Chemistry Reference
In-Depth Information
inclusions, occurring only in magmatic rocks, it has been transformed into
a more or less devitrified glass. The overall aspect of both types is strik-
ingly similar, to the point that it might be not easy in some cases to distin-
guish between both types; moreover, the distinction is only sharp to surface,
effusive rocks (lavas). In deep-seated, plutonic varieties, hydrous melts may
recrystallize slowly to a mineral agregate
+
fluid, nothing more than a spe-
cial type of fluid inclusion.
2.7.2 Identification of the fluid/melt content
The identification of a fluid/melt inclusion is only possible if some gas/vapour
is present in the form of a bubble (e.g. biphase (liquid/vapour) fluid inclusion.
In fluid inclusion, the gas bubble is perfectly spherical, provided that it has
enough space to expand, unique (only one bubble), moving freely in a liquid.
In melt inclusions, several bubbles may be present, not systematically spheri-
cal (eventually elliptic in shape, due to magma flow), and, evidently, do not
move. The spherical shape of the bubble is characteristic for a disorganized
atomic structure (liquid or glass); inclusion shapes in minerals are always
influenced by the mineral hosts, either through zigzag-shaped irregularities
(mineral defects) or, when the inclusion content is thermodynamically in
equilibrium with its host, as spectacular «negative crystals» (C
), the shape
of the inclusion being the ideal growth form of the mineral host.
Further identification of the nature (chemical composition) of the inclu-
sion content is highly specialized work. A first indication is given by the col-
our (beware of possible influence of the colour of the host), refractive index
(related to the colour, see below) and, above all, the temperature at which the
gas bubble disappear, either by shrinking (homogenization to liquid), expan-
sion (homogenization to vapour), or sudden disappearance of the gas/liquid
limit (critical homogenization). A precise determination requires a tempera-
ture controlled microscopic stage (microthermometry), but a first indica-
tion can be given by the temperature of the room, eventually raised on the
preparation by a simple hair-dryer. If the gas bubble remains above
−
31°C,
the fluid inside can only be aqueous, of variable salinity (would be evalu-
ated by freezing). If the gas bubble disappear at exactly
+
31°C, by critical
homogenization, then the fluid can only be CO
2
of critical density (0.473 g/
cm
3
). If the gas bubble disappears at lower temperature, in the range possi-
bly reached in a lab (let us say
+
0°C), then the fluid is dominantly CO
2
, low
density if it homogenizes to vapour, high density if it homogenize to liquid.
But it may also contain other components (CH
4
and or N
2
, notably), only
evidenced by chemical analysis (Raman microspectrometry). These rough
estimates are somewhat enhanced by the colour of the different phases: The
refractive index (N) of gases is uniformly 1, resulting in a dark, almost black
colour under the microscope. Liquid CO
2
(N
>
=
1.11) is distinctly darker
than liquid H
2
O (N
=
1.33).
