Chemistry Reference
In-Depth Information
Inclusions, either fluid of melts, can also contain solid minerals often
referred to as «daughter» phases. Their identification is a major aspect of
any inclusion studies, not easy in reason of their small size. Again a highly
specialized work, especially as spot analyses are commonly not possible,
because of the mobility of the solids within the enclosing liquid. Optical
characteristics are the first parameters to evaluate (see Table), as well as few
general remarks:
i Except for H
2
O-soluble species (halides) (very common in aqueous flu-
ids), many «daughter» phases a can also be s commonly found in the
groundmass, notably as mineral inclusions,
ii Many daughter phases are well crystallized, with well-developped
crystalline faces. These are sometimes sufficient to identify the mineral
species,
iii When several daughter phases are present (up to about 30 in some
topaze from Volhyn, Ukrainia!), each crystal corresponds to a differ-
ent mineral species. If they fill most of the cavity, a good trick may
be to freeze a section or a mineral grain in liquid N
2
, break it and put
the broken face as fast as possible in EDS-equiped Scanning Electrom
Microscope. Not easy, requires an extremely skilled technician, but not
impossible, as I have experienced in Amsterdam.
Most difficult inclusions to identify are high-density single phase
gazeous, like those found in high-pressure/high-temperature metamorphic
rocks (granulites/eclogites).Their content homogenize at very low tempera-
ture (less than
143°C for N
2
inclusions in eclogites) and, until this tem-
perature, they cannot be distingusih from empty cavities (holes), present in
virtually any rock section, filled with air at atmospheric pressure. Experi-
ence can be of some help, as well as very simple crushing tests under the
microscope (crushing a small grain in a droplet of oil or glycerine between
two glass plates) releases large gas bubbles.
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2.7.3 Primary versus secondary inclusions
A most important, much debated aspect of fluid inclusion studies is the
timing of inclusion formation in respect to its mineral host. Primary inclu-
sions are made at the time of the mineral growth, whearas secondary inclu-
sions may be formed much later (sometimes million years later!), in healed
microfractures. Detailed criteria are found in the inclusion literature. It will
be sufficient here to say that primary inclusions are either isolated, corre-
sponding to perturbations during the mineral growth or, e.g. in «phantom»
idiomorphic crystals, aligned along growth structures. Secundary inclusions,
on the other hand, are distinctly aligned and regularly spaced along trails,
not immediately related to the host mineral structure (except in the case of
