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trace of the optic axes. In dispersive minerals branches of hyperbola are
more or less open depending on the wavelength:
￿
If 2V is larger for red light than blue/(purple) light, noted r
b, there is
a red fringe in the concavity of the branches of the hyperbola, and a blue
fringe on the convexity of the hyperbola:
>
￿
In the opposite case, b noted b
>
r or more often r
<
b, the color fringes
is inverted.
2.7 FLUID AND MELT INCLUSIONS IN ROCK-
FORMING MINERALS
By Jacques Touret
Most rocks are formed or have been influenced by a fluid phase, which may
leave remnants in the form of inclusions in a number of minerals, first of all
quartz. These may be of any size, from mineral defects up to huge cavities
(liters in size!) contained in some pegmatite giant crystals. These inclusions,
sometimes mentioned under the very improper name of «bubbles», are pres-
ently the object of extremely active research. 1 They are most (not to say
only) useful in a rather restricted size range, roughly between 5 and 30
m
in diameter: smaller, they can hardly be seen under current petrographic
microscope, larger, they are likely to have suffered later perturbation.
This small size requires observation under high power objectives, com-
monly used in routine practice for convergent figures: ×25 or ×40. It must
be observed that larger inclusions (20 to 30
μ
m) are destroyed during the
preparation of standard thin sections, only 30
μ
m thick. Specialized fluid
inclusion studies are done on double-polished loose plates (wafers), whose
thickness, depending on mineral optical quality, can be
μ
m thick. These
wafers are only necessary if adapted fluid inclusion techniques (microther-
mometry, Raman microspectrometry) are to be used. These are extremely
time-consuming and require a special expertise. But before engaging in this
field of research, any petrologists should know that he can done a number of
interesting observations, which will notably indicate if this type of study is
simply possible: no need to call for the most advanced analytical instrumen-
tation, if inclusions are not there. A good alternative, which I use systemati-
cally for all reconnaissance work, is to use thin sections approximately 50%
thicker than usual, quartz polarizing in bright yellow.
>
100
μ
2.7.1 Definitions
A fluid/melt inclusion is a cavity entirely contained in a mineral host, filled
by a fluid phase at the time of the formation of the cavity. In fluid inclusion,
this filling phase has remained fluid at room temperature, whereas in melt
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