Diallage is an inosilicate, meaning it is a chain silicate, and is a part of the pyroxene group. Diallage is a junction between augite and diopside, just like fassaite. It was named in 1801 by René Just Haüy. Its name derives from the Greek word diallaghé (meaning change, transform, difference), as its composition differs from that of the other minerals in the pyroxene group. It is a fairly common mineral, and is cheap.
Properties
Diallage is typically an opaque mineral, but translucent specimens can occur. This mineral forms tabular crystals, meaning the crystals are flatter, and slightly longer than wide. These crystals occur as lamellar masses, meaning the crystals stack on top of each other, and form big chunks. It is a heavy, hard mineral, however, it is fragile. It is not soluble in any acid. According to the Von Kobell-scale, its melting point is high, though the exact melting point of said mineral is not specified. Due to alteration, diallage can grow spots on its surface, which are made out of smaragdite (the green variety of amphibole). It is not surprising, due to both minerals being inosilicates. This alteration is caused by metamorphosis.
Identification
If inspected without the use of tools, it is easy to confuse diallage with other members of both the orthopyroxenes However, pseudohyperstene is also a synonym for diallage, therefore it is not a hypersthene, as pseudo means not genuine. Furthermore, opinions whether it is a distinct species or a variant differ. Even the reason for why it's a variant, or which mineral's variant it is, differs. Mindat claims it is a variant since the parting is different. Furthermore, the formula of the mineral differs from site to site. On mindat.org it has the formula of CaMgSi<sub>2</sub>O<sub>6</sub>.