Diopside is a mineral of the pyroxene group. It is closely related to the most common pyroxene augite. Composition of pure diopside is CaMgSi2O6 but there is a complete solid solution to hedenbergite CaFeSi2O6 and also to augite (Ca,Mg,Fe2+,Fe3+,Al)2(Si,Al)2O6. These three are collectively named calcic clinopyroxenes and the boundaries between them are arbitrarily chosen to be percentages of Fe/Mg and Ca. How the classification works is shown on the diagram below.
The classification principles of pyroxenes. Diopside contains more magnesium than iron and its Ca content is above 45% of the sums of Fe+Mg+Ca. This diagram does not have enough variables to display all the pyroxene group minerals. There are also solid solutions toward Na-clinopyroxenes (Na substituting for Ca) which gives rise to more members of the group. Wollastonite shown in the upper corner is not a member of the group. There is no solid solution between pigeonite and augite. The same diagram is available in PDF version also.
The diagram above may leave an impression that what is diopside and what is not is completely artificial but it is not that bad. All three end-members have clearly distinct rocks where they occur, although there are some overlaps – sometimes wrong minerals occur in the wrong rocks. Just in case someone thinks that I am serious I will explain: minerals never occur in the wrong rocks. It is our knowledge and understanding that is incomplete or our classification principles are inflexible and cumbersome. What I want to say is that although pyroxene in gabbro is usually augite, sometimes it is diopside. And pyroxene in skarn, although usually diopside, may sometimes turn out to be augite. But eventually we have to decide where to draw a line when there is a solid solution case and it is often decided that 50% or other round numbers are the way to go.
Diopside in an ultramafic rock wehrlite (peridotite). The yellowish green mineral is olivine, purple is pyrope. In mafic and especially ultramafic rocks it may contain significant amount of chromium which gives it intense green color. Åheim, Norway. Width of sample 16 cm.
Crystals of Ca-clinopyroxenes are short and not as slender as, for example, crystals of Na-pyroxene aegirine. This is a crystal of augite (35 mm across).
Pure diopside contains no iron which means that there are no chromophore elements in its composition and the crystal should be colorless. That is indeed the case, although pure diopside is somewhat rare. It usually does contain variable amounts of chromophores like iron and chromium, which give it a greenish hue. Mg-rich variety with well-developed crystal faces is a semi-precious gemstone as is the chromian variety because of its intense green color.
Density of the mineral is 3.19 g/cm3, but it increases with increasing iron content. Hardness is about 6 and the crystals have two cleavage planes intersecting at 93/87 degrees as is typical to all the pyroxenes.
Diopside occurs mostly in basic and ultrabasic igneous rocks or in metamorphic rocks which are compositionally mixtures of silicates and carbonates. The first group includes rocks from the mantle which may be colorful as are the example pictures of pyrope- and diopside-rich peridotites in this article. It also occurs in kimberlites and it is an indicator mineral in diamond prospecting because intensely green chromian varieties are easily spotted in the heavy mineral fraction of sand. It is a common mineral in these rocks because the mantle is rich in magnesium when compared to the crust. Calcic clinopyroxenes in mafic rocks like basalt and gabbro are usually augitic in composition, but some contain iron- and aluminum-rich diopside (especially alkali basalts).
The latter group of metamorphic diopside-bearing rocks includes skarns and some marbles. These rocks form when silicate magma reacts with surrounding carbonate rocks (dolomite, limestone, marble) or when carbonate rocks with a significant silicate component are thermally metamorphosed. Skarns frequently contain ore minerals like magnetite and various sulfides and are therefore often mined. Skarn is a Swedish mining term for the gangue minerals surrounding the valuable ore. It is a major mineral in many skarn samples.
Diopside just as most pyroxenes has few uses. It is a semi-precious gemstone and it also has been used in dental ceramics.
Diopside as a Ca-bearing silicate is a common mineral in skarns which form when silicate magma reacts with carbonate rocks. This sample is composed of calcite (pink), actinolite (black slender amphibole) and dull green diopside. Tapuli, Sweden. Width of sample 10 cm.
Abundant pyrope (Mg-rich garnet) and chromian diopside in peridotite from Åheim, Norway. Width of view 20 cm.
Diopside forming rims around pyrope in peridotite from Åheim, Norway. Width of sample 10 cm.
Layered ultramafic intrusion where olivine-rich dunite alternates with pyrope-diopside pyroxenite layers. Width of view is about 30 cm. Åheim, Norway.
Green mineral here is not diopside, but it is also monoclinic pyroxene and compositionally not far from it. This mineral is omphacite and the rock is eclogite. Nordfjord, Norway.