Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Ferrous iron (Fe2+) ions are located in the A site, shown in bold italics on the graph. The chemical formula represents the pure form of Almandine, which is its end-member form. That means, if only iron and aluminum are found in the A and B sites, the gem is pure Almandine. A pure Almandine gem would have a graph point located at the end of the line that runs from Pyope to Almandine, precisely at the apex of the Almandine trisection of the graph (upper corner). However, all Garnets are hybrids of various Garnet species. Gem Almandines generally contain significant amounts of Pyrope, and Almandine graph points are never close to the pure apex point. Minor amounts of Spessartine and Andradite Garnet are also present.
Grape Garnet (Almandine from Orissa, India) #10
Almandine Graph Points
Pyrope Graph Points
Spessartine is one of the most sought-after Garnets due to its brilliance and bright color. Orange color is the rarest and most valuable, but red Spessartines are also desirable, and well-cut specimens show more brilliance than red Almandines and red Pyropes. Spessartine colors range from light "fanta" orange to dark "mandarin" orange to orange-red to red. The color of orange gems could possibly be due to a blending of yellow from manganese (Mn2+ and perhaps charge transfer processes involving Mn2+) in conjunction with red from iron (Fe2+ in Almandine). Spessartine derives most of its magnetic susceptibility from manganese (up to 40% manganese oxide by weight), and Garnets with Spessartine as the primary component are the only primary orange gemstones of any kind that pick up with a magnetic wand. Spessartine has the chemical formula shown below.
Violet Anthill Chrome Pyrope & Orange-red Standard Pyrope
The only thing that makes pure Spessartine chemically different from pure Almandine is that manganese occupies the A site instead of iron. Red Spessartine can at times be difficult to distinguish from red Almandine with the naked eye, but only red Spessartine is over the limit (OTL) of measurable refractive index. Magnetic susceptibility measurements (SI) also easily separate the two. Spessartine is the most magnetic transparent gemstone of any type, always surpassing Almandine. As you can see in the graph below, some Spessartines plot near the pure end member at 47.5 magnetic susceptibility. The purest Spessartines are dark "mandarin" orange. Graph point #10 in the graph below is an example of near-pure Spessartine.
Red and orangey-red Spessartines tend to mix with Almandine along the Sp-Al line, while orange and reddish orange gems tend to be in a solid solution series with Pyrope along the Py-Sp line. As you can see by graph point #14 below, Spessartine can contain up to 50% Almandine. The #14 point falls at the midpoint along the line between the Almandine end member and Spessartine end member (midpoint is indicated as .5). The #14 point has 50% of its A site occupied by iron and 50% by manganese.
Spessartine Colors: Light Orange, Dark Orange & Red
Spessartine Graph Points
As you can see, the Low anthill Garnet point has a magnetic susceptibility of less than 5, which is the lowest magnetic measurement this researcher has found for any Pyralspite Garnet. Because the gem is very small, it picks up with a hand-held magnet, but most Chrome Pyropes of average size show only a Drag response.
On the other end of the Pyrope range is the High aqua-blue point, a Standard Pyrope that is 52% Pyrope, with most of the remaining composition as Almandine (48%). This gem has an unusually high magnetic susceptibility measurement of 19.71.
On the graph below, the Low gray graph point to the far left represents a 0.25ct Chrome Pyrope anthill Garnet from a Navajo reservation in Arizona. This Pyrope has a composition closer to the pure end member than any other Pyrope tested. It is approximately 88% Pyrope, which means 88% of site A is filled with magnesium. The remainder of site A is filled with calcium (Grossular), iron (Almandine) and chromium (Uvarovite).
Spessartine: Fanta Garnet (trade name)
Spessartine: Mandarin Garnet (trade name)
Purplish Red Almandine
Pure Almandine gems are not known to exist in nature. The average Almandine gem contains approximately 66% Almandine and 34% Pyrope Garnet (66% of site A is iron and 34% is magnesium,). But any particular Almandine gem will generaly fall in the range of 50% to 75% Almandine, with 25-50% of the A site iron ions replaced by magnesium ions. Magnesium is magnetically inert (diamagnetic). The greater the replacement of magnesium by iron, the more magnetic Almandine becomes, and the further to the right toward the pure end-member its graph points plot.
The graph below shows actual plot points for 27 Almadnine Garnets that come from around the world, from the North America to Africa to India. Notice that Almandine graph points never reach the pure end-member at the upper right apex. Mid-point #10 on the graph is the Grape Garnet (Orissa Almandine) pictured above.
Almandine Cabochon Ring
© Kirk Feral 2011, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder.
Pure Red Almandine
Star Almandine, Idaho
Star Pyrope from Sri Lanka
Exceptionally purple Almandine with the trade name Grape Garnet comes from the state of Orissa, India. A dichroic (two colors) Almandine that changes from red to bright purple in incandescent light has been recently mined in the state of Tocantins, Brazil. Grape Garnet and Tocantins Garnet can be very similar in color to Rhodolite Garnet, which belongs to a different species. Gem dealers and gemologists often refer to purple Almandines as Rhodolites. It is our view that the term Rhodolite should not be used to describe every Garnet with purple color, but should be restricted to the purple variety of Pyrope Garnet for which it was originally intended, and which has its own distinct range of chemical composition.
Tocantins Garnet (Almandine from Brazil)
Pyrope, Almandine & Spessartine
Almandine is the most common gem Garnet. Most gems have a dark purplish red or reddish purple color (due to Fe2+ iron), but some gems may appear pure red (possibly due to additional chromium content). All have a high concentration of ferrous iron (Fe2+) as the distinguishing feature (see chemical formula below). An uncommon phenomenon in Almandine is asterism due to rutile needles. These Garnets are given the trade name Star Garnet (shown below). Star Almandine is the state gem of Idaho, and is also found in Sri Lanka, India and African nations. Shown below right is the apex section of the Almandine Garnet composition graph.
The chemistry of different Garnet species mix and blend while they are being geologically formed, and the chemical elements in the A and B sites interchange in different ratios. This is called isomorphous replacement. The crystal structure of Garnet remains unchanged, while chemical elements in the A and B sites of a pure end-member is substituted with chemical elements from other Garnet end-members in infinite variation to create intermediate varieties with different colors.
Anthill Garnets (shown below) are small rare Pyropes that are cleared out of underground ant tunnels by ants who carry them to the top of their mounds. Gem prospectors collect these tiny Garnets, but they are not commercially mined. Not all anthill Garnets are Chrome Pyropes, and not all Chrome Pyropes come from anthills.
The gems that gemologists commonly refer to as Pyrope actually represent just one of six different varieties of Pyrope: 1) Standard Pyrope, 2) Chrome Pyrope,3) Rhodolite Garnet, 4) Malaya Garnet, 5) Color Change Pyrope, and 6) Pastel Pyrope. In addition, dense rutile inclusions in Pyrope can result in the phenomenom of asterism in Star Pyrope Garnet, as identified in this study. Very rare Cat's Eye Pyrope is also known. Pyrope gems of all varieties tested in this study range from just 12% Almandine in a Chrome Pyrope (low refractive index), to 48% Almandine in a Standard Pyrope (high refractive index).
To calculate the percentage of end-members for any particular gem, we measure the length of the boundary line that connects the Pyrope end-member to the Almandine end-member. Then we measure the distance between the graph point and the end-member apex furthest from it, and divide this number into the total line distance to arrive at the percentage composition of the primary end member species.
For example, let's pretend that the length of the Pyrope-Almandine boundary in the graph above is 100 mm long. The high point farthest to the right on the graph above is 73% Almandine and 27% Pyrope. Its graph point would be measured at 73 mm to the right of Pyrope, because it is 73% of the total length of the line between pure Pyrope and pure Almandine. The low Almandine, the far left point above, would be located 50 mm along the boundary, the half-way point, indicating it is 50% Pyrope and 50% Almandine.
"Pyr" is the Greek word for fire, alluding to the fire-red color for which this Garnet species is known. The pure red to orange-red appearance is due to minor amounts of chromium in addition to iron. Pyrope gems can sometimes be indistinguishable in apearance from Ruby gems, which are also colored by chromium and iron. Since there are a number of varieties of Pyrope, we will refer to typical red and orange-red Pyrope gems as Standard Pyrope. Most Standard Pyropes have much lower RI's than Almandines. They contain an average of 74% Pyrope and 36% Almandine. Pick-up responses tend to have low strength relative to Almandine.
Almandine End Member
Cat's Eye Pyrope from Sri Lanka
The fact that color intensity of Almandine remains relatively constant regardless of the concentration of iron suggests that color could also be influenced by a charge transfer process such as iron to iron (Fe2+-Fe3+). In addition, intervalence charge transfer involving titanium ions (Fe2+-Ti4+) might also influence color toward darker tones (Hoover, pers. comm. 2012).
Almandine Chemical Formula