Structural derivation and crystal chemistry of apatites

Abstract

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.