|Chemical composition|| L3M2(SiO4)3
|Lustre||Vitreous to sub-adamantine|
Garnet is the family name given to a group of members with a common crystal habit and slightly different chemical makeup (isomorphous). The following are the 6 endmembers of the garnet group:
- Pyrope (magnesium aluminum silicate)
- Almandine (iron aluminum silicate)
- Spessartite (manganese aluminum silicate)
- Uvarovite (calcium chromium silicate)
- Grossular (calcium aluminum silicate)
- Andradite (calcium iron silicate)
In total, there are 15 members of the garnet group. In gemology, we traditionally disregard the other 9 because they do not produce gem quality minerals.
All the above members are rarely found with an ideal chemical makeup. Instead, they form an isomorphous series. Most gem quality garnets belong to either of the following 5 isomorphous series [Hanneman,2000] and their chemical composition is an intermediate between the two endmembers mentioned.
According the whether the L or the M component in the chemical composition of the species is constant, we can divide the members of the garnet family into two groups.
- Pyralspites (Pyrope, Almandine, Spessartite)
- Ugrandites (Uvarovite, Grossular, Andradite)
Classification of gem garnets
No other gemstone gives rise to so much controversy as the species of the garnet group.
The garnet group consists mainly of isomorphous series with end members that never occur in its pure form in nature. This makes it almost impossible to assign definite values of physical and optical properties to each species.
The major gemological institutes (GIA and Gem-A) as well as the Mineralogical Society seem to be in disagreement about when a garnet should be named a pyrope, an almandine or a pyrope-almandine.
Traditionally, mineralogists use the 50%-50% rule. If there is over 50% of pyrope in the chemical composition, it will be a pyrope and vice versa. They do not recognize the intermediate values of the isomorphous series. It is either a pyrope or an almandine, never a pyrope-almandine [Hanneman, 2000]. In gemology, we do accept the latter.
The physical and optical properties of the members of the garnet group are therefore not to be taken too literally until a clear, unified system of naming gem garnets is accepted worldwide.
The physical and optical properties given are not definite values; they overlap.
Specific gravity is in general not regarded as a primary means of separation between species of the garnet group. The combination of color (eye and spectroscopy) with RI however is.
The table below gives the refractive indices taught currently (2006) by the two major gemological institutes compared to Dr. Hanneman's unified system of classifying garnets.
|Refractive indices of gem garnets|
|* depending on isomorphous series|
An indepth study on garnets has been carried out by Carol M. Stockton and Dr. D. Vincent Manson at the GIA laboratory in the 1980's which resulted in a final paper on the classification of gem garnets in 1985. In this final paper gem quality garnets were divided into 8 species according to chemical and physical properties, viz. grossular, andradite, pyrope, pyrope-almandine, almandine, almandine-spessartine, spessartine and pyrope-spessartine.
Qualifications were made on color, refractive index and spectral analyses (supported by chemical analyses).
|Refractive indices according to Stockton and Manson|
|Grossular||1.730-1.760||Green through reddish-orange, colorless|
|Andradite||1.880-1.895||Very slightly yellowish green through orangy yellow|
|Pyrope||1.714-1.742||Purplish red through reddish orange, colorless|
|Pyrope-almandine||1.742-1.785||reddish orange through red-purple|
|Almandine||1.785-1.830||Orange red through purplish red|
|Almandine-spessartine||1.810-1.820||Reddish orange through orange-red|
|Spessartine||1.780-1.810||Yellowish orange through reddish orange|
|Pyrope-spessartine||1.742-1.780||Greenish yellow through purple|
During the study, Stockton and Manson published 4 earlier, intermediate, articles in Gems & Gemology which grabbed the attention of Dr. Hanneman. Although the Stocktom/Manson papers brought great new insights in the classification of garnets in gemology, Hanneman proposed a system that is perhaps more easily understood by gemologists.
It should be noted that any classification system is under debate and the reader should make the decision on which system is most appropiate/logical.
Dr. Hanneman believes the classification of garnets should be based on the 30-70% rule instead of the 50-50% rule mineralogists use. This system is similar to that used for plagioclase feldspar, with the note that garnets can form series with all (or most) members of the garnet group instead of a static system between two end members.
As the differences between two end members differ, so will the 30% and 70% of each "timeline", hence lowering or raising the values. Thus, instead of assigning a definite value (or a range of values) to a particular species, the values are flexible and are directly related to the isomorphous series the species belongs to.
This seems to be a complicated system, yet it could provide for a very good alternative to the vague values assigned to gem garnets as described in textbooks and syllabuses today while giving room for varieties (marketable names) such as rhodolite, malaia and future discoveries.
|Refractive indices according to Hanneman|
|Series||Name (species)||Refractive index||Varieties|
|Almandine-Spessartite||1.809-1.821||Mandarin, Kashmirine, Hollandine|
|Andradite||1.841-1.887||Melanite, Topazolite, Demantoid|
Hanneman's concept illustrated
On the left are "timeline" examples of two isomorphous series with flexible values.
It is unlikely that this system will ever be adopted by the major gemological institutes (in fact, they have rejected it) yet it does provide us with some insight in the complexity of trying to create a universal system for garnet classification.
Some of the intermediate species (such as pyrope-almandine) are already accepted by both Gem-A and the GIA, but not all schools. Rhodolite and malaia garnet are (or will probably be) given "species" instead of "variety" status by the GIA.
Valency in isomorphous replacement
The chemical formula of garnet is L3M2(SiO4)3, which means that the first element has a valency of 2+ and the second element has a valency of 3+. 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. Trace elements are not part of the "ideal" chemical makeup.
- Naming Gem Garnets (2000) - W.Wm. Hanneman, Ph.D
- A Proposed New Classification for Gem Quality Garnets - Stockton & Manson, Gems & Gemology Winter 1985, pp205-217
- Schematic overview of the garnet group (doc format)