macla nº 13. septiembre ‘10 revista de la sociedad española de mineralogía 217

A visual Method for Separating Amphibole

(Kaersutite) and Mg-Clinopyroxene / TERESA UBIDE (1,*), ROEL V. ELSAS (2), JAN R. WIJBRANS (2), ENRIQUE ARRANZ (1), MARCELIANO LAGO (1), CARLOS GALÉ (3), PATRICIA LARREA (1)

(1) Universidad de Zaragoza, Facultad de Ciencias. C/ Pedro Cerbuna 12. 50009, Zaragoza (Spain) (2) Vrije University Amsterdam, Faculty of Earth and Life Sciences. De Boelelaan 1085. NL-1081 HV Amsterdam (The Netherlands) (3) Cátedra de Residuos Sudismin, Universidad de Zaragoza, Facultad de Ciencias. C/ Pedro Cerbuna 12. 50009, Zaragoza (Spain)

INTRODUCTION. very similar black, somewhat elongated using a Frantz® separator. The fraction and shiny appearance. magnetically susceptible to a current Amphibole and clinopyroxene are intensity between 410 and 500 mA was common minerals in igneous rocks. In In this paper we describe an easy and selected. It was sieved over 400 µm for alkaline rocks, their respective cheap method to optically discriminate easier hand-picking and washed with compositions correspond mainly to kaersutite and diopside-augite, applied demineralized water again. kaersutite and diopside or Mg-rich to separate these minerals in samples augite (depending on their Ca content). of Cretaceous alkaline lamprophyres The resulting fraction was studied under from the Catalonian Coastal Ranges. the stereomicroscope. It contained a Analyzing separately these minerals high proportion of black, somewhat from the same rock may be interesting THE PROBLEM. elongated, shiny crystals, so the sample for mineralogical and petrological could be easily purified hand-picking the studies (e.g. mineral chemistry, K40/Ar40 Several Cretaceous lamprophyre dykes remaining fragments of groundmass or Ar40/Ar39 dating of kaersutite). If thin (camptonite) crop out in the northern (grey) and megacrysts of olivine (yellow- section point analysis is not analytically sector of the Catalonian Coastal Ranges green) and oxides (red) with the pick- possible or economically feasible, bulk (NE Spain). They usually carry millimetre- pen. However, taking into account that analyses have to be carried out. Then, to centimetre-sized megacrysts of both kaersutite and augite-diopside mineral separation between kaersutite kaersutite, Mg-clinopyroxene, olivine were present as megacrysts in the and augite-diopside is needed. (pseudomorphosed) and oxide minerals. original samples and that their densities Unfortunately, this is not an easy task, These megacrysts define a porphyritic and magnetic susceptibilities overlap due to their similar physical properties texture, where the groundmass is (see Introduction), we could not be sure (e.g. Wysoczanski, 1993). composed of plagioclase, kaersutite, that the black crystals of the purified Mg-clinopyroxene, oxide minerals, fraction were only kaersutite, but maybe Their magnetic susceptibilities overlap, accessory acicular apatite and minor also augite-diopside. It was crucial for us so the electromagnetic separator is amounts of glass. to isolate kaersutite for Ar40/Ar39 unable to separate them. Regarding analyses. Unfortunately, the visual heavy liquid separation, the density of Some camptonite dykes where selected criteria for differentiating these minerals kaersutite ranges from 3200 to 3280 for Ar40/Ar39 dating of the kaersutite macroscopically were not applicable on kg/m3, while the densities of augite and megacrysts, which are K-rich (wt. K2O up the separated crystals. diopside are 3200-3600 and 3250- to 2%) and big enough to allow their 3550 kg/m3, respectively. Besides, separation from the groundmass. THE SOLUTION. inclusions of ilmenite and magnetite are very common in these minerals, Kaersutite megacrysts were separated The separated fraction, magnetically modifying in variable amounts their at the Mineral Separation Laboratory of susceptible between 410 and 500 mA, density values. Therefore, separation the Vrije University (Amsterdam). Each with 3200-3300 kg/m3 density, 400- with heavy liquids is neither trustworthy. sample was splitted, crushed and sieved 500 µm size, purified hand-picking the to select the 250 to 500 µm fraction, remaining groundmass and megacrysts Separation of individual grains under the which was washed with demineralized fragments, was entirely composed of stereomicroscope is another possibility, water. The LOC-50 (Liquid Overflow black, somewhat elongated, shiny although much more time consuming. Centrifuge model-50; Ijlst, 1973) was crystals. They all had the same Theoretically, kaersutite crystals are used for heavy liquid mineral separation. appearance and we could not assess if dark brown or black, long and sharp- Heavy liquids were Diiodomethane they were kaersutite or clinopyroxene. ended due to their perfect cleavage at diluted with 1,2Dichlorobenzene. In 124º-56º angles. On the contrary, Mg- order to purify the sample for kaersutite, The problem of visual recognition clinopyroxenes have a greenish hue to the floating fraction using a 3200 kg/m3 contrasted with the simplicity of black and perfect cleavage at right density heavy liquid (groundmass) and differentiating these minerals in thin angles, so they should appear as the sinking fraction using a 3300 kg/m3 section: under plane polarized light, prismatic short crystals. However, these density heavy liquid (olivine and oxide kaersutite is brown, while clinopyroxene guides are not applicable in most real minerals) were removed. The 3200- is colourless to light-green. samples, where both minerals present a 3300 kg/m3 fraction was still purified

palabras clave: separación mineral, kaersutita, clinopiroxeno. key words: mineral separation, kaersutite, clinopyroxene.

resumen SEM 2010 * corresponding author: [email protected]

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thinned rims of the crystals, we tried the the tray. Kaersutite and clinopyroxene observation with the finer fraction we should appear as brown and green had rejected because it was harder to crystals, respectively (Fig. 1c). If the hand-pick (same density and magnetic studied fraction contains a high susceptibility ranges, but 250-400 µm proportion of the desirable mineral particle size). We made the fraction (kaersutite or clinopyroxene), go even finer, sieving it under 300 µm. In ahead to the next point. If not, study this case, all the crystals were coloured, fine fractions of different densities or brown in most cases, light-green in magnetic susceptibilities to identify others (e.g. Fig. 1c). the most suitable one. • Once the right density and magnetic We carried out a series of experiments susceptibility is identified on the finest on the mentioned 250-300 µm fraction, fraction, start the hand-picking on the always poured on a transparent tray. We corresponding coarsest one (400-500 used liquids with different refractive µm), in order to isolate the desirable indexes, namely: acetone (1.359 at mineral. This step has to be also 20ºC), ethanol (1.361 at 20ºC) and carried out under a stereomicroscope ethylene glycol (1.438 at 20ºC). Water using the white surface or, preferably, (1.333 at 20ºC) was ruled out due to its under the petrographic microscope. In high surface tension, which made the any case, pour the sample on a studied fraction to float. We tried the transparent tray. To make the picking observations both under the easier, do not add any liquid. Using stereomicroscope and the petrographic high magnification objectives, pay microscope. Finally, we also tried adding attention to the thinnest areas of each a tracing paper below the transparent crystal (e.g. the rims), where the tray which contained the fraction and brown (kaersutite) or light-green the liquid, in order to disperse the light. (clinopyroxene) colours can be identified (Fig. 1a). Best results (Fig. 1c) were obtained adding acetone or ethanol to the studied If the phases to separate displayed fraction. Acetone is preferable as it small crystal sizes, finer fractions would evaporates quicker. Both acetone and have to be sieved to obtain ethanol evaporate much quicker than homogeneous mineral fragments. In ethylene glycol. The tracing paper below that case, the colour differentiation the transparent tray also helped to show would be easier (smaller size allows fig 1. Microphotographs of the 3200-3300 kg/m3 the brown or light-green colours. Finally, better transmission of light, Fig. 1c) and density and 410-500 mA current intensity magnetic the observations were clearer under the both mineral recognition and hand- susceptibility fraction on a transparent tray under petrographic microscope, although picking would be done in the same the petrographic microscope, plane polarized light. a) 400-500 µm fraction. b) 400-500 µm fraction, acceptable under the stereomicroscope. fraction, although hand-picking would be with ethanol. c) 250-300 µm fraction, with acetone harder due to the small particle size. and tracing paper; note the easily recognizable THE PROPOSED METHOD. brown and light-green colours in this photograph, in spite of its lower magnification. Our method is as simple as effective. It When dealing with kaersutite and Mg- represents a quick and cheap alternative Following this idea, we poured the clinopyroxene separation we propose to XRD or microprobe analyses when the separated fraction on a transparent tray the following methodology: separated fraction cannot be assured to and observed it both under the be clinopyroxene or kaersutite by the stereomicroscope (choosing the white • First, traditional separation has to be classical physical or optical methods. surface) and under the petrographic carried out. Split, crush and sieve the microscope with plane polarized light sample. Select the 250 to 500 µm ACKNOWLEDGEMENTS. (Fig. 1a). Some crystal rims (thinner than fraction and purify it with heavy the cores) were brown-coloured, as liquids, the electromagnetic separator This work is part of the project CGL- kaersutites are when observed in thin and/or other methods (e.g. Faultabel), 2008-06098. It is included in the scope section. A few others were identified as when necessary. Sieve it to make of the pre-doctoral grant BF106.189, clinopyroxene, due to their colourless to three fractions: 400-500 µm, 300-400 Gobierno Vasco (Teresa Ubide Garralda). light-green hue on their thin areas. µm and 250-300 µm. The former is Identification was not possible for thick easier to hand-pick, while the latter is REFERENCES. crystals without thin rims. These easier to recognize under the observations were clearer if acetone or microscope. Ijlst, L. (1973): A Laboratory Overflow- alcohol was poured within the tray, as • Study the finest fraction (250-300 Centrifuge for Heavy Liquid Mineral µm) under the stereomicroscope Separation. Amer. Miner., 58, 1088-1093. refraction of light inside the liquid Wysoczanski, R. (1993): Mineral and rock helped transmitting the light (Fig. 1b). using the white surface or, preferably, analyses of lithospheric xenoliths from under the petrographic microscope. In Marie Byrd Land, West Antartica. Victoria Given that the brown or light-green any case, pour the sample on a University of Wellington, Research School colours were only recognized in the transparent tray and add acetone. If of Earth Sciences, Geology Board of possible, locate a tracing paper below Studies Publication No. 13.