Inclusion in Gemstones & Their Types
General Definition of an Inclusion:
(a) A crystal or fragment of one substance enclosed within another
crystal;
(b) A cavity enclosed in a crystal. Such a cavity may also contain a gas, a
liquid, or a crystal;
(c) An "optical illusion" which is enclosed within a crystal; A fragment of an older rock
enclosed in a younger rock (the term
"xenolith" is more correct in this instance).
Basic
"Types" of Inclusions :
In the simple sense there are three basic "types" of Inclusions
and these are as follows :
(a)
Antegenic Inclusions :
These are formed before the Host Crystal.
(b)
Syngenetic Inclusions :
These are formed at the same time, as the Host Crystal.
(c)
Epigenetic Inclusions :
These are formed after the Host Crystal.
Any
given Host Crystal may contain any one (or more) of these "types" of
inclusions. Determining "which type" is present is usually beyond the
scope of most Hobbyist Mineral Collectors - as it requires both !
"Specialised Knowledge" and "Specialised Equipment".
Some
crystals which one would normally expect to be transparent are opaque because
of the presence of Inclusions of one type or another. These may be of one
specific type or they may be in combinations of two or more types. A classic
example of this is "Milky Quartz" in which the milky-white colour can
be caused by any one (or more of the following Inclusions
(a) Myriads of very tiny internal cracks; and
(b) Myriads of very tiny gas bubbles; and
(d) Myriads of very tiny grains or flakes of an opaque white mineral, such as
Feldspar or Mica or Clay,
Nature
of Inclusions :
In the simple sense there are four basic nature of Inclusions and these are as
follows :
(a)
Solid Inclusion :
An enclosed "Solid Inclusion", when present, can be virtually any
Mineral Species whatsoever - including the "Host Mineral",
Normally only one such Included Species would be present, but crystals have
been observed which have contained several different species.
(b)
Liquid Inclusion :
It is by no means unusual for mineral crystals to contain internal cavities
(see "Negative Crystals" below), usually small but often quite large.
These cavities can be full (or nearly full) of a liquid.
That liquid is usually just "plain water", but it can be a saline
solution, or be liquid carbon dioxide, or even a naturally occurring
hydrocarbon compound - and - in rare instances, it can be a more
"exotic" fluid.
That included liquid, in turn, may also contain (include) a bubble of gas,
and/or a floating crystal of another mineral,
(c)
Gaseous Inclusions :
The internal cavities in crystals may be filled (or partly filled) with a gas.
This is usually "normal air" or is Carbon Dioxide. However, it can be
a "compound gas" or some "exotic" gas .
That gas may be present "in solution" in the liquid in a cavity in
the Host Crystal. In this instance it may be able to be moved - thus forming a
"natural spirit level''.
(d)
Optical Inclusions :
The "classic" example of this type of inclusion is the so-called
"Phantom Inclusion" - where one can see, within the crystal, one or
more "images" which more or less replicate the external shape of the
Host Crystal,
Each such "image" represents one stage in the growth of that crystal
(a "stop and start" process as it were).
The
Host Crystal grows to a certain stage and then stops - and - then starts to
grow again, enclosing the previous outer surfaces of that crystal. During that
"Stop & Start" process those preexisting surfaces (or part of
them) will usually receive a very fine "coating" of another substance.
They thus become "visible" (forming the "Phantom"
Inclusion) once overgrown by the Host Mineral. In some instances it is possible
to identify this "coating" material.
Minor changes in the internal structure and/or chemical composition of a
crystal can result in corresponding changes to the colour of that crystal (or
part of it). This results in another type of Optical Inclusion - that of Colour
Zoning.
Some
types of mineral crystals may contain (or may have contained) small crystals of
a radio-active mineral. The effects of that radioactivity may result in another
type of Optical Inclusion - that of the Radiation Halo.
Negative
Crystal Inclusions :
This is the general term for a cavity within a crystal. Such a cavity may be of
a completely irregular form, or it may be a regular form. If it has a regular
form then it may be possible to determine that it has a shape that can be
ascribed to one of the seven Crystal Systems..
(a) Such an "empty
cavity" is known as a "Negative Crystal" - and - it is also known as a
"Single-Phase Inclusion (as is also any "Solid "Inclusion"
on its own in a Host Crystal),
(b) Such a "Negative Crystal" containing a liquid (or a gas) on its
own (within that cavity) is known as a "Two-Phase Inclusion".
(c) When a "Negative Crystal" contains a
liquid plus a gas - or - a liquid plus a solid, then it is known as a
"Three-Phase Inclusion".
(d) One can also have a "Four-Phase
Inclusion" - this is a "Negative Crystal" plus a liquid plus a
gas plus a solid.
"Negative Crystals" are considered to have resulted from one (and/or
more) of the following situations :
(a) Growth in several directions at the same time - with some form of
"interruption" resulting in the cavity.
(b) Inhibited growth in certain regions of the crystal.
(c) Solution (or dissolving) of a preexisting crystal including part of the
Host Crystal.
In some instances it is possible to determine what species that dissolved
crystal was. One then has what is technically known as being
"Hollowmorph/Pseudomorph".
Liquid
Inclusions :
A liquid can be "trapped" within the growing crystal at any time
during its growth - or - at some later date it can infiltrate into the crystal.
In some instances it can result from the chemical dissolution of portion(s) of
the Host Crystal and/or an included mineral.
The chemical composition and even the temperature of such an included liquid
(and/or gas) can be studied - and - the information obtained can provide
invaluable details relating to the growth, etc., of that crystal and/or the
"environment" in which it grew.
Gaseous
Inclusions :
These occur in essentially the same way as for liquid inclusions - and - in
some instances one can be derived from (and/or changed to) the other as a
result of pressure and/or temperature change.
Solid
Inclusions:
The situation regarding an enclosed Solid Inclusion is, perhaps, even more
complex :
(a) It could be a preexisting crystal (floating in a solution), which has been
"entrapped" by the growing Host Crystal. This make a it an example of
an Antegenic Inclusion.
(b) It may have been growing on the wall of a cavity and was, again,
"entrapped" by the growing Host Crystal. Again, this is an example of
an Antegenic Inclusion.
(c) It can occur as a result of the crystallisation of part of the
liquid/gaseous chemical solutions "entrapped" within the Host
Crystal.
Depending on "when" this took place would determine whether it was an
Antegenic Inclusion or Syngenetic Inclusion or Epigenetic Inclusion.
(d) It can occur as a result of crystallisation or recrystallisation of portion
of the Host Crystal as a result of changes in temperature and/or pressure of
the surrounding geological environment. This would make it an example of an
Epigenetic Inclusion.
"Random"
or "Regular" Arrangement of Inclusions :
Inclusions are generally randomly (irregularly) arranged in most instances -
with no "rhyme or reason" being evident in their position within the
Host Crystal.
In some instances, however, shore is often clear evidence that the inclusions
are positioned in some definite relationship to the crystal structure of the
Host Mineral.
One of the most familiar examples of this is the mineral Andalusite variety
Chiastolite. This mineral contains carbonaceous inclusions which are regularly
arranged in the shape of a cross (or other symmetrical shape formed of small
oblongs or squares) when viewed in a transverse section of the Host Crystal.
Gemmological
Importance of Inclusions:
In the field of Gemmology the presence (or absence) of various natural
inclusions - which are frequently quite "characteristic'' of a specific
gemstone and/or locality - are regularly used to differentiate between natural
and synthetic gemstones.
In many instances they can also be used to determine "where" a
specific gemstone came from (its origin or locality).
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