Garnet

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From The Gemology Project
Garnet
Chemical composition L3M2(SiO4)3

Isomorphous series

Crystal system Cubic
Habit Dodecahedra
Cleavage None
Hardness 6.5-7.5
Optic nature Isotropic
Refractive index 1.74-1.89
Birefringence None
Specific gravity 3.60-4.20
Lustre Vitreous to sub-adamantine

Contents

Chemical composition

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:

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.

  • Pyrope-Almandine
  • Pyrope-Spessartite
  • Spessartite-Almandine
  • Pyrope-Grossular
  • Grossular-Andradite

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
Hanneman Gem-A GIA
Pyrope 1.714-* 1.74-1.76 1.720-1.770
Almandine *-1.830 1.76-1.81 1.760-1.820
Spessartite *-1.800-* 1.79-1.82 1.790-1.814
Grossular *-1.734-* 1.73-1.75 1.730-1.760
Andradite *-1.887 ±1.89 1.855-1.895
* 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
Species Refractive index Hues Varieties
Grossular 1.730-1.760 Green through reddish-orange, colorless Tsavorite, hessonite
Andradite 1.880-1.895 Very slightly yellowish green through orangy yellow Demantoid, topazolite
Pyrope 1.714-1.742 Purplish red through reddish orange, colorless Chrome pyrope
Pyrope-almandine 1.742-1.785 reddish orange through red-purple Rhodolite
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 Malaia, color change

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
Pyrope-Almandine Pyrope 1.714-1.749
Pyrope-Almandine 1.749-1.795 Rhodolite
Almandine 1.795-1.830
Pyrope-Spessartite Pyrope 1.714-1.740
Pyrope-Spessartite 1.740-1.774 Malaia (malaya)
Spessartite 1.774-1.800
Almandine-Spessartite Spessartite 1.800-1.809
Almandine-Spessartite 1.809-1.821 Mandarin, Kashmirine, Hollandine
Almandine 1.821-1.830
Grossular-Almandine Grossular 1.734-1.763
Grossular-Almandine 1.763-1.801
Almandine 1.821-1.830
Grossular-Spessartite Grossular 1.734-1.754 Tsavorite, Hessonite
Grossular-Spessartite 1.754-1.780
Spessartite 1.780-1.800
Pyrope-Grossular Pyrope 1.714-1.720
Pyrope-Grossular 1.720-1.728
Grossular 1.728-1.734
Grossular-Andradite Grossular 1.734-1.770
Grossular-Andradite 1.770-1.841 Grandite
Andradite 1.841-1.887 Melanite, Topazolite, Demantoid

Hanneman's concept illustrated

The concept of intermediate species between endmembers is not new and has been earlier proposed by Anderson and Stockton/Manson. Following is an illustration according to Hanneman's calculations.

Timeline illustration for Hanneman's concept of the Pyrope-Almandine series (in % almandine)


Timeline illustration for Hanneman's concept of the Pyrope-Spessartite series (in % spessartite)


On the left are "timeline" examples of two isomorphous series with flexible values.
Pyrope will have an RI range of 1.714-1.749 in the pyrope-almandine series, while having 1.714-1.740 in the pyrope-spessartite series.

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.

Related topics

G&G Articles on Garnet 1934-1980

The GIA has published all the G&G's from 1934 until 1980 online. The organization of the list by subject was done by Joseph Gill.

References

  • 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

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