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P.2.4.1 Apatite Family-Ca 5 (PO 4) 3 (0H,F)
P.26
Apatite is the most abundant phosphorus-bearing mineral. It is a common accessory min-
eral in many types of rocks (acid to basic). Apatite can take up significant amounts of Sr
(up to 11.6 wt% SrO) and also rare earths (up to 11 wt% REE) and so may be a suitable host
for nuclear waste elements. The rare earths, predominantly Ce, may replace Ca in apatites
of alkaline igneous rocks. U + 4 (r = 0.97 A) can also substitute for Ca 2 + (r = 0.99 A).
Natural apatites have %0.01% U, if primary igneous apatite, or slightly richer; 0.02% U if
sedimentary marine apatite. Thorium is more abundant than U by a factor of 3 or 4 (Deer
1962). Apatites can contain CO 3 , SO 4 and SiO4 groups replacing P0 4 . In sedimentary phos-
phorites, the apatite can have up to 7 to 8% CO 3 content, with much lesser SO 4 or SiO4 sub-
stitution. The carbonate content of onshore phosphorites is less (3%) than that of sea
floor phosphorites, suggesting that weathering reduces the carbonate content.
In terms of geologic evidence for stability to weathering apatite is not uncommon in
sedimentary rocks where it occurs both as a detrital mineral and as a primary deposit. It
is not classified as a placer deposit-forming mineral, however. On the weathering stability
list of Pettijohn (1941), apatite has an index of 6, putting it beneath biotite and garnet.
Smithson (1941) from a study of Jurassic sandstones in Yorkshire, England, lists apatite as
stable in unweathered rock but decomposed in weathered rocks. Graham (1950) lists apatite
with olivine as least stable and Jackson (1953) puts it low in the second stage of the
weathering sequence of clay-size mineral particles. Thus, the stability of apatite has yet
to be firmly shown.
Strontium apatite results in the solid solution:
Ca 5 (P0 4) 3F -NaCeSr 3 (PO 4) 3 (OH).
fluor-apatite belovite
However, belovite is unstable under surface conditions and is readily replaced by rhabdo-
phanite, CePO 4 H 2 0; Sr and Na are then rapidly lost (Vlasov 1966). There is unlimited sub-
stitution in the systems Ca 5(PO 4) 3F-Sr 5 (PO 4 ) 3F and Ca 5 (PO 4 ) 3(OH)-Sr 5 (PO 4 ) 3 (OH). Sr-apatite,
found in alkali pegmatites, is readily soluble in acids (Vlasov 1966).
We can use the solubility criteria laid out in the introduction to this appendix to
examine the stability of apatite minerals. Although thermodynamic data for Sr-apatite are
lacking, there are data for fluorand hydroxy-apatite (Naumov et al. 1974). Using these
data, we can compute the following:
Ca 5(PO 4) 3F + 3H + - 5Ca 2+ + 3HPO + F" (P.2)
K 1 = 10- 3 3 . 3 3 x 1 01997.71/T-1/298]
Ca 5(P0 4) 3F + 6H + - 5Ca 2 + + 3H 2 PO4 + F- (P.3)
K 2 = 10- 11 . 7 0 x 102646.8[1/t-1/298]