71st Annual Meteoritical Society Meeting (2008) 5232.pdf

MULTIMINERAL INCLUSIONS IN THE MORASKO COARSE OCTAHEDRITE Ł. Karwowski1 and A. Muszyński2. 1 Faculty of Earth Sciences, University of Silesia, ul. Będzińska 60, 41-200 Sosnowiec, Poland; E-mail: [email protected] 2 Institute of Geology, Adam Mickiewicz University, ul. Maków Polnych 16, 60-606 Poznań, Poland; E-mail: [email protected]

Introduction: The Morasko (IAB-MG) iron is known since 1914. New specimens were found recently including the 164 kg mass found in 2006. The specimens contain irregularly scattered, mostly oval inclusions composed mainly of troilite and graphite, often rimmed with schreibersite and cohenite. Many accessory minerals were found in the inclusions including sphalerite, whitlockite [1], altaite, daubreelite, kosmochlore [2]. Analytical methods: 20 recent finds of Morasko were cut in half, sliced and 50 partslices containing inclusions were exam- ined with optical microscope. Then detailed examination was car- ried out with the CAMECA SX 100 microprobe. Results and discussion: Troilite is associated with pyrrothite and daubreelite, which forms elongated grains in troilite or grains intergrown with troilite and graphite usually in outer parts of in- clusions. There are two varieties of sphalerite: Fe- and Mn-rich (19 – 20 at% and 0.3 – 4.0 at% respectively) and Fe-poor. Rare in the schreibersite rims is djerfisherite usually adjoining troilite. Altaite (PbTe) is situated usually in the discontinuity zone be- tween troilite and the schreibersite rim, composed of cataclased mixture of troilite and graphite with tiny grains of schreibersite. Altaite was found too in tiny grains in kamacite together with schreibersite and sphalerite. Chromites are associated with phos- phates: buchwaldite, brianite and phases: Na2MgPO4 and Na(Ca0.75Mg0.25)PO4. Rutile containing Cr – 0.24 at%, Nb –0.27- 0.29 at% and Fe –0.13-0.19 at% appears in form of grains poikil- ically enclosing graphite. Kosmochlore is the most common py- roxene. Enstatites occur separately from kosmochlore, sometimes they are contacting with feldspar or phosphates. They sometimes contain numerous small inclusions of troilite or silica. The most interesting are Cr-rich Na-Ca pyroxenes we call kosmochlore- augite [3]. Sometimes grains of K-feldspar are associated with pyroxenes. Olivines are very scarce in form of tiny isolated grains in troilite. Silica (unidentified) appears quite often together with chromite and pyroxenes. The origin of pyroxenes seems to be more complex than is suggested by [4]. In our opinion two types of silicate melts occurred during pyroxene formation. The melts were rich in Na, K, Ca, Cr, Fe and phosphate ions. From the first one, chemically close to aubrites, crystallized two ensta- tites, silica and olivine. The second one gave rise to kosmochlore. When fractionated (rich in Ca, Na and Cr), it reacted with previ- ously formed enstatites leading to kosmochlore-augite. References: [1] Dominik B., 1976. Prace Mineralogiczne PAN 47, 7-53. [2] Muszyński A. et al., 2001. Pol. Tow. Mineral. Prace Spec. 18, 134-137. [3] Karwowski Ł. and Muszyński A., 2006. Mineralogia Polonica, Special Papers. 29, 140-143. [4] Benedix G.K. et al., 2000. Meteoritics and Planetary Science 35, 1127- 1141.