por- by Pentti Ojanperä, the other mineralogical phyritic microcline-biotite granite with some studies and the writing of the manuscript by topaz). The mineralogy of the greisens and Ilmari Haapala. associated fracture fillings is very variable. In addition to quartz and mica minerals (in part Li-bearing), they contain commonly chlorite, Petrography beryl, almandine, fluorite, topaz and apatite as gangue minerals. The most common ore miner- In the greisen-bordered veinlet under con- als are sphalerite, chalcopyrite, cassiterite, wolf- sideration there is a definite zonal structure 100 Ilmari Haapala and Ventti Ojanperä

altered into sericite, although all the other minerals are unaltered.

Triplite

wolframite The central veinlet of the greisen consists of almost pure triplite. Along the cracks triplite has altered to a slight degree into an extremely fluori te fine-grained opaque pigment, obviously con- triplite sisting of iron and manganese oxides. Along the chlorite vein margins triplite is replaced by apatite, which has w = 1.636 ± 0.002 and e = 1.633 ± 0.002. The apatite was also identified with an X-ray powder photograph. Apatite replacing triplite has been described by Pehrman (1943), Volborth (1954) and Fisher (1957). The colour of triplite is brown with a greasy Fig. 1. The greisen-bordered veinlet with triplite and lustre. The mineral has three cleavage directions, wolframite. Eurajoki, BH 8, depth 21.4 m. 2/3 X nat. which were identified by single-crystal X-ray studies from a cleavage fragment as {001J (good), {010} (moderate) and {l00}(poor). The optical (Fig. 1). The central part is occupied by a mas- orientation was measured from the same cleavage sive triplite veinlet, about 1 cm in width. Usually fragment with a universal stage. A thin section, in the Eurajoki area the central veins of the made from crushed triplite grains, was also used greisens consist essentially of quartz with or in the optical study. without beryl, but in this « ase these minerals are A crystal splinter of triplite was used for the lacking. A flattened wolframite crystal, about determination of the space group and the unit 2 cm X 2 cm X 1/2 cm in size, is situated in the cell dimensions. From this splinter an a-axis triplite veinlet. There are fluorite and chlorite zero-level Wrissenberg photograph was taken crystals, about 1—3 mm in diameter, as well as with Ni-filtered Cu radiation, as well as b- and some small spessartine crystals on the borders of (•-axis zero-, first- and second-level precession the triplite veinlet. On both sides on the triplite photographs using Zr-filtered Mo radiation. veinlet there is normal greisen, which grades The choice of the axes was the same as that of into unaltered granite. The total width of the Richmond (1940) and Wolfe and Heinrich (1947). greisen and the central veinlet is only about Among the general reflections (hkl) only those 5—6 cm. The chief minerals of the greisen proper with h + k + I even appeared. No other con- are quartz, mica minerals, chlorite, fluorite and ditions for the non-extinction of the reflections spessartine. The mica consists in part of green, were noticed. These data are compatible with brown or pale brown mica with high bire- space groups I 2, I2jm, and Im, in accordance fringence, in part of sericite. Chalcopyrite, with the studies of Wolfe and Heinrich, (op. cit.) wolframite and sphalerite occur in small amounts. who gave the space group I 2jm for triplite. The border between the greisen and the granite If a base-centered instead of body-centered is gradual. The most easily altered mineral in the cell is chosen, the space group is C 2, Cljm, or Tärkki granite is plagioclase, which has partly C m. The transformation matrix from the I Triplite and wolframite from a greisen-bordered veinlet ... 101

• * O • O O •t c•* O o

o o o. o. • \ • • 4 / o o o I o o o \ i . . / .

» . "Ni \ \ 4 s" . o o o w, ^ o a o

o o o o o O O O O O a*

« /«

» • •

• / *

» * Fig. 2. i-axis zero-level precession photograph of triplite from Eurajoki. Zr-filtered Mo radiation, /u = 30°. Natural size, F = 6.0 cm. The weak reflections with h and / odd are marked with circles on the upper side of the film. cell to the C cell is 101/010/001. Thus the {001} The unit cell dimensions (Table 2) were cal- cleavage of the I cell changes to {101} in the culated from the zero-level precession photo- C cell. graphs. These films were calibrated with the The reflections of the hOl type gave a reflections of an oriented silicon crystal, exposed well pronounced pseudo space group I 2jc or I c on the same films with the same instrument because the reflections with h and I odd were settings. For the base-centered cell the cell di- extremely weak. In fact they were noticed only mensions should be a0 = 13.348 A, b0 = 6.505 A, in over-exposed films (Fig. 2). The pseudo Co = 10.069 A and ß = 119°47'. No reflections space group corresponds to the space group I indicating the doubling of ba could be detected, 21 c given by Waldrop (1968) for triplite on the as is the case with mineral wagnerite in the basis of crystal structure analysis. Both the triplite group (see Strunz, 1966). The indexing studies of Wolfe and Heinrich (op. cit.) and of the powder pattern and the calculation of the Waldrop (op. cit.) referred to the triplite from interplanar spacings, given in Table 1, was based the Mica Lode pegmatite, Fremont County, on the crystallographic data obtained from the Colorado (see Appendix). single-crystal studies. The powder pattern was 102 Ilmari Haapala and Ventti Ojanperä

TABLE 1 It was not possible to completely separate the X-ray powder diffraction data for the triplite from Eura- oxide pigment from the triplite. For this reason joki. Mn-filtered Fe radiation, silicon as an internal the contents of the iron and manganese oxides standard. The 20 values smaller than 60° are given for in the analysis (Table 2) are probably slightly Ka radiation (A = 1.937 2 8 Å), those greater than 60° for too high for pure triplite. The chemical formula Kax radiation (A = 1.93597 Å) for the mineral, calculated on the basis of 5(0, F, OH), is hkl i meas. d meas. d calc.

202 4.369 (Mnx. 02, Fe" ' 0-73» Ca0.12, Fe ' o-o9> 211 10 26.02 4.303 4.304 -^§0-07) 2-03 Po-93 03.78 (F0.96, OH 0.26)X.22, 211 25 30.66 3.664 3.667 112 20 32.7 2 3.439 3.446 202 50 34.5 0 3.267 3.277 compared with the ideal formula of triplite 020 3.253 (Mn, Fe)2P04(F, OH). The compositions derived 121 60 37.10 3.045 3.048 121 2.914 from the measured specific gravity and cal- 400 2.896 culated density (ca. 40 and 48 mole per cent 402 100 39.4 5 2.870 2.870 Fe2P04F respectively) by using Winchells' 220 15 40.00 2.832 2.836 411 20 41.46 2.737 2.740 (1958) diagram are in fairly good agreement 022 10 42.20 2.691 2.697 with the chemical analysis (100-Fe/(Mn+Fe) = 222 20 43.6 2 2.607 2.609 44.5). Using Heinrich's (1951) triangular dia- 321 20 45.17 2.522 2.5 23 grams relating MnO, FeO + equivalent Fe203 204 5 45.71 2.494 2.494 411 5 47.7 5 2.392 2.398 and MgO + CaO to the refractive indices a and 413 2.368 y, the estimated a 1.674 and y 1.691 closely agree 213 2.353 with the measured values. The calculated density 114 10 48.8 7 2.342 2.341 321\ 2.309 was obtained from the analysis and cell dimen- 10 49.7 5 2.303 222/ 2.309 sions by assuming 40 (O, F, OH) per unit cell. 512 15 51.63 2.224 2.225 420 10 53.3 3 2.158 2.163 422 10 53.5 4 2.151 2.152 231 10 57.0 8 2.027 2.028 Wolframite 521 1.938 15 60.15 1.932 024 j 1.938 523 20 64.2 5 1.820 1.822 A flattened wolframite crystal, about 2 cm x 415 10 65.28 1.795 1.798 2 cm x V 2 cm in size, was situated in the triplite 521 10 66.19 1.773 1.776 veinlet. Because the crystal faces were striated 314 15 66.7 6 1.759 1.758 and rounded, no exact measurement of the faces 325 10 70.47 1.678 1.682 433 15 72.14 1.644 1.650 was possible. In addition to this large crystal 233 25 72.49 1.637 1.645 there were some very small wolframite crystals also in the greisen proper. The large wolframite crystal was used for recorded with a Philips wide-range goniometer detailed mineralogical studies. This crystal was by using Mn-filtered Fe radiation, a scanning especially suitable for chemical analysis, because speed of 1° per 4 minutes and silicon as an it had not altered to any observable degree into internal standard. The intensities of the reflec- scheelite, as the wolframites in the Eurajoki tions were measured from the heights of the greisens usually have. Slight quantities of a peaks on the chart. yellow earthy alteration product were sometimes For the chemical analysis, triplite was crushed observed on the fractures of this wolframite and hand-picked under a binocular microscope. crystal. As cleavage directions were identified Triplite and wolframite from a greisen-bordered veinlet ... 103

TABLE 2 Chemical composition and physical properties of the triplite from Eurajoki. Chemical analysis by Pentti Ojanperä

Number of ions on the basis of 40 Wt% (O, OH, F) Physical properties

Fe203 3.12 Mn 8.177 The unit cell: FeO 23.97 Fe2+ 5.877 Space group 12, I2Jm or Im MnO 32.93 Ca 0.955 16.22 (Pseudo space group I 2jc MgO 1.19 Fe3 + 0.688 or I c) CaO 3.04 Mg 0.520 aa = 12.084 ± 0.012Å

P205 29.8 9 P 7.418 ba = 6.505 ± 0.007Å H„0+ 1.07 O 30.212 Co = 10.069 ± 0.010 Å H20— 0.03 F 7.695 40. oo ß = 106°32' ± 05' F" 8.30 OH 2.092 a0:b0'. c0 = 1.858 : 1 : 1.548 3 103.54 100.01 V = 758.8 Å Sp. gr. meas- = 3.82 —O = F2 3.50 (Berman balance) 100.04 Density cale. = 3.85 g/cm3 The optical properties:

y = 1.692 |(Na h«ht) 2 Vymeas. = 85—88° 2Vycaic. = 86° Axial plane {010} ZAa^- 45°

by single-crystal X-ray studies {010} and {l00}. Before the wet chemical analysis the wolframite One cleavage fragment was used to obtain a was qualitatively tested with an X-ray fluores- f-axis rotation photograph with Ni-filtered Cu cence spectrograph by Mr. Väinö Hoffren, radiation as well as a- and £-axis zero-, first- Mag. Phil, (quantitative analysis of Nb2Os), and and second-level precession photographs with with an optical spectrograph by Mr. Arvo Löf- Zr-filtered Mo radiation. The systematic extinc- gren, Mag. Phil, (quantitative determination of tions are in agreement with the space group P Sc203, ln203, Ti02 and Sn02). According to 2\c given by Broch (1929) on the basis of crystal the chemical analysis (Table 3), the Eurajoki structure analysis. wolframite contains 75 mole per cent ferberite 2+ The unit cell dimensions a0, bQ and c0 (Table 3) (total iron as Fe ), 22 mole per cent hybnerite were calculated by the least-squares method and 2 mole per cent scheelite. The Sc203 content from the X-ray powder pattern reflections 100, (0.23 weight per cent) is somewhat greater than 110, 011, 020, 002 and 030. The powder pattern is usual in iron- rich wolframites (see Leutwein, was recorded with a Philips wide-range gonio- 1951). Yttrium and rare earths were not detected. meter by using Mn-filtered Fe radiation, scan- The relationships between the unit cell dimen- ning speed 1° per 4 minutes and silicon as an sions and the compositions of wolframites have internal standard. The 20 values used were the been studied by several authors. Obviously the averages of five records. The angle ß was meas- most accurate are the curves given by Sasaki ured from the &-axis zero-level precession photo- (1959) for synthetic wolframites. Plotting the graph. Because the powder pattern did not cell dimensions as well as the values of 2(9110— markedly differ form those given in literature 20oll and 20lxl—20^ (20„i—20xll in Sasaki, (e.g. Sasaki, 1959) for iron-rich wolframites, it op. cit.) given in Table 3 on the curves, we get is not repeated in this paper. the following mole per cents of MnWO 4 for the 104 Ilmari Haapala and Ventti Ojanperä

TABLE 3

Chemical analysis and physical properties of the wolframite from Eurajoki. Chemical analysis by Pentti Ojanperä

Number of ions on the basis Wt % of 8 (O) >) Physical properties

2+ SiOa 0.16 Fe 1.371 The unit cell: TiOa 0.08 1) Mn 0.425 a0 = 4.755 ± 0.006 A 3+ A1203 0.31 Fe 0.099 bo = 5.720 ± 0. 003 A Fe203 1.31 Ca 0.038 1.96 C0 = 4.979 ± 0. 006 A FeO 16.37 Mg 0.008 ß = 90°10' MnO 5.01 Sc 0.020 a0:b0:c0: = 0.8313 : 1 :0.8705 MgO 0.05 In 0.002 V = 135.4 A CaO 0.35 Sn O.ooo Sp.gr.meas* = 7.34 Sc203 0.23 1) W 1.961 (Berman balance) Al 0.037 3 ln203 0.0 5 !) 2.0 2 Densitycalc. = 7.3 45 g/cm Sn02 O.oi i) Nb 0.011 2@110—2©0ll = 0.85° WO, 75.57 Ti 0.006 2 0llt—2 ©in = 0.10° Nb205 0.2 5 2) O 8.000 H20+ 0.16 H20— 0.02 99.93 100.01 i) Spectrographic determination by Arvo Löfgren a) X-ray fluorescence analysis by Väinö Hoffren 3) Si02 and H20+ omitted

Eurajoki wolframite: 23 (a0), 26 (bQ), 41 (c0), mela—Somero (Mäkinen, 1913), Kemiö (Pehr- 19 (ß), 7 (20110—20oli) and 21 (26^-26^). man, 1945) and Eräjärvi (Volborth, op. cit.), The most accurate values are those derived and mentioned from Helsinki (unknown occur- from «o, bo, ß and 201U-—20^, because their rence; Laitakari, 1967). Sometimes triplite is variation range is comparatively great. The found, however, in greisens or related rocks in average of these values (22 mole per cent association with wolframite and/or cassiterite in a MnWO,) is in very good agreement with the similar way to that in Eurajoki (e.g. Heinrich, composition obtained from the chemical ana- 1951; Oelsner, 1952, pp. 49-51; Fisher, 1957). lysis. Fluorite, topaz and apatite are, however, by far the most common fluor minerals in greisens. In Finland wolframite in outcrop has been Conclusions described in only one locality (Knorring, 1955) and in glacial boulders in three localities (Laita- The greisen-bordered veinlet described is very kari, op. cit.). The composition of wolframite small and, of course, of no economic importance. varies very strongly depending on the mineral A short description of it is given, because it is paragenesis and consequently on the crystalliza- the only triplite-bearing vein of this type known tion temperature (Oelsner, op. cit.; Schröcke, in Finland. Triplite is typically a pegmatite op. cit.). Thus the ratio hybnerite: ferberite mineral, usually occurring in pegmatites in late (H/F coefficient) is greater than 1 in pegmatites, stage assemblages (e.g. Wolfe and Heinrich, and smaller than 0.05 in hydrothermal deposits 1947; Volborth, 1954). In Finland triplite has (T < 350°C). In greisen veins and quartz veins been described from the pegmatite areas Tam- (not those with pyrrhotite) which have crystal- Triplite and wolframite from a greisen-bordered veinlet... 105 lized between about 400° and 350°C the ratio is APPENDIX between 0.5 and 0.1. In the wolframite from the greisen-bordered veinlet now described, the After the manuscript went to press, one of the writ- 2+ ers (I. H.) received a letter from Mrs. Lyneve Waldrop, ratio is 0.29 (total iron as Fe ) which is in good Massachusetts Institute of Technology, giving further agreement with the diagrams of the cited authors. data on the crystallography of the Mica Lode triplite. According to Mrs. Waldrop the triplite studied by her Acknowledgements — The authors are indebted to appears to have an approximately random distribution of Mr Arvo Löfgren, Mag. Phil., for the spectrochemical Mn-Fe, Ca and Mg between the two metal sites. She analysis, to Mr. Väinö Hoffrén, Mag. Phil., for the X-ray states that with the same coordinates for all atoms, the fluorescence analysis, and to Miss Karin Dahl for drawing symmetry could be reduced to / 2 (Eurajoki triplite) if the Fig. 1. Sincere thanks are also due to Dr. Atso Vorma the metal atoms adopt ordered arrangement among sites for reviewing the manuscript. which are equivalent in I 2jc.

REFERENCES

BROCH, E. K. (1929) Untersuchungen über Kristallstruk- PEHRMAN, G. (1945) Die Granitpegmatite von Kimito turen des Wolframmittypus und des Scheelittypus. (SW-Finland) und ihre Minerale. Acta. Acad. Aboen- Vidensk.-Akad. Oslo Skr. Mat.-nat. Kl., No. 8. sis, Math. Phys. 15,2. FISHER, D. J. (1957) Isokite and triplite from Bohemia. RICHMOND, W. E. (1940) Crystal chemistry of the phos- Mineral. Mag. 31, pp. 587—602. phates, arsenates and vanadates of the type A2X04(Z). HEINRICH, E. WM. (1951) Mineralogy of triplite. Am. Am. Mineral. 25, pp. 441—479. Mineral. 36, pp. 256—271. SASAKI, A. (1959) Variation of unit cell parameters KAHMA, A. (1951) On contact phenomena of the Sata- in wolframite. Mineral. Journ. (Japan) 2, pp. 375— kunta diabase. Bull. Comm. géol. Finlande 152. 396. VON KNORRING, O. (1955) A mineralogical and geochemi- SCHRÖCKE, H. (1960) Isomorphiebeziehungen in der cal study of the metamorphic iron ores of S. W. Fin- Wolframitgruppe. Beitr. Mineral. Petrogr. 7, pp. 166 land. An unpublished Ph.D. thesis, University of —206. Leeds. WALDROP, L. (1968) Crystal structure of triplite. Natur- LAITAKARI, A. (1967) Index of Finnish minerals with wissenschaften 55, H. 4, p. 178. bibliography. Bull. Comm. géol. Finlande 230. WINCHELL, A. N. and WINCHELL, H. (1959) Elements of LEUTWEIN, F. (1951) Die Wolframitgruppe. Mineralo- optical mineralogy. II. Fourth edition. John Wiley and gisch-lagerstättenkundliche Untersuchungen. Frei- Sons, New York. berger Forsch. -H. C 3, pp. 8—19. WOLFE, C. W. and HEINRICH, E. WM (1944) Triplite MÄKINEN, E. (1913) Die Granitpegmatite von Tammela crystals from Colorado. Am. Mineral. 32, pp. 518 und ihre Minerale. Bull. Comm. géol. Finlande 35. —526. OELSNER, O. (1952) Die pegmatitisch-pneumatolytischen Lagerstätten des Erzgebirges mit Ausnahme der Kon- taktlagerstätten. Freiberger Forsch. -H. C 4. Manuscript received, September 20, 1968.

14 13902— 68