Red Corundum, Sapphirine and Kornerupine from Kittilä, Finnish Lapland

RED CORUNDUM, SAPPHIRINE AND KORNERUPINE FROM KITTILÄ, FINNISH LAPLAND

ILMARI HAAPALA, JAAKKO SIIVOLA, PENTTI OJANPERÄ and VEIJO YLETYINEN

HAAPALA, I., SIIVOLA, J., OJANPERÄ, P. and YLETYINEN, V.: Red corundum, sapphirine and kornerupine from Kittilä, Finnish Lapland. Bull. Geol. Soc. Finland, 43, 221—231.

Mineral assemblages containing corundum, sapphirine and kornerupine are present in a high-grade metamorphic amphibolite-hornblendite formation near the great granulite complex of Finnish Lapland. A low-grade metamorphism has slightly retrogressed the mineral assemblages and partly destroyed corundum, sapphirine and kornerupine. Chemical analyses and physical properties including single-crystal data are presented for hornblende, gedrite, sapphirine and kornerupine.

Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen, Geological Survey of Finland, Otaniemi, Finland.

Introduction conrundun (ruby), panning was carried out on the spot. A great deal of corundum was obtained In September, 1965 the Geological Survey of by panning the soft rock weathered during pre- Finland received a small boulder containing red glacial time and now located beneath the normal corundum (Fig. 1) from a place called Paaraskalla moraine. Many of the corundum crystals were in the Kittilä wilderness in Finnish Lapland. This weakly translucent and of a pink or red colour, sample was found by a school-boy, Mikko Tervo. but no precious corundum was found. Six During short periods in 1966—68 the area was crystals were polished by Mr. Tauno Paronen, studied for economic corundum deposits, and but no star figures were visible. The amount of many tens of boulders and some outcrops con- corundum in the rock is so small and the crystals taining corundum were found. The corundum so impure that quarrying for industrial corundum occurs in strongly tectonized amphibolitic- would not be economic. Nevetheless, the occur- hornblenditic rocks. When studying the Paa- rence is interesting because of its mineralogy, raskalla samples in the laboratory, sapphirine chemistry and metamorphic facies. and kornerupine were identified. Kornerupine is The field work for this study was done by a mineral new to Finland, and sapphirine has Haapala and Yletyinen. The chemical analyses been described from only one locality (Rouhun- were made by Siivola and Ojanperä, while Haa- koski 1969). Because it was considered that the pala is responsible for other mineralog:cal and gravel at Paaraskalla could contain some precious petrological studies and the conclusions. 222 Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen

Fig. 1. The boulder sample containing red corundum and kornerupine from Paaraskalla, Kittilä. Natural size. Photo Erkki Halme.

Geology 1. Hornblende-plagioclase ^ chlorite (most common type) The corundurr, sapphirine and kornerupine occur in medium-grained metamorphic mafic- 2. Hornblende-plagioclase-corundum ^ chlo- ultramafic rocks with indistinct layered or gneiss- rite ose structure. The mineral composition of the 3. Hornblende-gedrite-sapphirine-corundum rocks is variable but in general they are amphi- (-chlorite-plagioclase) bolite or, rarely, hornblendite. The main mine- 4. Hornblende-corundum-kornerupine-sericite- rals are hornblende and plagioclase. In places clinozoisite-chlorite (only one boulder) hornblende forms phenocrysts up to several centimeters in diameter. Plagioclase is often con- Alteration products of plagioclase, and alu- centrated in thin bands parallel to the foliation minous chromite are common accessories. The and to the elongation of the complex. However, kornerupine-bearing sample also contains some plagioclase veinlets cutting the foliation have dravite. also been found. Plagioclase of the amphibolite- Corundum occurs as crystals detectable with hornblendite formation may be almost fresh the naked eye in only a few outcrops (Nos. 40, anorthite, but often it is strongly altered to 41 and 44 in Fig. 2). Microscopically, small sericite, clinozoisite and, in small amounts, to remnants of replaced corundum can be detected carbonate. The unaltered plagioclase was identi- in outcrop 3. The corundum crystals often form fied as anorthite by microprobe technique. rows of grains parallel to the foliation of the rock On the basis of thin section studies, the follow- (Fig. 3). Boulders containing corundum can be ing main mineral associations were distinguished traced over a long narrow zone parallel to the in the amphibolite- hornblendite formation: formation, part of which is visible in Fig. 2. This Red corundum, sapphirine and kornerupine from Kittilä . . . 223

25°| 30' suggests that the corundum-bearing rock forms a conformable zone in the amphibolite-hornblendite formation. Even in the richest part, the corundum content is only a few per cent. A hand specimen rich in corundum was polished and the corundum content (3.5 per cent) was determined with a planimeter. A corundum-bearing sample (type 2) taken from outcrop 41 was analysed chemically by Pentti Ojanperä. The result was: SiO, 45.09, TiOa 0.18, A1203 19.78, Cr203 0.05, Fe203 1.52, FeO 3.84, MnO 0.09, MgO 14.67, CaO 10.77, NaaO 1.29, K20 0.26, P2Oä 0.01, C02 0.00, H20+ 2.24, H20— 0.04, total 99.83. Sp. gr. 3.03. The corresponding Niggli numbers are: si 88.64, al 22.95, fm 51.58, c 22.68, alk 2.78, mg 0.87, k 0.12, ti 0.27 and p 0.01. The titanium content of this rock is much lower, and the alumina content higher, than in normal magmatic basic rocks (see e.g. Turner and Verhoogen, 1960). On the western side of the amphibolite-hornblendite formation is granite gneiss — a strongly 25 "I 30' tectonized rock with quartz, microcline, plagioclase, biotite and hornblende as the main mine- Fig. 2. A map showing the locations of corundum- bearing outcrops and boulders. 1. Granite gneiss; rals. 2. Amphibolite and hornblendite; 3. Banded hornblende In the east, the amphibolite-hornblendite for- gneiss and amphibolite; 4. Corundum in outcrop; 5. Corundum-bearing boulder; 6. Outcrop; 7. Foliation mation is in contact with a finer grained banded and lineation; 8. Number of important sample locality. amphibole gneiss-amphibolite formation. The banded structure is due to the concentration of quartz and plagioclase,and hornblende in different bands. Some quartz in these rocks occur as thin veins and is younger than the other main minerals. Hornblende is much darker than in the amphibolite-hornblendite formation. Plagioclase is commonly strongly altered. In some thin sections almost fresh andesine, and areas in which andesine is replaced by albite and clinozoisite-epidote, can be seen. Titanite, calcite and opaque minerals occur in small amounts.

Corundum Fig. 3. Corundum-bearing amphibolite, outcrop 41. The corundum crystals found in the outcrops Corundum is visible as white roundish spots, some of are generally about one centimeter or less in which are marked with a small arrow. 1/10 of natural size. Photo Ilmari Haapala. diameter, though crystals as large as 3—4 cm 224 Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen

Hornblende In general, hornblende represents the most abundant mineral of the amphibolite-hornblendite formation, though in some samples the amount of plagioclase exceeds that of hornblende. The colour of hornblende is green in a hand specimen, but in thin section the mineral is colourless. Hornblende was separated for chemical analysis from a sapphirine-bearing sample (40/IH/67) taken from outcrop 40. The physical properties of this material are given in Table 1. The unit Fig. 4. Remants of corundum in a chlorite matrix. One nicol, magn. 50 x. Photo Erkki Halme. cell dimensions were determined by the Buerger precession method using Zr-filtered Mo radiation. The film skrinkage was calibrated by in diameter can be found. By far the largest and recording silicon [111]-axis zero-level precession most striking corundum crystal is that of the photographs on the same films. The specific original boulder sample (Fig. 1). The colour is gravity was determined with Clerici solution normally pink, white or red, but under the bino- and a Westphal balance. Refractive indices were cular microscope also some deep blue corundum measured in Na light by the immersion method. grains were detected in the material crushed for A universal stage was used in measuring the optic mineral separation. At the margins, the crystals angle. The same methods were also employed in are usually replaced by colourless optically posi- determining the physical properties of gedrite, tive chlorite (Fig. 4), and sometimes also by sapphirine and kornerupine. hornblende or sericite. The replacement is often On the basis of the formula calculated from almost complete so that only scattered remnants the chemical analysis (Table 1), the mineral is with a uniform orientation remain (Fig. 4). magnesio-hornblende (Leake 1968). In the three-

TABLE 1 Chemical composition and physical properties of hornblende. Chemical analysis by Pentti Ojanperä

Number of ions on the basis Wt% of 24 (O, OH) Physical properties

Si 6.690 Si02 48.19 • Z = 8.00 The unit cell: Al[4] 1.310 Space group C2\m A1203 13.46 Al[6] 0.892' "a = 9.77 ± 0.01Å TiOä 0.20 Ti 0.021 b0 = 17.97 ±0.02A Cr203 0.17 Cr 0.019 f0 = 5.30 ± .005Å Fe203 1.18 Fe3+ 0.123 • Y = 5.09 ß = 104°58' ± 05' FeO 3.56 Fe2+ 0.413 V = 898.9Å3 MnO 0.0 9 Mn O.oii Sp.gr.meas = 3.086 ± 0.005 MgO 17.43 Mg 3.607 3 CaO 12.5 5 Ca 1.867' Dcalc. =3.09 g/cm NaaO 1.16 Na 0.312 • X = 2.21 The optical properties: = 1.628 K20 0.17 K 0.030 a 1 P205 O.oi /Scare. = l-"8 [ ± 0.002 y = 1.652 H20+ 2.14 OH 1.982 ] J • 24.00 2Va = 101 ± 3° H20— 0.04 O 22.018 100.35 100 • Mg/(?v lg + Fe2+) = 89.7 Red corundum, sapphirine and kornerupine from Kittilä . . . 225 dimensional coordinate system proposed by than in hornblende (cf. Klein 1968). The optical Whittaker (1968) for the classification of amphi- properties are in good agreement with the boles the mineral falls into the composition space diagrams for the relationships between these centered on coordinates 211. The alumina con- variables and the chemical composition of tent is remarkably high, reflecting the high antophyllite-gedrite (Seki and Yamasaki 1957). alumina content of the rock as a whole. This mineral is biaxially positive and thus differs from Sapphirine most common hornblendes. Hornblende from the kornerupine-bearing Deep blue sapphirine is found in one outcrop sample has refractive indices a 1.630 and y 1.654, (40) and in one boulder (7). In both cases, the suggesting a slightly lower Mg:Fe ratio than in rock has the mineral assemblage hornblende- the hornblende of the sapphirine-bearing sample. gedrite-sapphirine-corundum (-chlorite-anorthite). Sapphirine is typically intergrown with hornblende (Fig. 5) or corundum, and is in many Gedrite cases partially replaced by chlorite. Some larger Gedrite is found as long prismatic crystals in sapphirine crystals are repeatedly twinned on. the sapphirine-bearing rock. The crystals may From a crystal splinter of the sapphirine, r-axis be several centimeters in length. Macroscopically, zero-, first-, second-, and third-level Weissenberg the mineral is beige in colour but in thin section photographs were taken with Ni-filtered Cu it is colourless. It is very difficult to distinquish radiation, as well as b- and [101] -axis zero-, first-, gedrite from hornblende in thin section, because and second-level precession photographs with both minerals appear to be colourless. For the Zr-filtered Mo radiation. The systematic extinc- universal stage measurements the gedrite grains tions (hkl no conditions,^)/ absent if h is odd, had to be identified from a polished thin section 0/é0 absent if k is odd) agree with the require- by microprobe. ments of the space group Pl^ja, recorded by A partial chemical analysis was made from a Moore (1969) for Fiskenaesset sapphirine. The polished thin section of the sample 40/IH/67 systematic extra-extinctions recorded for sapphi- using the microprobe technique (Table 2). The rine from four localities by Fleet (1967) do not ratio Mg/(Mg + Fe) is 84.7, i.e. somewhat lower exactly fit the Kittilä material. After long expo-

TABLE 2 Chemical composition and physical properties of gedrite. Electron microprobe analysis by Jaakko Siivola

Number of cations on the Wt% basis of 23 O. Physical properties

Si02 46.6 Si 6.28] The unit cell: • 8.00 TiO, n.o. Al[4] 1.72 Space group Pnma 6 AI2Ö3 20.4 Al[ ] 1.52 a0 = 18.52 ± 0.02Å Cr203 trace Mg 4.3 8 b„ = 17.79 ±0.02Å FeO 7.0 Fe 0.79 •7.10 C„ = 5.27 ± 0.005Å MnO trace Ca 0.12 V = 1736Å3 MgO 21.8 Na 0.29 Sp.gr.m,as = 3.14 ± 0.01 CaO 0.8 The optical properties K2O trace A = 1.6311 Na20 l.l ßc,lc.= 1-641 ± 0. 002 100 Mg/(Mg + y = 1.650 97.7 2VA = 89 ± 2°

n.o. not observed 226 Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen

Sapphirine was analysed from a polished thin section of sample 40/IH/67 by microprobe (Table 3). The silica content is a little lower and the alumina content somewhat higher than in the previous sapphirine analyses (Deer, Howie and Zussman 1963, Wilson and Hudson 1967, Lutts and Kopaneva 1968, Forestier and Lasnier 1969) suggesting a pronounced coupled substitution AI, Al for Mg, Si. The asymmetric formula unit contains Mg3.38Fe2+0.i5Fe3+0.20 Alj.gjSii.jjOjo-

Kornerupine Fig. 5. Needle-like sapphirine grains (dark grey) inter- Kornerupine was recognised only in the origi- grown with hornblende. Sample from boulder No. 7. One nicol, magn. 50 x. Photo Erkki Halme. nal boulder sample found by Mikko Tervo (Fig. 1). The main minerals in this sample are hornblende, corundum, kornerupine, sericite, sures some very weak reflections incompatible clinozoisite and chlorite. Fresh plagioclase with Fleet's rules were visible on the films. When (anorthite), tourmaline (dravite) and chromite comparing the extinction rules, the different occur in smaller amounts. Kornerupine occurs choices of axes in Fleet (1967) and in the present as pale bluish green crystals up to 1 cm in dia- study were taken into account. Thus, for ex- meter. In thin section, the mineral is colourless ample, the following »forbidden» reflections and has indistinct {110} cleavage. The korne- were noticed: 602 (402), 12.312 (0312), 12.712 rupine crystals are veined and strongly replaced (0712), 312 (112), and 332 (132). The indices by chlorite and fine-grained mica (Fig. 6), and in brackets refer to the Fleet cell. the associated plagioclase grains are in general

TABLE 3 Chemical composition and physical properties of sapphirine. Electron microprobe analysis by Jaakko Siivola

Number of cations on the Wt% basis of 80 O Physical properties

SiOa 12.2 Si 5.80 The unit cell Ti02 n.o. AL 35.3 5 Space grop P2,!a AI2O3 63.1 Cr 0. 02 a0 = 11.28 3 Cr203 0.04 Fe + 0.82 ba = 14.42 • ± O.01Å 1 2 Fe203 2.3 ) Fe + 0. 60 c0 = 9.95 FeO 1. 5 1) Mg 13.53 ß = 125°28 '±05' MnO n.o. V = 1318Å J

MgO 19.1 Sp-gr.meas= 3.518 ± 0.005 CaO n.o. The optical properties: Na20 n.o. a = 1.713 K2O n.o. ß =1.716 • ± O.002 Loss y = 1.718 of ign. 0.371) 2VA = 65 ± 2° 98.61

l) Wet chemical determination by Pentti Ojanperä, n.o. not observed Red corundum, sapphirine and kornerupine from Kittilä . . . 227

completely saussurized. Idiomorphic twinned clinozoisite crystals are present and may be up to two millimeters long (Fig. 7). Remnants of strongly altered idiomorphic plagioclase crystals are common inclusions in kornerupine (Fig. 8). Unaltered plagioclase can be found only as enclosed, shielded crystals. A microprobe analysis of such a crystal showed it to be anorthite. Corundum may also occur as inclusions in kornerupine. A series of precession photographs about all the crystallographic axes were taken to determine the space group and unit cell dimensions of Fig. 6. Remnants of kornerupine in a chlorite-sericite matrix. Crossed nicols. magn. 50 x. Photo Erkki Halme. kornerupine. The conditions for non-extinction of the reflections were: hkl h + k = 2n, hQl I = 2n. These data are compatible with space groups Cmcm, Cmc21 and C2cm, conforming to the studies of Bartl (1965), McKie (1965) and Knorring et al. (1969). Moore and Bennet (1968) recorded space group Cmcm on the basis of a crystal structure analysis of the Mautia Hill kornerupine. The Kittilä kornerupine was so strongly meta- somatized that it was not possible to obtain sufficient pure material for a complete wet chemical analysis. Therefore, the main part of the analysis was made by microprobe. H20-|- was Fig. 7. Clinozoisite crystals in a chlorite-sericite matrix. determined using a Penfield tube from a small Crossed nicols, magn. 50 x. Photo Erkki Halme. amount (60 mg) of carefully hand-picked material. Iron was detemined by a wet chemical method and boron spectrochemically from material containing small amounts of alteration products as impurities. The amount of impurities was so small that their influence on the iron and boron contents is believed to be insignificant. However, the water content may be slightly high. According to the microprobe analysis, the total iron content calculated as FeO is 4.9 %. The boron content (Table 4) is somewhat lower than in other contemporary kornerupine analysis (cf. Knorring et al. 1969). Correspond- igly, the silica content is s'ightly greater than usually, suggesting an unusually low substitution Fig. 8. Plagioclase crystals in kornerupine. Completely of B for Si. Moore and Bennett (1968) suggested altered crystals on the border of the kornerupine grain on the left, shielded fresh anorthite crystal on the right. the simplified formula Mg3Al6(Si, A1)5021(0H) Crossed nicols, magn. 50 x. Photo Erkki Halme.

29 8210—71 228 Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen

TABLE 4

Chemical composition and physical properties of kornerupine. Electron probe micro- analysis by Jaakko Siivola

Number of ions on the basis Wt % of 88 (O. OH) Physical properties

SiO, 31.7 Si 15.62 The unit cell: B2O3 L.OI) B 0.85 Space group Cmcm, TiOa n.o. AI 25.27 Cmc2x or Clem AI2O3 43.5 Cr 0.13 a0 = 16.09 ± 0.02Å 3+ Cr203 0.33 Fe 0.85 b0 = 13.73 ± 0.01Å 2+ Fe2Oa 2.3 2) Fe 0.78 C0 = 6.73 ± 0.005Å FeO 1.9 2) Mn 0.04 V — 1487Å3 MnO 0.1 Mg 12.78 Sp.gr.meas= 3.266 ±0.005 MgO 17.4 Ca 0.04 Dca.c. = 3.30 g/cm» CaO 0.07 OH 4.93 The optical properties: NaaO trace O 83.07 a = 1.674 1 K2O n.o. ß = 1.686 I ± 0.002 H2O+ 1.5 2) y = 1.6 87 J 2VA = 15 ± 2° 99.80 Spectrochemical determination by Arvo Löfgren ") Wet chemical determination by Pentti Ojanperä n.o. not observed for kornerupine. The formula calculated for the composed of sapphirine-bearing metamorphosed

Kittilä kornerupine is Mg3.2Fe+20.2Al6.3Fe3+0.2 basic-ultrabasic rocks of granulite terrains. The

Si3. QBo^OgQ.^OH)^. minerals commonly associated with sapphirine A partial semiquantitative microprobe analysis are orthopyroxene, hornblende, gedrite, phlogo- of the colourless, optically positive chlorite pite, plagioclase, cordierite, spinel, corundum associated with kornerupine gave Si02 29.3, and rarely kornerupine. Recently, sapphirine has A1203 26.6, FeO 6.8 and MgO 21.9 wt %. This also been described from rocks of the skarn type indicates that the mineral is clinochlore. An ana- (Knorring 1967, Rouhunkoski 1968, p. 29). lysis of tourmaline by a similar method gave In his study on the sapphirine occurrences, Si02 35.6, A1203 35.6, FeO 2.0, MgO 10.8, Sorensen (1955) discussed the stability relations CaO 1.5—1.8, and Cr203 0.41 wt % showing of sapphirine. He concluded that sapphirine was the mineral to be dravite. Tourmaline often formed in rocks rich in alumina and magnesia occurs at the margins of the corundum crystals but poor in silica, which have been metamor- and may penetrate them as thin veinlets. This phosed under the P-T conditions corresponding suggests boron metasomatism. to high amphibolite facies or low granulite facies. His study largely conforms to the conclusions of Vogt (1947) and Ramberg (1948). Discussion According to Turner and Verhoogen, (1960) The literature dealing with the rare rock- sapphirine is formed under conditions of horn- forming mineral sapphirine is fairly comprehen- blende-granulite subfacies. Herd et al. (1969) sive. Types of sapphirine and kornerupine made a detailed study of the petrology of different occurrences and their stability relations are sapphirine-bearing rocks of the Fiskenaesset summarized by Vogt (1947), Sorensen (1955), region in West Greenland. These rocks were Deer, Howie and Zussman (1963), Herd et al. derived from layered spinel-bearing ultramafic (1969), Forestier and Lasnier (1969) and Schreyer rocks which formed a minor part of chromite- and Seifert (1969). The most common type is bearing layered anorthosite complex. The comp- Red corundum, sapphirine and kornerupine from Kittilä . . . 229 lex has undergone hornblende-granulite fades metamorphosed under at least two different P-T metamorphism followed by cordierite-amphi- conditions; (a) under high-grade amphibolite — bo'ite facies metamorphism. Herd et al. distin- low-grade granulite facies, and (b) with slight guished enstatite, pargasite, gedrite and phlogo- retrogression under greenschist facies. According pite types of sapphirine-bearing assemblages, to Winkler (1967) gedrite is not stable under which represent increasing degrees of Si, Ca, K granulite facies conditions. The sapphirine- and H,0 metasomatism related to shearing and bearing assemblage (type 3, p. 222) corresponds deformation. Herd et al. conclude (op. cit, p. 41): to a transition between the pargasite and gedrite »Sapphirine was thus formed under hornblende- types of Herd at al. (1969). granulite facies conditions, was stable in reduced Zakrutkin (1968) has shown that the compo- amounts under cordierite-amphibolite facies con- sition of hornblende depends on the meta- ditions and was stable even in relatively calcium morphic facies. Plotted on his diagram based on rich assemblages where it was able to crystallise Al[4] and Al[0] contents, the analysed hornblende under amphibolite facies conditions». Accord- from the sapphirine-bearing assemblage falls into ingly they state that sapphirine is stable under the amphibolite facies field but very near the a wide range of P-T conditions, but within a junction of the albite-apidote-amphibolite facies. limited range of chemical environments. A similar result is obtained from Zakrutkin's According to the experimental studies of diagram based on the contents of (Fe8+ + A[61 + Schreyer and Seifert (1969) of the system Ti) and (Mg + Fe2+ + Mn). The high content t6] Mg0-Al,03-Si02-H20 assemblages containing of Al suggests unusually high pressure during sapphirine, gedrite and kornerupine may be metamorphism (Kostyuk and Sobolev 1969). formed at elevated pressures and intermediate It is not clear what the rocks were before temperatures. Sapphirine, corundum and gedrite the high-grade metamorphism. The Niggli appear as stable coexisting phases in the system numbers (p. 223) suggest the formation to be Mg0-Al203-Si02-H20 at a pressure of 14 kb an »ortho-amphibolite». Compared with c and and temperature 850° (Schreyer 1968, Fig. 27). al-alk the mg number is somewhat higher than At higher temperatures (810°—910°C, depending it should be in mixtures of normal pelites or on the pressure) gedrite breaks down to enstatite- semipelites and dolomites or limestones (cf. Leake bearing assemblages. Boronfree kornerupine 1964). Also the occurrence of chromite as an appears to be stable at 8—13 kb and 850° ± 50°C. accessory mineral points to igneous origin. The Because of the greater complexity of natural large hornblende phenocrysts are possibly systems, however, the phase relations in the pseudomorphs after pyroxene. The high alumina system Mg0-Al203-Si02-H20 can be applied and magnesia contents may be derived from the only with reservations. For example, the Kittilä primary spinel-rich layers in the mafic-ultramafic material contains notable amounts of calcium, formation, as is the case in the Fiskenaesset iron and alkalies, which would change the PT- region (Herd et al. 1969). Spinel has been de- conditions. scribed from some ultramafic rocks near to the The corundum-sapphirine-kornerupine occur- granulite complex of Lapland (Mikkola and Sa- rence described in this paper is located just out- hama 1936). However, the data are insufficient side the great granulite complex of Lapland for excluding the possibility of metasedimentary (Mikkola 1937, Mikkola and Sahama 1936). Poor origin. exposures make it difficult to draw any detailed It appears that the main metamorphism took conclusions concerning the genesis and petrology place under high pressure and high temperature of the amphibolite-hornblendite formation. In conditions at or near the boundary between every case it is apparent that the complex was amphibolite and granulite facies. Greenschist 230 Ilmari Haapala, Jaakko Siivola, Pentti Ojanperä and Veijo Yletyinen facies metamorphism has slightly retrogressed In this sample unshielded plagioclase is com- the mineral assemblages. Anorthite has altered pletely replaced by sericite and clinozoisite, and in varying amounts to sericite and clinozoisite. kornerupine very strongly by chlorite. Thus, the Corundum, hornblende, sapphirine and korne- retrograde metamorphism shows a tendency to rupine are partly replaced by chlorite. The destroy corundum, sapphirine and kornerupine. retrograde metamorphism (and metasomatism) appears to have been most intense in the rocks Acknowledgements — The authors are indebted to Mr. represented by the kornerupine-bearing sample. Erkki Halme for taking the photographs.

REFERENCES

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