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POLYTYPISM AND TWINNING IN MICAS FORM BLACK HILL (AUSTRALIA) AND ALTO PARANAIBO (BRAZIL).
No full textPolytypism in micas from Black Hill and apparent polytypism in micas from Alto Paranaibo
No full textMicas are commonly found minerals of igneous and
metamorphic rocks which form in a wide range of
chemical and physical conditions. This particular feature can be understood on the ground of the compositional
variation and different cation arrangement connected with
the intensive parameters change during the crystallization
process. Recent studies of cation site partitioning in Tirich
1M polytype from Black Hill (Australia) using a
multi-technical approach [1] showed the occurrence of
Fe, as Fe2+ and Fe3+, and Ti, as Ti3+ and Ti4+. While
Titanium populates both octahedral and tetrahedral site, Fe
is located only at the octahedral position. In addition Mg
and Fe distribution was shown to be shared out in
approximately equal amounts across M1 and M2 site.
The present study investigates the 2M1 polytype, coexisting
with 1M form, from Black Hill. Structural analysis showed
similar M1 and M2 cation distribution in both polytypes
with some noteworthy differences regarding occupancy of
the octahedral cations. More specifically, while Mg and Fe
randomly populate both M1 and M2 site in the 1M
polytype, these cations are distributed in orderly manner
in the 2M1 polytype, resulting in symmetry change from
C2/m (1M) to C2/c (2M1) as well as c parameter doubling.
This work also investigates an apparent polytype,
observed in the trioctahedral micas from Alto Paranaibo
igneous province (Southeastern Brazil). Crystal chemistry
of 1M polytype from this locality was investigated by [2].
By making use of the multitechnical approach, [3] recently
reanalysed one of the sample quoted in [2] and labelled
Ma1. A number of interesting new results were found: Na
is lodged at octahedral site, Ti at both octahedral and
tetrahedral sites and all Fe3+ at tetrahedral site and Fe2+
populating the same octahedral site. Further investigations
showed that the Ma1 can also form apparent polytypes, i.e.
twinnings of 1M polytype which simulate diffraction of
polytype patterns with long periodicity (c ≈ 30 Å).
Structure refinement of the twinned crystal (R = 3.9%,
carried out on the family reflections) indicated that the
twin law is [310], the twin index is 3 and the volume
element ratio is roughly 0.85 : 0.09 : 0.06. The same
conclusions were reached by [4] when studying ferric
phlogopites from Ruiz Peak (Northern Chile)
Studio in situ del comportamento termico di miche triottaedriche del Mt. Vulture mediante diffrazione X da cristallo singolo.
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A new application of SIMS to the analysis of nitrogen in mica minerals: tobelite.
No full textMuscovite, 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
SIMS investigation of nitrogen in tobelite. Constraints from EMPA and crystal chemical charge-balance
No full textPossibility to develope reference materials (standards) for quantitative SIMS
analysis of nitrogen in mica
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