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Most Mineral Collectors are quite familiar with crystal specimens which enclose Inclusions of another mineral. The following are a few "classic" examples:
* Rutile in Quartz (Rutilated Quartz), Tourmaline in Quartz (Tourmalinated Quartz), Pyrite in Fluorite
* Sand in Gypsum ("Sand Rose")
* Rutile in Corundum (Star Sapphire)
General Definition of an Inclusion:Return to Top
The following general (broad) definition of an Inclusion is provided as a basic guideline only - and - will be expanded on later in the article :
(a) A crystal or fragment of one substance enclosed within another crystal; and
(b) A cavity enclosed in a crystal. Such a cavity may also contain a gas, a liquid, or a crystal; and
(c) An "optical illusion" which is enclosed within a crystal; and
A fragment of an older rock enclosed in a younger rock (the term "xenolith" is more correct in this instance).
Which Came First?Return to Top
The "usual question" that one gets asked about such specimens is on the lines of "But which came first - the inclusion or the crystal which contains it?".
Unfortunately, this is NOT always easy to answer, "Mother Nature" - who makes the rules - also likes (quite often) to confuse the issue somewhat. The "answer" as to "Which came first?" - in the simple sense - will depend on what "type" of Inclusion that we are talking about.
Basic "Types" of Inclusions :Return to Top
In the simple sense there are three basic "types" of Inclusions
and these are as follows :
(a) Antegenic Inclusions :Return to Top
These are formed before the Host Crystal.
(b) Syngenetic Inclusions :Return to Top
These are formed at the same time, as the Host Crystal.
(c) Epigenetic Inclusions :Return to Top
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,
Types of Inclusions :Return to Top
In the simple sense there are four basic types of Inclusions and these are as follows :
(a) Solid Inclusion :Return to Top
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 :Return to Top
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 :Return to Top
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 :Return to Top
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 :Return to Top
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 :Return to Top
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 :Return to Top
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:Return to Top
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 :Return to Top
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.
One explanation(?) I have been given of this (which sounds quite reasonable) is that the carbonaceous compound has been "squeezed out" of the centre of the silicate structure of the Host Mineral.
Gemmological Importance of Inclusions:Return to Top
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.
For the complete article please contact Patrick C Murphy, 59 Harvey Rd., Elizabeth Grove, South Australia, 5112 Australia or e-mail to firstname.lastname@example.org