Formula: CaMgSi2O6
Inosilicate (chain silicate), one of the most common members of the clinopyroxene group. It forms a series with hedenbergite and with johannsenite.


Salite is an iron-bearing variety of diopside
Schefferite is a brown Mn2+-bearing variety of diopside

Crystal System: Monoclinic
Specific gravity: 3.22 to 3.38 measured, 3.278 calculated
Hardness: 5½ to 6½
Streak: White
Colour: Green, brown, colourless
Luminescence: Often fluorescent bright powder blue
Melting point: About 1,400oC at atmospheric pressure (JVW p275)
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Fe,V,Cr,Mn,Zn,Al,Ti,Na,K

Plutonic igneous environments
Metamorphic environments

Diopside is a common metamorphic mineral formed by the metamorphism of siliceous, magnesium-rich limestone or dolostone. It may be found in granite, skarn, marble, eclogite and kimberlite.
It often occurs in marble associated with spinel, phlogopite, tremolite and grossular.
In hornfels of contact and regional metamorphic rocks diopside is found in association with phlogopite, chondrodite and actinolite.
In carbonatites it occurs in association with dolomite, fluorite and andradite.
Other associations include tremolite, scapolite, vesuvianite, garnet and titanite. It is a mineral of the hornblende-hornfels, pyroxene-hornfels, greenschist, amphibolite and granulite facies.


The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. At the Two Mile deposit diopside is a primary silicate that occurs in silicate-rich skarn associated with grossular, actinolite and minor magnetite. At the Three Mile deposit, diopside occurs with grossular, calcite and magnetite adjacent to marble, and also as an alteration product of clinopyroxene (AJM 22.1.35).

At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is located in the Grenville Geological Province, which consists mostly of marble, gneiss, and quartzite. Syenite-migmatite was also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the fact that perfectly formed euhedral crystals of different minerals can often be found floating in calcite with no points of contact with the walls. Sometimes these crystals have inclusions of calcite, irregular or rounded in shape. It has been argued that at least some of the vein-dikes were formed as a result of melting of Grenville marble.
Diopside is common. It occurs as dark green to nearly black, short-prismatic crystals to 3 cm. More rarely, diopside forms pale yellow or pale green equant crystals to 5 mm in calcite or green crystals to 2 mm with scapolite (R&M 97.6.556-564).

At Sha Lo Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, the exposed skarn zone is about 5 m wide, and is composed mainly of garnet, vesuvianite, diopside and epidote, with scattered magnetite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet. Diopside is an important constituent of the skarn, occurring as pale green to greenish grey prismatic crystals up to 4 cm long, and as small dark green grains. Associated minerals include actinolite-tremolite, garnet, epidote, chondrodite, fluorite and magnetite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Diopside appears as dark green translucent crystals to 5 cm in length. Colourless crystals of apophyllite occur sometimes as overgrowths (R&M 96.5.401).

Amity, Town of Warwick, Orange county, New York, USA, is an area of granite intrusions into marble and associated gneiss. The marble is mostly composed of white crystalline calcite that often has small flakes or spheres of graphite and phlogopite. Diopside forms grey or greenish-grey prismatic crystals. An unusual habit of diopside from this occurrence consists of white to grey, radiating, curved crystal clusters that fluoresce a bright greenish-blue (R&M 96.5.436).

At Rose Road, Pitcairn, St. Lawrence county, New York State, USA, diopside occurs at the skarn deposit as pseudomorphs after wollastonite, either as isolated crystals in areas of coarsely crystallised calcite or as crystals lining the walls of a diopside-albite rock that faces into coarsely crystallised calcite (R&M 97.5.434-444).

The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in marble. The development of veins of large crystals probably occurred as a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during this primary event, with additional species resulting from the subsequent alteration of scapolite. There seems to be little, if any, secondary, late-stage mineralisation present.
Diopside is common as purple crystals to 4 cm in calcite and as massive fine-grained lavender material suitable for cutting and polishing. Most crystals are prismatic and form interlocked networks in massive calcite (R&M 96.6.549). The diopside has variable fluorescence from faint to bright white under short wave UV (R&M 97.5.442).

At the Pyrites Mica mine, St Lawrence county, New York, USA, diopside often forms a matrix for larger meionite crystals, and sometimes has associated titanite and pyrite. (R&M 93.4.339)


During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

åkermanite and CO2 to diopside and calcite
Ca2MgSi2O7 + CO2 ⇌ CaMgSi2O6 + CaCO3
The maximum stability limit of åkermanite in the presence of excess CO2 is about 6 kbar. Below that pressure, at relatively lower temperatures, åkermanite reacts with CO2 to form diopside and calcite according to the above reaction. (JVW p144)

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan. (DHZ 2A p512)

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction. (DHZ 2A p272)

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction. (DHZ 2A p272)

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2 (DHZ 2A p274)

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar pressure and 400 to 500oC (greenschist facies). (DHZ 1A p263)

Fe-rich cordierite and diopside-hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2 (DHZ 2A p126)

diopside, CO2 and H2O to tremolite, calcite and quartz
5CaMgSi2O6 + 3CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2
Diopside is produced by the metamorphism of siliceous dolostone, and if water is introduced at a later stage tremolite may be produced from the above reaction, or by the reaction of diopside with dolomite. (DHZ 2A p276)

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2 (DHZ 2A p453)

diopside and antigorite to forsterite, Mg-rich tremolite and H2O
2CaMgSi2O6 + 5Mg3Si2O5(OH)4 ⇌ 6Mg2SIO4 + Ca2Mg5Si8O22(OH)2 + 9H2O
At 10 kbar pressure the equilibrium temperature is about 580oC (amphibolite facies). (SERC)

diopside and dolomite to forsterite, calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 → 2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC. (MOM, DHZ 5B p213)

diopside, dolomite, CO2 and H2O to tremolite and calcite
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3
diopside with CO2 and H2O. (DHZ 2A p276)

diopside, dolomite and H2O ⇌ hydroxylclinohumite, calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ Mg9(SiO4)4(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, clinohumite associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction (DHZ 1A p264).

diopside, forsterite and calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 + 2CO2
This reaction requires a high temperature. (DHZ 2A p271)

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2 (DHZ 2A p136)

dolomite and coesite to diopside and diamond and oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2
The coexistence of diamond and carbonate minerals in mantle eclogite is explained by the above reaction.

dolomite and quartz to diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
In the metamorphism of siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite. (DHZ 2A p270, MOM p482)

dolomite, tremolite and forsterite to diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
At a pressure of 4 kbar the equilibrium temperature is about 840oC (granulite facies) (JVW p97).

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above. (DHZ 2A p135)

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above. (DHZ 2A p135)

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure.
(DHZ 1A p233)

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4 (DHZ 2A p258)

forsterite and åkermanite to diopside and monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 + 3CaMg(SiO4)

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4
The reaction can proceed in either direction, depending on the ambient conditions.

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8)= Mg2Si2O6 + CaMgSi2O6 + MgAl2O4 (DHZ 1A p242)

forsterite, calcite and SiO2 to diopside and CO2
Mg2SiO4 + 2CaCO3 + 3SiO2 → 2CaMgSi2O6 + 2CO2
In high temperature environments with excess SiO2 diopside may form accoring to the above reaction. (DHZ 2A p271)

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 = 3CaMg(SiO4) + 2CO2

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) + 2 CO2
This reaction occurs during contact metamorphism of magnesian limestone. (DHZ 1A p353)

grossular, diopside,monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises. (DHZ 1A p714)

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical. (DHZ 2A p272)

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical. (DHZ 2A p272) jadeite, diopside, magnetite and quartz to aegirine, kushiroite (pyroxene) and enstatite-ferrosilite
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction. (DHZ 2A 512)

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure. (DHZ 2A p449)

monticellite and diopside to åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi27 + Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar (granulite facies). (DHZ 1A p357)

nepheline and diopside to åkermanite, forsterite and albite
3NaAlSiO4 + 8CaMgSi2O6 ⇌ 4Ca2MgSi2O7 + 2Mg2SiO4 + 3NaAlSi3O8
This reaction is in equilibrium at about 1180oC, with lower temperatures favouring the forward reaction. (DHZ 4 p251)

phlogopite, calcite and silica to diopside, K-feldspar, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction. (DHZ 2A p272)
The association of phlogopite and calcite is stable only in the absence of excess silica. (DHZ 3 p51)

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction (DHZ 2A p123)

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌ Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite and forsterite according to the above reaction. (DHZ 2A p271)

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4 This reaction occurs at fairly low temperature and pressure.

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