Series

It is very hard to define a mineral series in writing. The following is a bit confusing but is the simplest way to put it in words.

A series is a mineral group consisting of minerals that have a nearly identical crystal structure. The only difference between the minerals in a series is the type of elements contained in the crystal structure of the mineral. This means that there must be at least two defined minerals in a series: one mineral containing one particular element, and one mineral containing a different element. These two minerals are known as the end members, or anchor members. In-between the end members minerals as a combination of both anchor members exist. Sometimes, these intermediary minerals are given names. An example of this is a mineral series known as the Olivine group. In the Olivine group, the two end members are Forsterite and Fayalite. Forsterite and Fayalite have a crystal structure that is identical, except that Forsterite is composed of magnesium (Mg), and Fayalite is composed of iron (Fe).

Another member of the Olivine group is Chrysolite. Chrysolite is an intermediary member, so it has a varying amount of magnesium and iron.

The amount of magnesium to iron in Chrysolite is not defined, and can range from 1:99 to 99:1. However, in certain mineral series, intermediary members are given names based on defined percentages of the elements they contain. For example, if an intermediary member contained 20% Mg and 80% Fe, it might be classified as a different mineral than 80% Mg and 20% Fe.

Some mineral series, such as the Spinel series, contain more than two end members. This means there can be many different combinations of intermediary members.

Some possible combinations might be:

Most of these possible combinations do not have names, but a few, such as Gahnospinel, (Mg,Zn)Al₂O₄, do.

In a few complex mineral series, such as Garnet, more than one element may be replaced by another.

The mineral Pyrope and the mineral Uvarovite contain TWO elements that are different, and yet they are still in the same series, since they are identical in crystal structure, and can (theoretically) form intermediary members.
In large series such as Garnet, nearly hundreds of possible intermediary combinations can exist. However, this number is greatly restricted since the laws of chemistry only allow certain members to combine with others.

A mineral series may contain a member with the same name. For example, the Skutterudite series contains three minerals, and the name of one of them is Skutterudite.

For more information, see "The Chemical Properties of Minerals"