A Range of Compositions

Many minerals are not restricted to a single chemical composition, but instead can have a range of compositions. The pyroxene minerals are an example of silicate minerals with magnesium and/or iron and/or calcium cations bonded to the Si-O structure. There are two ways to think about the mineral's composition -- you can refer to the cations plus anions as a total of 100%, or you can refer only to the total cations (in this case, Mg+Fe+Ca) as being 100%. For this discussion, let's just think about the total cations equalling 100%.

It's A Little Bit Like Mixing Paint

Orthopyroxenes generally have only magnesium or magnesium plus iron. One way to think about this is to imagine a line representing a range of paint colors, with black at one end, white at the other, and shades of gray in between. If black is 100% and white is 0%, 90% would look nearly black, but 30% would be a pale grey. Now imagine that instead of black, we have magnesium, and instead of white we have iron. 100% magnesium would yield a composition called "enstatite", and a composition of 0% magnesium (100% iron) would be called "ferrosilite".

In the clinopyroxenes, calcium is a third cation that is always present in varying abundances, and is in addition to the variability possible in the Mg/Fe ratio, although the total amount of Ca cannot be more than 50%; the sum of Mg+Fe cannot be 100% anymore because Ca is included too (i.e., Mg+Fe+Ca = 100% of the cations). These variations in composition can be represented by a ternary diagram, which is commonly used by geologists to graphically represent the compositions possible in minerals and rocks.  Below is the ternary diagram for Mg-Fe-Ca pyroxenes:

At the apex of the triangle is Ca -- the composition of that point on the diagram is 100% Ca. At the lower left is Mg -- that point has a composition of 100% Mg (enstatite). The same follows for the lower right corner, with 100% Fe (ferrosilite). A point on the line between Mg and Fe has a composition that includes Mg and Fe and has 0% Ca; if the point is closer to Mg, then there is more Mg than Fe. (Similarly, points along the other sides of the triangle are lacking in either Mg or Fe.) If the point on the Mg-Fe join moves straight up from the line towards Ca, then Ca is being added to the composition. The star in the middle of the diagram represents a composition that is 33.3%Mg, 33.3%Fe, and 33.3%Ca. The diamond at lower left represents a composition that is 80%Mg, 18%Fe, and 2%Ca. The clinopyroxene end members diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6) are found at the center of the Mg-Ca join and Fe-Ca join, respectively. The compositions that would be in the upper half of the triangle are compositions that are not generally found in nature, although the apex of the triangle represents a real, “pyroxenoid” mineral called wollastonite.

Numerous silicate and non-silicate mineral series (e.g., olivine, amphibole, carbonate) exhibit solid solution, and the cations they contain are not limited to Mg, Fe, and Ca.  Cations such as Na, K, Mn, and others also are common.