NANOCRYSTALS of NATIVE MOLYBDENUM, IRON and TITANIUM WITHIN IMPACT GLASSES of LUNAR REGOLITH Andrey V
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14 New Data on Minerals. 2014. Vol. 49 NANOCRYSTALS OF NATIVE MOLYBDENUM, IRON AND TITANIUM WITHIN IMPACT GLASSES OF LUNAR REGOLITH Andrey V. Mokhov, Tatyana A. Gornostaeva, Pavel M. Kartashov, Enver E. Asadulin, Oleg A. Bogatikov Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences, Moscow, [email protected] This paper discusses the results of study of the impact glass fragments from the Mare Fecunditatis and Mare Crisium regolith samples brought on the Earth by the Soviet Automatic stations Luna-16 and Luna-24. Nanoinclusions of native molybdenum, iron and titanium were found in the samples with scanning and trans- mission electron microscopy (SEM, TEM). It is shown that these inclusions are single crystals. A natural high pressure native w titanium was identified for the first time. The composition and structure of glass matrix of the regolith samples indicates its extreme micro and nano heterogeneity. The probable formation mechanisms of the studied nanocrystals are discussed. 3 tables, 10 figures, 26 references. Keywords: native molybdenum, native iron, native w titanium, nanophases, single crystals, impact glasses, lunar regolith, Mare Fecunditatis, Mare Crisium. The main goal of the investigation of mi- Therefore, the aim of this study is investi- ne rals from lunar regolith (average fine lunar gation of structure and formation mecha- matter) was detetermination of the composi- nisms of metal inclusions in lunar glasses. tion and structure of microphases reflecting specific character of mineral-forming proces- Analytical techniques ses at high temperature and pressure gradient in oxygen-free and anhydrous atmosphere. Glass fragments of the regolith samples The impact events determining chemi cal and from the Mare Fecunditatis and Mare physical properties of lunar regolith made a Crisium brought to the Earth by AS Luna-16 substantial contribution to the formation of and Luna-24 were examined with transmis- these phases. However, large grains of native sion (TEM) and scanning (SEM) analytical metals have not been found on the Moon yet. electron mic roscopy using a JEM-2100 These are fine particles up to 150 µm in size microscope equip ped with an IETEM spec- (Frondel, 1975), that predetermined methods trometer INCA-250 and a JSM-5610LV scan- of our study. ning electron microscope equipped with an Particularly, native molybdenum as thin INCA-450 EDS. films and spherical submicron grains was fre- Supporting of purity and prevention of quently found in the Mare Fecunditatis and con tamination by foreign phases has been Mare Crisium regolith samples studying with regarded in the preparation of fine-dispersed SEM (Bogatikov et al., 2001; Mokhov et al., samples of lunar glasses for the SEM analy- 2007; Kartashov et al., 2010). sis. A conductive double-sided tape in whose Iron is the most abundant metal on the com position only carbon and oxygen were Moon; it occurs as oxidized and native spe - detected (within detection limits of EDS) was cies (Frondel, 1975). Irregular-shaped, rib- stuck on ordinary aluminum tables. A tape- bon-like, spherical, and drop-like particles of free surface of the table was coated with gra - native iron are the most abundant in lunar phite glue to avoid a fluorescence of alumi - regolith (Mokhov et al., 2007). num. Immediately after opening, regolith Nonvalent iron, aluminum, silicon and ti - sample was spread as a thin layer on the tanium were identified with the X-ray photo inner side of the tape with removed protec- electron spectroscopy studying condensate tive film and the table with the prepared sam- glass film coating the lunar regolith samples ple was enclosed in clear Petri dish and was from the Mare Fecunditatis (Yakovlev et al., placed in a special storage till the start of 2009). However, these data only indirectly ve - electron mic roscopic study. Thus, a contami- rify me tallic titanium in lunar glass and do not nation of sample by artifacts is eliminated as provide reliable information on its structure. far as possible. Minerals and metals already Nanocrystals of native molybdenum, iron and titanium within impact glasses of lunar regolith 15 Table 1. Compositions of glasses and pyroxene (wt.%) in samples of studied lunar regolith Analysis 12 3* 45678 Na2O 3.9±0.5 2.3±0.4 –––4.2±0.5 1.4±0.4 – MgO 1.3±0.2 2.3±0.2 6.7±0.3 14.8±0.3 4.9±0.3 2.7±0.3 – 6.21±0.3 Al2O3 7.3±0.3 24.5±0.5 18.0±0.4 – 13.0±0.4 22.8±0.5 25.6±0.6 13.6±0.4 SiO2 53.9±0.5 58.1±0.7 43.2±0.6 51.9±0.7 36.7±0.5 54.0±0.8 61.7±0.8 42.4±0.6 K2O––––0.3±0.2 3.7±0.6 –– CaO 5.8±0.2 5.2±0.3 14.6±0.4 8.2±0.3 17.4±0.4 – 11.3±0.4 14.4±0.4 TiO2 0.9±0.1 –––5.6±0.3 ––4.2±0.2 FeO 26.8±0.5 5.1±0.3 17.1±0.5 25.1±0.4 22.1±0.6 12.6±0.5 – 19.1±0.4 BaO – 2.6±0.4 –––––– Notes: Total is normalized to 100%. *– Contents of Na2O, K2O, and Cl below 0.2 wt.%. Analysts A.V. Mokhov and T.A. Gornostaeva. analyzed were prepared according to the given in Table 1 (an. 1). Secondary and back - procedure of Lapi na and Mokhov (1995) for scatte red electron images (Figs. 1a, 1b) clear- using as refe rence samples. ly show a glass coating that eliminates con- A possible contamination of the samples tamination of sample by man-made molyb- with foreign matter is the most dangerous in denum. How ever, because of glass coating, these studies and put in question these fin - the identified inclusions did not show elec- dings. It refers first of all to micro and na - tron backscattered diffraction (EBSD) pat- noparticles, which could easily contaminate terns. These patterns (Fig. 2) supporting me- a sample. We confidently can state that the tallic state of molybdenum grains (Table 2) phases studied are not impurities only if they were obtained only after ion beam cutting of were coated with glass or shallowly sank into the glass coat. it. Fre quently, these phases are sufficiently Nanocrystals of native molybdenum contrasted to be detected because back-scat- (Fig. 3), which occur as chains to form tri- tered electrons are generated and drift out of choid structures (Fig. 4) in glass or as mas- the sample from the relatively great depth. sive segregations also in glass (Fig. 5) were To demonstrate that the particles are coated identified as a result of the TEM study. In the with glass layer an imaging in secondary first case, the microdiffraction patterns show electrons is used because these electrons low intensive discrete-ring reflections of drift out of the sample from quite shallow native molybdenum, whereas in the second depth and allow obtaining information of the case, the micro diffraction patterns are absent morphological features of the sample regard- because of great thickness of the samples. less its mean atomic weight. The composition of glass is close to that Samples were prepared for TEM according determined with SEM (Table 1, an. 1), how- to suspension method of Gritsaenko (1969). ever, the higher content of silica, much lower This method is the most common be cause it concentration of Fe, and small amount of Ba is very informative and simple. A glass frag- were measured (Tab le 1, an. 2). ment nearly hundred microns or larger in Numerous micro- and nanoinclusions of size was placed into clear test mic rotube. native iron were found in the Mare Fe cun - Then, the test tube with a piece of glass in ditatis glass samples with SEM and TEM. few amount of distil analytically pure water Matrix glass containing these inclusions was placed into sonicator for disintegration. inhomogeneous even in scanning electron A sample crushed by ultrasound in water microscope is moreover inhomogeneous in drop was placed on a special cooper grid with transmission electron microscope with the film of formovar and collodion and then was composition significantly different from that dried at 35°С in drying box. de termined with SEM (Table 1, an. 3). Nu - merous fragments of this glass contain both Results spherical nanoparticles and larger irregular- shaped particles. Point TEM analysis showed As a result of the SEM study massive that these larger irregular-shaped microin- spherules of native molybdenum (Fig. 1) we - clusions are different in composition corre- re identified in glass of the Mare Crisium sponding to the rock-forming silicates (pla- rego lith samples with average composition gioclase, pyroxene, olivine), whereas nanoin- 16 New Data on Minerals. 2014. Vol. 49 1a 1b 1c Fig. 1. Native molybdenum coated with film of glass. (a) Secondary electron image, (b) backscattered electron image, (c) energy-dispersive spectrum. 0 2a 2b 2c 2d Fig. 2. A particle of native molybdenum. (a) After ion beam etching; (b) analyzed point; (c) electron backscattered diffraction patterns; (d) energy-dispersive spectrum (contains glass elements peaks induced by fluorescence). clusions are enriched only in Fe in compari- ments containing the largest inclusions of son with glass. this type were obtained to determine the Figure 6 shows the glass fragment contai - exact composition of these globules. The pat- ning large irregular-shaped inclusion whose terns obtained verified an absence of oxygen composition given in Table 1 (an. 4) corre- and appreciable contents of other elements sponds to pigeonite (Mg0.85Fe0.81Ca0.34)2(Si2O6), than iron in glo bules.