Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 85

Orthopyroxene
Plagioclase
Amphibole
Figure 1.17 The graphs of coefficients of REE the distribution between garnets and coexisting
orthopyroxenes from garnet lherzolites, and coexisting amphiboles and plagioclases from
the granulites.
initial geochemical balance, which was achieved during the formation of inner zones
of these crystals, has been upset, and therefore garnets turned out to be much poorer
in HREE [Lesnov et al ., 2005a].
Despite the fact that data on the REE distribution between garnets and coexisting
orthopyroxenes, plagioclases and amphiboles currently are of very limited amount,
they show that due to preferential accumulation of HREE in garnets, the graphs of
change of K d (garnet/orthopyroxene), K d (garnet/amphibole) and K d (garnet/plagioclase)
mainly have a positive slope, and the values of K d (garnet/orthopyroxene) for all the
elements are usually greater than 1, the values of K d (garnet/amphibole) and K d (garnet/
plagioclase) for most of the REE do not reach 1, and exceed 1 only for Yb and Lu
(Figure 1.17).
1.6 SOME CRYSTAL-CHEMICAL ASPECTS OF THE REE
ISOMORPHISM IN GARNETS
The available data on this issue indicate that the crystal structure of garnets, regard-
less of their chemical composition and genesis, is able to accumulate a higher amount
of REE, mainly of heavy elements. Parameters of their distribution in this mineral
contain very important, but not meaningful enough genetic information. In addition,
the forms of REE presence in garnets structure and the mechanisms that caused their
isomorphic occurrence to this structure, haven't been sufficiently explored as of today.
Nevertheless, already at this stage of study of this question we can assume with a
sufficient confidence that one of the determinants of preferential accumulation of
isomorphous admixture of HREE in the crystal structure of garnets are intrinsic prop-
erties of the structure, which is characterized by a relatively small size of a unit cell.
Another important factor that plays an essential role in this isomorphism is appar-
ently some of the properties of trivalent ions of HREE, namely the smaller size of their
ionic radii compared with the trivalent ions of LREE. In this regard, the opinion that
the crystal structure of garnets is the 'perfect place to find heavy lanthanides' is quite
legitimate [Pride & Muecke, 1981].