Apatites - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 173

Apatites

As an accessory mineral apatite is almost always present in many varieties of magmatic
and some metamorphic and metasomatic rocks. In ultramafic rocks the apatites are
quite rarely present, more often - in gabbro rocks and very often - in the granitoids
and various rocks of heightened alkalinity. Depending on the composition of the ani-
ons the apatites are divided into two subgroups: fluorapatites (Ca 5 [PO 4 ] 3 F) and chlo-
rapatites (Ca 5 [PO 4 ] 3 Cl), and the latter might contain some amount of F, OH and CO 3
as anions. It is estimated that in fluorapatites the average contents of the following
elements are (wt%): CaO (55.5), P 2 O 5 (42.3), F (3.8), and in chlorapatites - CaO
(53.8), P 2 O 5 (41), Cl (6.8). Apatites are able to concentrate in their structure signifi-
cant amounts of REE, among which the leading role belongs to LREE. In addition to
REE there are sometimes Na, Fe, Al and some other elements presented as an impurity
[Betekhtin, 1956].

5.1 THE REE COMPOSITION OF APATITES

In general, the regularities of REE distribution in apatites are studied a bit better com-
pared to some other accessory minerals but samples from various petrographic and
genetic types of rocks are characterized unevenly by these studies. The total REE con-
tent in apatites varies widely and the chondrite-normalized contents of LREE mostly
prevail over HREE contents (Table 5.1, Figure 5.1).
Among the first samples of apatite, which were determined to contain some REE,
is the sample from gabbro of Skaergaard massif, the total REE content of which was
3000 ppm [Paster et al ., 1974]. Chondrite-normalized contents of certain elements in
it were (t.ch): La

300. The REE pattern
of the apatite was a relatively shallow negative sloping and slightly convex upward
line complicated by weak negative Eu anomaly (Figure 5.1, 1). Slightly lower total
REE content and a steep negative slope of REE patterns were determined in apatites
from hawaiites from the State of Texas (USA)

∼ 1300, Ce ∼ 1550, Nd ∼ 1800, Sm ∼ 1800, Yb ∼

∼ 2700 ppm and from basanites from

the State of Arizona (USA) 2600 ppm [Irving & Frey, 1984]. The level of La in apa-

tite from hawaiite was 2100 t.ch., from basanite - 2900 t.ch., while the level of Yb
accumulation in them was slightly lower (70-120 t.ch.) than in minerals from gab-
bro of Skaergaard massif. REE patterns of apatites from hawaiites and basanites are
almost straight lines with a steep negative slope, on which there are no Eu anomalies,

∼