Some general regularities of REE distribution in the minor and accessory minerals of ultramafic, mafic and some other rocks - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 221

that are much less compatible with the structure of apatite compared to Eu 2+ ions. In
contrast to the apatite from many other rocks, in the spectra of REE patterns of the
samples from some of lherzolites, kimberlites, basanites and hawaiites the negative
Eu anomalies were not identified. This characteristic of patterns of apatite suggests
that they crystallized in a neutral or reducing environment, when a substantial part
of Eu ions was in the form of Eu 2+ ions, which are better compatible with the crystal
structure of apatite. However, the values of K d (apatite/melt) for all REE, which were
calculated based on the results of physical experiments, were usually greater than 1.
This indicates that during crystallization of apatite the REE elements had the proper-
ties of the elements that are compatible with the structure of this mineral.
Titanites are among the accessory minerals that make up so many magmatic
rocks, as well as some rocks of different origin. The structure of this mineral is able
to concentrate a very large amount of REE. Their total content ranged from tenths
fractions wt% to the first wt%. For some titanite crystals along with the zonal distri-
bution of elements such as Ca, Ti, Al and Fe a zonal REE distribution was observed,
what is more, these crystals got inversely proportional relationship between the con-
tents of REE and Ca. The chondrite-normalized REE patterns of titanite often have
a form of subhorizontal and slightly convex upward lines, indicating relatively high
concentrations of MREE. Titanite got a high chemical resistance, which is why there
is sometimes an incomplete decomposition during the preparation of rock samples for
analysis and, therefore, under determination of REE in these samples.
Perovskites. This accessory mineral is commonly represented in kimberlites,
lamproites, melilite and leucite basalts, sometimes it can be found in the rocks of
titanomagnetite and chromite deposits, in chlorite schist and meteorites. Perovskites
usually accumulate significant amounts of REE with substantial predominance of
light elements. For example, in kimberlites from South Africa and India the total REE
content ranges from 9500 to 49800 ppm. At higher modal amounts of perovskite
in the rocks the vast majority of REE in their overall balance is concentrated in this
mineral. The values of K d (perovskite/silicate melt) for LREE and MREE 1. During

the isomorphous incoming into the perovskite structure these REE had properties of
compatible elements.
Micas . As a minor mineral, mica is presented in many magmatic and metamor-
phic rocks. Total REE content in them usually ranges from the first ppm to 100 ppm,
more rarely it is slightly more. In the studied phlogopites from kimberlites, peridotites,
pyroxenites and gabbros the total REE content is generally lower than in biotites that
reside in granitoids and metamorphic rocks. Chondrite-normalized LREE contents in
micas in many cases are slightly higher than the contents of MREE and HREE. The
chondrite-normalized REE patterns of some of the studied micas have a common
negative slope and winding configuration, which is probably due to inexact determi-
nation of some elements. The patterns of biotites from granites showed positive Eu
anomalies, while the patterns of biotites from rhyolites are complicated by an intense
negative Eu anomaly.
These are the most significant features of REE distribution in the accessory and
minor minerals of ultramafic, mafic and some other types of rocks. It should be
noted that to date the REE composition of accessory minerals from rocks of mafic-
ultramafic complexes has been studied unevenly and is generally much less detailed
than the composition of rock-forming minerals.

>