ANOMALOUS NIR AND MÖSSBAUER SPECTRA OF HIGH-Ca PYROXENES: THE EFFECT OF MINOR

Introduction: Some terrestrial high-Ca pyroxenes (clinopyroxenes) produce anomalous near-infrared (NIR) reflectance spectra, “Type B” patterns, with absorption at about 2 um [1, 2]. This absorption band seems to indicate that Fe ion is in the silicate crystal’s larger, M2 octahedral site; Fe cannot actually be there, however, because Ca fills that site [3]! Pyroxenes are the principle phases detected by NIR spectroscopy of igneous rock whether remote or in the laboratory so the problem is of cosmochemical significance. For three well-studied angrites, meteorites containing large abundances of high-Ca pyroxenes, for example, two lack the anomalous absorption, but the third, Angra dos Reis , is of Type B [4]. Assignment of cations to crystal sites can of course be made by means of x-ray diffraction, but for low-Ca pyroxenes this task was facilitated by Mössbauer spectroscopy, an additional Fe-sensitive method [5]. Mössbauer spectroscopy applied to some 45 high-Ca pyroxenes has led to the surprising result that most NIR Type B samples show an anomalously-intense ferric-ion signature [6-9]. For one Type B sample, PYX018, detailed scanning electron microscopic (SEM) analysis suggests that the presence of minute amounts of andradite [Ca3Fe 3+ 2(SiO4)3] may be responsible [1, 9, 10]. To test the latter hypothesis we are experimentally mixing small amounts of andradite into PYX020, a Type A (“normal”) pyroxene of composition almost identical to that of PYX018. This report presents preliminary results. Experimental: PYX018 (“Diopside, San Bernardino County, California”) was kindly supplied by Dr. E. Cloutis. PYX020 (“Diopside, Horado Mines, Japan”) and andradite were obtained from Minerals Unlimited, Ridgecrest, CA. Crystals were ground in an agate mortar and passed through a 45 um sieve. Table 1 shows the chemical analysis for the pyroxenes [1]. Samples were subjected to Fe-Mössbauer spectroscopy at room temperature and sent to the RELAB at Brown University for NIR spectroscopy. Results and Discussion: Figure 1 presents Mössbauer spectra for the pure samples and one experimental mixture. Spectra for the pyroxenes are as previously presented [6], and that for andradite is consistent with literature parameters [11]. The Mössbauer spectrum for the mixture is about as expected; the NIR results should be very interesting. References: [1] Cloutis E. A. and Gaffey M. J. (1991) JGR, 96, 22809-22826. [2] Schade U. et al. (2004) Icarus, 168, 80-92. [3] e.g., Hazen R. M. and Finger L.W. (1977) EPSL, 35, 357-362. [4] Gaffey M. J. (1976) JGR, 81, 905-920; Burbine T. H. et al. (2001) LPS XXXII, Abstract #1857. [5] e.g., Dowty E. and Lindsley D. H. (1973) Am. Min., 58, 850-868. [6] Hoffman E. J. et al. (2004) LPS XXXV, Abstract #1128. [7] Hoffman E. J. and Cloutis E. A. (2004) MAPS, 39, A47 . [8] Hoffman E. J. et al. (2005) LPS XXXVI, Abstract #2096. [9] Hoffman E. J. (2005) MAPS, 40, A69. [10] Housley, R. M., personal communication. [11] e.g., Murad, E. (1984) Am. Min., 69, 722-724.