Difference between revisions of "Garnet"

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(Physical and optical properties)
(Physical and optical properties)
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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.<br />
 
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.<br />
As the differences between two end members differ, so will the 30% and 70% of each "timeline" differ. Hence lowering or raising the values.<br />
+
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.<br />
 
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.
 
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.
  
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! Name (species)
 
! Refractive index
 
! Refractive index
 
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Revision as of 11:04, 12 December 2006

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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

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:

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.
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.

In addition to this, Hanneman proposes that intermediate species be given a separate (intermediate) name, giving room for varieties (marketable names) as rhodolite, malaia and future discoveries.

Refractive indices according to Hanneman
Series Name (species) Refractive index
Pyrope-Almandine Pyrope 1.714-1.749
Pyrope-Almandine 1.749-1.795
Almandine 1.795-1.830
Pyrope-Spessartite Pyrope 1.714-1.740
Pyrope-Spessartite 1.740-1.774
Spessartite 1.774-1.800
Almandine-Spessartite Spessartite 1.800-1.809
Almandine-Spessartite 1.809-1.821
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
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
Andradite 1.841-1.887

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