Isomorphous replacement

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Isomorphous replacement is the replacement of certain elements in a gemstone by another element of the same valency, without the gemstone changing in form.
The word isomorph comes from iso (same) and morph (form). Thus although the chemical makeup of the gemstone changes, the crystal form does not.

The favorite gemstone variety used to explain this phenomenon is the Garnet group.

Basic

When explaining isomorphous replacement the Garnet group is most suitable for illustration.

One can divide the Garnets into two main groups (refer to garnet for a detailed explanation).

  1. Pyralspite (Pyrope - Almandine - Spessartine)
  2. Ugrandite (Uvarovite - Grossular - Andradite)

Each member of these groups has an "ideal" chemical makeup. For instance for the Pyralspites this will be:

  • Pyrope - Magnesium Aluminum silicate
  • Almandine - Iron Aluminum silicate
  • Spessartine - Manganese Aluminum silicate

From the above it is clear that the first element mentioned changes from member to member in the pyralspites. In nature, however, one will never find a pyrope garnet with such a pure chemical makeup.
Instead during growth other elements, which are present in the melt from which the pyrope grew, were mixed in. When Iron (as seen in Almandine) was mixed in during the growth of the Pyrope Garnet, one would get a Magnesium-Iron Aluminum silicate. So part of the magnesium content in the Pyrope was replaced by iron.
It is the amount of Magnesium/Iron content that defines the distinguishing line between Pyrope and Almandine. When there is an abundance of magnesium in the chemical makeup, it is a Pyrope, when the iron is dominant, it is Almandine Garnet and somewhere in between we name it Pyrope-Almandine Garnet (with some exotic names as Rhodolite that are not of importance here).
It should be noted that the boundaries where it is a Pyrope or where it is an Almandine is under heavy debate.

Timeline: Isomorphous replacement of magnesium/iron in Pyrope-Almandine


As can be seen in the "timeline" image, with the replacement of magnesium with iron, the physical and optical properties of the gemstone change as well.
In this case (Pyrope-Almandine), the index of refraction, as well as the specific gravity, increase.

In the above example Pyrope (magnesium) and Almandine (iron) are two "end members" with a whole possible range of values in between, depending on how much magnesium or iron is present in that specific gemstone.


Pyralspite triangle


One could draw likewise "timelines" between Pyrope and Spessartine, or between Almandine and Spessartine.

From the "Pyralspite triangle" it is obvious that between the 3 "end members" of the Pyralspite group every value is possible and that physical and optical properties vary at great length.
The latter is very important to remember when testing Garnets as no value will be the same. Instead it will fall within a "range". In fact there may even be a mix of all three of the end members.

Likewise timelines and triangle can be drawn for the Ugrandite garnets and there may even be elements of the Pyralspite present in the Ugrandite garnets. The possibilities are almost limitless.

The "form" of the garnet however does not change, it remains cubic.

Note: the mixing of elements occurs at the time of formation of the gemstone, not afterwards. So the isomorphous replacement does not happen after crystallization.

Valency in isomorphous replacement

The chemical formula of Garnet is L3M2(SiO4)3 which means that the first element has a valency of 2+. Elements with the same valency can easily replace each other to form new chemical bonds as in the case of Garnet.
One should not confuse the presence of trace elements with isomorphous replacement as trace elements are not part of the "ideal" chemical makeup.

Isomorphic Gemstones

Besides Garnet there are other gemstone varieties that show isomorphic replacement:

  • Garnet
  • Spinel

Advanced

When two ions are brought together, there is a distance where a force of repulsion abruptly sets in and resists any closer approach. It is convenient to consider the ions as "spheres" in contact, and the distance between their centers is taken as the sum of the radiis of the two ions. An anion, such as an oxygen ion, have large ionic radii, where cations have radii that are much smaller.
The common cations which enter the structure of crystals are often those with similar sized radii, and can replace each other within the structure without unduly straining the space lattice. This accounts for the phenomenon of isomorphism and isomorphic replacement.
In the garnet and spinel groups, magnesia (MgO) and ferrous oxide (FeO) replace one another extensively. Likewise, alumina (Al2O3) and ferric oxide (Fe2O3) can easily replace each other within a crystal's lattice. When such a replacement takes place there can be a significant variation in chemical composition, without much effecting the internal structure. This accounts for the fact that in certain gemstones of the same species, the specific gravity and refractive indices are not constant specimen to specimen. In some species like garnet, spinel, tourmaline, and beryl the values for refractive index and specific gravity can vary within wide parameters. This is an important factor to keep in mind when testing gemstones.


Sources

  • Naming Gem Garnets - W. Wm. Hanneman, Ph.D.