Garnet
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--Doos 07:59, 12 December 2006 (PST) |
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
Basic
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 species of the garnet group:
- Pyrope (magnesium aluminum silicate)
- Almandine (iron aluminum silicate)
- Spessartite (manganese aluminum silicate)
- Uvarovite (calcium chromium silicate)
- Grossular (calcium aluminium 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 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)
Physical and optical properties
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) aswell as the mineralogical society seem to be in disagreement about when a garnet should be named a pyrope, an almandine or a pyrope-almandine.
Tradionally 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 to literally until a clear unified system of naming gem garnets is accepted worldwide.
The physical and optical properties given are not definite values, rather 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 serie |
Dr. Hanneman believes that 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 2 end members.
As the differences between two end members differ, so will the 30% and 70% of each "timeline" differ. 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. In addition to this, Hanneman proposes that intermediate species be given a separate (intermediate) name (those whose ranges fall between 30 and 70%).
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) 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 | |
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 | Grandite | |
Spessartite | 1.780-1.800 | Melanite, Topazolite, Demantoid | |
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 | ||
Andradite | 1.841-1.887 |
Hanneman's concept illustrated
|
"Timeline" examples of two isomorphous series with flexible values. |
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
References
- Naming Gem Garnets (2000) - W.Wm. Hanneman, Ph.D
External links
- schematic overview of the garnet group (doc format)