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| Titel |
Unit-cell data and XRD compositional indicators for fluorapatite-chlorapatite crystalline solutions |
| VerfasserIn |
Guy Hovis, Daniel Harlov, Matthias Gottschalk, Georg Schettler |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056294
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| Zusammenfassung |
| Before any mineral series can be utilized for petrologic interpretation, it should
be well-understood chemically, structurally, and thermodynamically. The apatite
[Ca5(PO4)3(F,Cl,OH,CO3)] mineral system promises to be useful in the study of both
terrestrial and planetary systems. As a first step in the investigation of these minerals,
Schettler, Gottschalk, and Harlov (2011, American Mineralogist) reported details of the
synthesis and chemical characterization of a fluorapatite-chlorapatite solid solution series.
Enthalpies of solution and F-Cl mixing behaviour also have been investigated for these
synthetic samples (Hovis and Harlov, 2010, American Mineralogist). The former
paper reports structures and unit-cell dimensions that were characterized at the
GeoforschungZentrum-Potsdam both by Rietveld analysis of powders and single crystal
XRD. In the present work, we report unit-cell results and XRD compositional indicators
based on measurements at Lafayette College. For present work, XRD measurements were
made on powdered samples for twenty members of the same series, employing a
Scintag PAD V system, CuKα radiation, and NBS (NIST) 640a Si internal standard.
Utilizing the software of Holland and Redfern (1997, Mineralogical Magazine),
unit-cell dimensions were refined from Si-corrected machine-measured 2Î values in
conjunction with manually-identified Miller Indices. Overall, the resulting unit-cell values
based on this methodology are in excellent agreement with those of Schettler et
al. (2011). In cases where the Rietveld and single-crystal data of Schettler et al.
(2011) produced slightly different results for the same mineral sample, present data
generally agree better with the Rietveld-based results than those from the single-crystal
data.
Given the different sizes of fluorapatite and chlorapatite unit cells, there are a number X-ray
peaks that change positions significantly with F:Cl ratio, making these good compositional
indicators. The latter include the {310}, {311}, {321}, {420}, {331}, {421}, and {502}
diffraction maxima, all of which are present across the entire solid solution series and change
position by more than 1° 2Î, and in three cases by more than 1.6° 2Î, from fluorapatite to
chlorapatite. Because different peaks move at different rates with composition, any peak may
be overlapped by a second peak over some portion of compositional space, but for all of
the latter diffraction maxima this range is relatively restricted. The {311} peak is
generally free from interference by other major peaks over the entire compositional
span.
One must be cautious in the interpretation of volume behavior for this series, as Cl-rich
synthetic samples contain minor amounts (mostly in the range from 4 to 8 mol%) of an
oxyapatite component (Schettler et al., 2011). For oxyapatite-free samples that exist over
65% of the compositional range, volume-composition relationships imply positive volumes of
mixing, whether data from Rietveld, single-crystal, or the present study are utilized. However,
it will not be possible to confirm this mixing behaviour until oxyapatite-free Cl-rich samples
become available. |
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