Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
MJO has sent me a box of lovely assorted garnets, so I think it’s a great time to explain:
The Garnet Group In Theory:
The Garnet Group is a series of silicates, all crystallizing in the cubic system, which can divided on the basis of their chemical composition. The group is arbitrarily divided into 6 species, but extensive substitutions are possible within each species. This is called isomorphic replacement.
The basic chemical composition for the members of the garnet group is:
Where R**, represents a divalent ion and R*** represents a trivalent ion.
As long as an ion is about the same size as another, it can easily be replaced by it.
R** ions that are similar in size are Calcium, Magnesium, Ferrous Iron or Manganese and R*** ions that are similar in size are Aluminum, Ferric Iron and Chromium.
To simplify, let’s divide our Garnet Group into 2 sub-groups each with three members, called species.
The first will be Pyrope, Almandine and Spessartine.These garnets all have the same R***: Aluminum, but have substitutions going on with the R** member of their formula.
Gee, that’s easy to understand right?
What’s the big deal?
There is a slight complication. There can be infinite substitutions between our Mg, Fe and Mn.
For example, Rhodolite garnets are chemically (Mg, Fe)3 AL2 (SiO4)3. Rhodolite garnets are a combination of pyropes and almandines. More on this later!
Our second sub-group of species all have the same R**: Calcium, but have substitutions with the R***.
The combinations of different elements, give garnets species different properties (refractive index, specific gravity, spectroscopic absorption) that can be used to identify them.
Now, let’s get Practical!
PYROPE Mg3 Al2 (SiO4)3
Color Range: Medium to dark reddish orange through slightly purplish red.
Clarity: Type II
Refractive Index: 1.714-1.742, normal: 1.74
Specific Gravity: 3.84 (+-0.10)3.78 (=.09,-.16)
Spectrum: Broad band about 564nm, fine material may show chromium lines in the red end of the spectrum.
Cause of Color: Iron and Chromium Note: Pyropes are NEVER pure Mg3 Al2 (SiO4)3.
If they were they would be colorless and they never are!
Microscopic Observations: Needle-like included xls and rounded, irregular included xls.
Phenomenon: Color change occasionally seen resulting from pyrope-spessartine mix.
RHODOLITE (Mg, Fe)3 Al2 (SiO4)3
Color Range:Light to dark purplish red, or reddish purple
Clarity: Type II
Refractive Index:1.760 (+0.10, -0.20)
Specific Gravity: 3.84 (+-0.10)
Spectrum: Similar to almandineCause of Color: Iron
Microscopic Observations: Needle-like included xls and rounded, irregular included xls.
ALMANDINE (Almandite) Fe3 Al2 (SiO4)3
Color Range: Reddish orange to Red, slightly purplish-red to reddish purple, usually dark in tone
Clarity: Type II
Refractive Index: 1.790
Specific Gravity: 4.05 (+0.25,-0.12)
Spectrum: BE strong bands 504, 520, 573, with fainter lines often present at 423, 460 & 610Cause of Color: Iron
Microscopic Observations: Needle-like included xls, zircon xls with strain halos, irregular rounded xls with low relief
Phenomenon: Asterism
MALAIA (Mg, Mn)3 Al2 (SiO4)3
Color Range: Malaia: Dark to light slightly pinkish orange, reddish orange or yellowish orange ;Color-change possible without blue tones
Clarity: Type II
Refractive Index: 1.760 (+0.20,-0.19)
Specific Gravity:3.78 to 3.85
Spectrum:Strong lines at 410,420 and 430nm.
Cause of Color: Malaia: manganese, iron; color change: chromium, vanadium
Microscopic Observations: Rutile, pyrite and apatite xls.
Phenomenon: Color Change
SPESSARTINE (Spessartite) Mn3 Al2 (SiO4)3
Color Range: Yellowish-orange to reddish orange
Clarity: Type II
Refractive Index: 1.810 (+.004,-0.020)
Specific Gravity: 4.15 (+0.05,-0.03)
Spectrum: Broad band about 564nm, fine material may show chromim lines in the red end of the spectrum.
Cause of Color: Iron and Chromium
Microscopic Observations: Needle-like included xls and rounded, irregular included xls.
Phenomenon: Color change occasionally seen resulting from pyrope-spessartine mix.
GROSSULAR (Grossularite) Ca3 Al2 (SiO4)3
Color Range: Light to dark green, light to dark yellow to reddish orange, occasionally colorless
Clarity: Type II
Varieties:Hessonite: yellow-orange to reddish orange; Tsavorite: intense green to yellowish green
Refractive Index: 1.740 (+.020,-0.010)
Specific Gravity: 3.61 (+0.12,-0.04)
Spectrum:Hessonite may show 407 & 430nm bands
Cause of Color:Hessonite: Iron; Tsavorite: Vanadium with a trace of Chromium
Microscopic Observations: Hessonite: Heat-wave effect
ANDRADITE Ca3 Fe2 (SiO4)3
Color Range: Yellow, green, brown, black; transparent to opaque
Clarity: transparent to opaque
Refractive Index: 1.888 (+.007, -.033)
Specific Gravity: 3.84 (+-.03); Melanite 3.90 (+-.20)
Spectrum:Demantoid: dark band at 440nm; may also have lines at 618, 634, 685, and 690nm
Cause of Color: Demantoid: chromium, iron; Topazolite: iron; Melanite: iron and titanium
Microscopic Observations: Demantoid: Horse-hair inclusions;
Phenomenon: Iridescence (opal-like) in semi-translucent to opaque andradite
HYDROGROSSULAR Ca3 AL2 (SiO4)3 with hydroxide (OH) partly replacing (SiO4), may be inter grown with idocrase
Color Range: Green to bluish green, pink, white & gray.
Clarity: transparent to opaque
Refractive Index: 1.720 (+.010, -.050)
Specific Gravity:3.47 (+-.08, -.32)
Spectrum: darker green material often shows a cut-off below 460nm
Cause of Color: Green: chromium and possibly iron; pink: manganese
Microscopic Observations: Black, peppery inclusions
Phenomenon: None
Last edited by Barbra Voltaire, FGG on Mon Mar 05, 2007 2:49 pm, edited 2 times in total.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Example 1.
The specs of the lovely oval in this suite are as follows:
Weight: 10.41 carats
RI: Over the limit
SG: 4.18
Anomolous double refraction
No diagnostic inclusions
Spectroscope
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Example 5
A mesmerizing concave cut, that couldn't be any more splendid! Like a tropical sunset.
Carat weight: 9.67
Specific Gravity: 4.20
RI: Over the limit
Spectroscope:
Joined: Mon Oct 17, 2005 10:56 am Posts: 6461 Location: The frozen north prairie :-/
Okay ... looking back at my posts, I see I've messed up the order of my answers . Let's try again:
#1: Spessartite
#2: Grossular (Tsavorite)
#3: Malaia ... hmmm, could be rhodolite from the absorption spectra
#4: Grossular (Tsavorite) The one I want!!!
#5: Spessartite
#6: Color-change (Pyrope-spessartite mixture)
_________________ IIJA Registered Gemologist GIA Graduate Gemologist
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