Garnet

From The Gemology Project
Revision as of 08:59, 12 December 2006 by Doos (talk | contribs)
Jump to: navigation, search
Exlamation mark.jpg
This section is currently under construction, do not edit until this message is gone
--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

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

Then there is a 6th serie that hardly produces any minerals large enough to be cut to gemstones.

  • Uvarovite-Grossular

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 no definite values, rather they overlap.

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

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