Muscovite, KAl2[AlSi3O10](OH)2, is a common rock-forming mineral in igneous and metamorphic-rocks, sediments, hydrothermal alteration and ore deposits. The site between two adjacent T-O-T (tetrahedral-octahedral-tetrahedral) layers is shared between K and NH4 in any proportion leading to the building of the “ammonium micas”. Mica with: (i) NH4>K, □ (vacancy); (ii) Si ≥ 3 apfu (atoms per formula unit); (iii) layer charge (T-O-T) less than one, is named tobelite [Brigatti M F and Guggenheim S 2002 Rev. Mineral. Geochem. 46 1-97]. The NH4-analog of muscovite, i.e., tobelite, has been predominantly associated to two distinct geological settings: a) diagenetic to low grade metamorphic shales from meta- anthracite and anthracite coal fields; b) hydrothermal areas alteration [Ruiz Cruz M D and Sanz de Galdeano C 2010 Clays Clay Miner. 58 558-572]. In this work three crystals labelled Tob_M2, Tob_M3, Tob_3 were investigated by electron probe microanalysis (EPMA) in terms of major constituents, and in terms of nitrogen by secondary ion mass spectrometry (SIMS) in order to gain information on the presence and amount of NH4. Nitrogen was detected as secondary positive ions by means of a Cameca IMS 4f ion microprobe installed at CNR-IGG, Pavia. SIMS analysis on 14N+ was performed with 16O- primary beam at a mass resolution (M/ΔM) of ~ 1250 required to discriminate the 28Si2+ and 12CH2 + interferences at the nominal mass number 14 (a.m.u.). In spite of the severe inhomogeneity of nitrogen in each crystal, the SIMS data put Tob_M2 as the N-richest crystal of the set. The crystal, analyzed at different spots, is characterized by an ion signal in the range 399 - 560 (c/s). For Tob_M3 the 14N+ average ion signal is 91 (c/s). In Tob_3 the N content is likely the lowest in the sample set with an average count rate of 61 (c/s). The lack of calibration standards did not allow so far to obtain quantitative results for N at the ion microprobe. Nevertheless, our SIMS data agree qualitatively with constraints derived from EPMA and charge-balance crystal chemical considerations, and point out that the ion probe is a valuable tool for the investigation of N in mica minerals.
A new application of SIMS to the analysis of nitrogen in mica minerals: tobelite.
MESTO, ERNESTO
2013-01-01
Abstract
Muscovite, KAl2[AlSi3O10](OH)2, is a common rock-forming mineral in igneous and metamorphic-rocks, sediments, hydrothermal alteration and ore deposits. The site between two adjacent T-O-T (tetrahedral-octahedral-tetrahedral) layers is shared between K and NH4 in any proportion leading to the building of the “ammonium micas”. Mica with: (i) NH4>K, □ (vacancy); (ii) Si ≥ 3 apfu (atoms per formula unit); (iii) layer charge (T-O-T) less than one, is named tobelite [Brigatti M F and Guggenheim S 2002 Rev. Mineral. Geochem. 46 1-97]. The NH4-analog of muscovite, i.e., tobelite, has been predominantly associated to two distinct geological settings: a) diagenetic to low grade metamorphic shales from meta- anthracite and anthracite coal fields; b) hydrothermal areas alteration [Ruiz Cruz M D and Sanz de Galdeano C 2010 Clays Clay Miner. 58 558-572]. In this work three crystals labelled Tob_M2, Tob_M3, Tob_3 were investigated by electron probe microanalysis (EPMA) in terms of major constituents, and in terms of nitrogen by secondary ion mass spectrometry (SIMS) in order to gain information on the presence and amount of NH4. Nitrogen was detected as secondary positive ions by means of a Cameca IMS 4f ion microprobe installed at CNR-IGG, Pavia. SIMS analysis on 14N+ was performed with 16O- primary beam at a mass resolution (M/ΔM) of ~ 1250 required to discriminate the 28Si2+ and 12CH2 + interferences at the nominal mass number 14 (a.m.u.). In spite of the severe inhomogeneity of nitrogen in each crystal, the SIMS data put Tob_M2 as the N-richest crystal of the set. The crystal, analyzed at different spots, is characterized by an ion signal in the range 399 - 560 (c/s). For Tob_M3 the 14N+ average ion signal is 91 (c/s). In Tob_3 the N content is likely the lowest in the sample set with an average count rate of 61 (c/s). The lack of calibration standards did not allow so far to obtain quantitative results for N at the ion microprobe. Nevertheless, our SIMS data agree qualitatively with constraints derived from EPMA and charge-balance crystal chemical considerations, and point out that the ion probe is a valuable tool for the investigation of N in mica minerals.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
