 |
| Titel |
Synthetic and natural plagioclases: iron variations and its influence on
VNIR reflectance spectra |
| VerfasserIn |
Cristian Carli, Andrea Orlando, Daniele Borini, Gabriele Giuli, Giovanna Serventi, Giovanni Pratesi, Maria Sgavetti |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144024
|
| Publikation (Nr.) |
 EGU/EGU2017-7802.pdf |
|
|
|
|
|
| Zusammenfassung |
| Besides being one of the most important rock-forming phases, plagioclase (pl) is a common
surface mineral in several Solar System bodies. In particular, pl is present in meteorites
and lunar samples, both in lunar Highland, where it is the dominant phase, and
Maria samples. Moreover, pl has been detected in Martian meteorites, as well as in
HEDs.
In visible and near-infrared (VNIR) reflectance spectroscopy this phase is characterized
by a crystal field (C.F.) absorption band in iron-bearing samples. In particular, Burns (1993)
summarized the electronic absorptions due to iron, pointing out: 1) a broad absorption around
1.25 μm related to a C.F. transition due to Fe2+ replacing Ca2+ in seven-fold coordinated
sites; 2) narrow absorptions around 0.4 μm related to tetrahedrally coordinated Fe3+ ions
replacing Al in the tetrahedral sites.
A better understanding of the spectral properties of Fe2+-pl can be an important tool to
investigate the spectral influence of pl in regolith material in which it can be mixed with
variable amount of other components with variable abundance. This goal can be reached
working on synthetic pl with variable FeO and An contents, which must be well characterized
to be sure about the attribution of absorption bands seen in reflectance spectra, as well as
working on well characterized (in term of An, iron amount and Fe2+/Fe3+) terrestrial
pl.
Future rover mission will have onboard hyperspectral instrument working in the VNIR
with relative high spatial resolution and, so, with the possibility of measured pl crystals. For
this reason, working more in detail on iron bearing plagioclase can be an important
task.
Here, we present our results on synthetic An90 mol% pl with different iron contents (0,
0.5 and 1.0 FeO wt%) with the aim to investigate the effects of iron substitution on the VNIR
spectra of pl. Reagent-grade oxides and carbonates reactants used as starting materials were
thoroughly mixed to ensure homogeneity. Each experimental charge weighed about 200 mg.
All the experiments have been carried out in a Deltech vertical gas-mixing (CO – CO2)
quenching furnace equipped with an oxygen fugacity probe. Temperatures were in
the 1395 – 1580 ˚ C range and run duration varied from 15 min to 48 hours. The
synthesis procedure envisaged two stages: 1) preparation of a glass from the starting
material at high temperature (1550 or 1580 ˚ C); 2) annealing at lower T (1395 ˚
C).
The produced pl were grinded and sieved at about <100 μm, VNIR reflectance and
EXAFS spectra have been acquired to define the reflectance spectra and the Fe2+/Fe3+ ratio
on each experiment. Moreover, the obtained products were characterized by several
techniques such as optical microscopy, XRD, SEM, EMPA, to assess the pl-crystallinity and
its chemical composition.
Spectra collected on synthetic samples are compared with those of natural pl, and the
broad 1.25gμm C.F. absorption parameters have been related to Fe2+ abundance and
Fe2+/Fe3+ ratio.
Reference: Burns, R.G., 1993. Mineralogical Applications of Crystal Field Theory.
Cambridge University Press, 551 pp. |
|
|
|
|
|
|