The concept of isomorphous replacement series gemstones is one that stumps many gemologists. Particularly those that have not had the opportunity to study the chemical structure of gemstones. But this is a very important area of study because it helps one understand how a single family of gemstones can offer such a wide variety of colors....and sometimes inclusions.

There are several gemstone groups that offer isomorphous replacement series of formations. Among them are the topaz family, the tourmaline family and the garnet family. For our purposes today we are going to work with the garnet family of gemstones.

Have you ever wondered how a gemstone can be an orange/brown hessionite garnet while a beautiful emerald green tsavorite can also be a garnet? Or how a pyrope garnet can have a refractive index in the 1.73 range while a uvarovite garnet is over the reading scale of a refractometer. Or perhaps why there is such a wide range of specific gravities listed for garnets while they are all still considered garnets? Well, the answers are easier than many people think....

The truth is, the garnet family is a large group of gemstones that all form in the same crystal system, all share a relatively common, but varying, chemical structure. As we will learn in a few minutes, garnets are actually the same general gemstone. But as small variations occur in their chemical structure, this causes variations in the properties and colors of the garnets. Such as when the magnesium in pyrope garnet is replaced with iron...what would you expect to happen? Get heavier? Your are exactly right. And when the magnesium in pyrope garnet gets replaced with iron the specific gravity goes from roughly 3.65 for pyrope to 3.95 for almandine. And the refractive index numbers go up as well due to the iron. However, you still have a gemstone that forms in the cubic crystal system. It they both still share the same basic chemical property since both are what we call silicates, since both have silicon oxide as a basic building block chemically.

What you have, however, are two gemstones with remarkably similar properties but slightly different chemical make-ups. Therefore, you have a isomorphous replacement series of gemstones that all are loosely related to one another, but differ slightly from one another.

Perhaps the most important part of the above is that this variable does not all happen at once. Sometimes you will find one garnet that is completely different, such as all of the iron being out of a pyrope garnet, to make it a pure almandine garnet. In these cases you have the classic concept of isomorphic replacement. The possibility of a gemstone that contains a little of both elements, thereby making it a transition garnet. Such as in the case of a pyrope/almandine garnet of rhodolite. This type of garnet can have both the magnesium content of pyrope but some of that magnesium...not all just some...has been replaced by iron. Giving the gemstone some of the properties of both members of the garnet family.

The importance of this is that there is no set listing of gemological properties for garnets. In spite of the fact that they form in the cubic system and have only one RI reading for each stone....if a pyrope garnet has some iron that has replaced some of the magnesium...then you will start getting a higher reading. Meaning that it is sometimes difficult to determine exactly which member of the garnet family you have since variable amounts of elements can make a stone actually test out in the middle of the properties.

This is the point that many students have problems with...the transition of elements in isomorphous replacement series stones. In order to better explain these concepts I have gone back to the drawing board to try to create some graphics to better illustrate these concepts for you. I hope they are of service. Let me know if there is something more that needs to be covered....


A gemstone is made up of a variety of elements. Sometimes only one or two, sometimes many. And these elements can differ from one location to another due to the available elements in the mix where the gemstones formed. In the case of garnets there are three basic elements that go into the make-up of each member of the garnet group.

In a perfect formation, a pyrope garnet will be magnsium, aluminum and silicon/oxygen, as shown by the blue, pink and red you see at left.

  In some formation locations, there may not be sufficient amounts of one element or another, or there may be far more of one element or another, and the result is a garnet that is only part of one element and part of another. The change occurs when another element takes some of the space within the garnet crystal. As in the case of iron replacing some of the magnesium as shown by the yellow at left.
  Depending on the amount of iron and/or lack of magnesium in a location, a garnet may have much more or much less of iron or magnesium. It is this transition amount of iron to magnesium that makes some garnets test in between the published property charts for the various garnet groups. As they cease being pure pyrope (for instance) and start to have more characteristics of almandine.

Eventually, in some locations, there is no magnesium for the garnet to use...and where the magnesium would have made a pyrope, there is now only iron. The lack of magnesium and presence of iron changes the properties and color of the garnets produced in that area to be of the property and color of almandine garnet.

Therefore, you now have an almandine garnet with a chemical structure of iron, aluminum, and silicon/oxygen.

 Garnet Isomorphous Replacement Series Gemstones
   Let's run through the garnet series and see if the changes will make all of this clear for you. At left you see a pyrope garnet and the chemical make-up.
  Now, you can see how the iron in this garnet has taken over some of the chemical make-up of the stone...literally replacing some of the magnesium with the iron. Remember, this is not something that actually changes within the stone. It is how the formation occurs when the crystal forms due to different elements being more available. And this simply causes a change in properties based on the different chemical composition of the garnet.
  Now the iron has completely taken the place of the magnesium in the chemical equation. This stone is an almandine garnet but still shares many properties with the pyrope.
  In spessartite garnets the iron and magnesium are no longer present, but now that third chemical place has been taken over by manganese. Changing the gemological properties of spessartite, changing the color offered due to the different elements in the spessartite, but maintaining the overall relation to the other garnets by the cubic formation and the aluminum silicate based for the other elements.
   In grossular garnets, rather than containing manganese, the third place of chemical composition is now taken up by calcium. Thereby maintaining the cubic crystal structure and the aluminum silicate based elements. But the calcium will cause a variable in colors of the gemstone, and a variable reading in gemological properties.
   No something unusual happens. Rather than the top element changing as we saw in the previous graphics, this time the calcium is staying and the aluminum is gone...replaced by iron. This is an andradite and is more often known by its demantoid variety with the horse tail inclusions of asbestos. Here the calcium element stays but we see a different type of isomorphous replacement as the aluminum that has been with us all along is now replaced by iron.
   And finally, we still keep the calcium but now the iron (that replaced the aluminum) has been replaced by chromium. Giving this garnet group member the chemical make-up of calcium, chromium, silicate. Still forming in the cubic cystal system, and still maintain its status as a silicate since the silicon oxide is still the basic building block. But now middle property has changed to include chromium...meaning that the gemological properties will also change.

  How about a YourGemologist Quick Quiz!


 The above is why the garnet group has such a wide variety of gemological properties that are possible. And you can have many, many members of the garnet group with properties that overlap one another.

I hope this overview of isomorphous replacement series stones has been of help. Those of you who are taking your FGA exams should be study this information carefully. It is usually on one of the theory exams every year.

Until next time......Robert James FGA, GG

 © 2004 and Robert James FGA, GG for all graphics and copy. Only private individuals may download this information for their own study. None of this information and none of the graphics may be reproduced for commercial purposes. And none may be reproduced on any internet website or other media without written permission of Robert James FGA, GG...YourGemologist


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