Structural derivation and crystal chemistry of apatites
The crystal structures of the [A(1)2][A(2)3](BO4)3X apatites and the related compounds [A(1)2][A(2)3](BO5)3X and [A(1)2][A(2)3](BO3)3X are collated and reviewed. The structural aristotype for this family is Mn5Si3 (D88 type, P63/mcm symmetr...
| Main Authors: | , |
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| Other Authors: | |
| Format: | Journal Article |
| Language: | English |
| Published: |
2011
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| Subjects: | |
| Online Access: | https://hdl.handle.net/10356/90582 http://hdl.handle.net/10220/6899 |
| Summary: | The crystal structures of the [A(1)2][A(2)3](BO4)3X apatites
and the related compounds [A(1)2][A(2)3](BO5)3X and
[A(1)2][A(2)3](BO3)3X are collated and reviewed. The
structural aristotype for this family is Mn5Si3 (D88 type,
P63/mcm symmetry), whose cation array approximates that of
all derivatives and from which related structures arise through
the systematic insertion of anions into tetrahedral, triangular
or linear interstices. The construction of a hierarchy of spacegroups
leads to three apatite families whose high-symmetry
members are P63/m, Cmcm and P63cm. Alternatively,
systematic crystallographic changes in apatite solid-solution
series may be practically described as deviations from regular
anion nets, with particular focus on the O(1)-A(1)-O(2)
twist angle ' projected on (001) of the A(1)O6 metaprism. For
apatites that contain the same A cation, it is shown that '
decreases linearly as a function of increasing average ionic
radius of the formula unit. Large deviations from this simple
relationship may indicate departures from P63/m symmetry or
cation ordering. The inclusion of A(1)O6 metaprisms in
structure drawings is useful for comparing apatites and
condensed-apatites such as Sr5(BO3)3Br. The most common
symmetry for the 74 chemically distinct [A(1)2][A(2)3]-
(BO4)3X apatites that were surveyed was P63/m (57%), with
progressively more complex chemistries adopting P63 (21%),
P 3 (9%), P 6 (4.3%), P21/m (4.3%) and P21 (4.3%). In
chemically complex apatites, charge balance is usually
maintained through charge-coupled cation substitutions, or
through appropriate mixing of monovalent and divalent X
anions or X-site vacancies. More rarely, charge compensation
is achieved through insertion/removal of oxygen to produce
BO5 square pyramidal units (as in ReO5) or BO3 triangular
coordination (as in AsO3). Polysomatism arises through the
ordered filling of [001] BO4 tetrahedral strings to generate the
apatite±nasonite family of structures. |
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