guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Inclusions, intergrowths, alteration products
Raith, Raase & Reinhardt – February 2012
3.4 Inclusions, intergrowths, alteration products
Additional characteristics that can be used for mineral identification are, on the one hand,
inclusions which have been incorporated during crystal growth (primary inclusions) and, on the
other hand, inclusions that formed from alteration of the host mineral (secondary inclusions).
Although primary inclusions are not mineral-specific, they can give clues to the growth conditions
of the host mineral (pressure-temperature conditions; changes of compositional parameters).
Primary fluid and melt inclusions are found in minerals that have grown in the presence of a free
fluid or from a melt (Fig. 3.21 A-D, L). Large crystals in metamorphic rocks (porphyroblasts) may
contain small-sized inclusions, the orientation and distribution of which provides evidence for
timing relationships between crystal growth and deformation (Fig. 3-21 E, F, K). In micaceous
metamorphic rocks, Al-rich minerals such as staurolite (Fig. 3.21 G), garnet, andalusite and
kyanite commonly form poikiloblasts rich in quartz inclusions. In quartz-rich rocks, skeletal
crystals of these minerals can be observed (Fig. 3-21 H). Porphyroblasts with growth sectors that
advanced much faster than other sectors of the crystal display a high density of minute primary
inclusions (hourglass structure: chloritoid, andalusite; Fig. 3.21 I, J).
Secondary inclusions are, for example, intergrowths of isomorphic minerals that are the result of
unmixing from solid solutions, such as pyroxenes, amphiboles and feldspars. The unmixed phases
which are commonly lamellar to spindle-shaped show a regular, structurally controlled orientation
within the host (Figs. 3.22, 3.23 J). The unmixed phase may, however, also be irregular in shape
(e.g., dolomite in calcite; Fig. 3.22 K, L). Other commonly occurring secondary intergrowths are
formed by the precipitation of Fe- and Ti-oxides in minerals from high-temperature rocks
(pyroxene, amphibole, biotite, garnet, quartz, plagioclase; Fig. 3.23). Precipitation follows the
cooling of the rocks whereby the solubility of Ti decreases. Although the crystal structures of host
mineral and secondary phases are not isomorphic, the precipitation of Fe- and Ti-oxides may still
be structurally controlled by the host mineral.
High-grade metamorphic rocks can show reaction textures that relate to decompression, particularly
to episodes of rapid exhumation at relatively high temperatures. Commonly intergrowths of
two new minerals form at the expense of a previously stable one (symplectites: Figs. 3.24, 3.25).
Less common are fibrous intergrowths of three newly formed minerals (kelyphite: Fig. 3.24 A).
Single-phase reaction coronas form during the pseudomorphic transformation of coesite to quartz
(Fig. 3.25 I, J), from the pseudomorphic reaction of corundum to spinel (Fig. 3.25 G), or from the
hydration of periclase to brucite (Fig. 3.25 E).
Characteristic replacement textures are also generated by retrograde reactions involving hydrous
fluids. In the presence of such fluids, hydrous phases grow at the expense of less hydrous or
anhydrous minerals. The primary mineral is replaced from the surface inwards, while the reaction
proceeds also preferentially along fractures and open cleavage planes (Figs. 3.26, 3.27). During
saussuritization and sericitization of plagioclase, consumption of the anorthite component produces
fine-grained clinozoisite, zoisite and sericite, without orientation relationships with the host
crystal. (Fig. 3.27 J, K). Apart from hydration, oxidation reactions may be involved as well in
such replacement processes (Fig. 3.26 A-E, I).
A special feature are pleochroic haloes around minerals containing a significant amount of radiogenic
isotopes. The most common minerals in this group are zircon, monazite and xenotime. The
radioactive radiation emitted from these minerals affects the crystal structure of the surrounding
host minerals, and these structural defects become visible as coloured concentric haloes around
the inclusion (Fig. 3-28). Over geological time, the effect intensifies, and the minerals that carry
the radiogenic isotopes may have their own crystal structure modified if not destroyed.
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