Scapolite

A series between Marialite and Meionite
Marialite Na4Al3Si9O24Cl
Meionite Ca4Al6Si6O24(CO3)
These are tectosilicates (framework silicates)
Specific gravity: 2.5 to 2.8
Hardness: 5 - 6½
Streak: White
Colour: Colourless, white, grey, yellow, green, blue, red to violet
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments

Scapolite is characteristic in limestone as a contact metamorphic mineral. It also occurs in regionally metamorphosed rocks. It is common in a wide variety of ore deposits, particularly skarn. It is also present in hydrothermally altered basic igneous rocks. Marialite is commonly associated with very high NaCl in hydrothermal fluids.
Scapolite may occur in schist and gneiss.
It is associated with diopside, amphibole, garnet, apatite, titanite and zircon.
Scapolite is a mineral of the amphibolite and granulite facies, but it has also been reported in rocks from the greenschist, eclogite and sanidinite facies.

In the Mount Lofty Ranges of South Australia, scapolite is associated with calcite, K-feldspar and pyroxene.

At Craigmont, Ontario, Canada, scapolite occurs in nepheline gneiss, associated with nepheline, corundum and plagioclase.

At Långö, Finland, scapolite occurs in a pegmatite dyke associated with biotite, allanite, quartz, fluorite, calcite and apatite.

In Yakutia, Russia, scapolite is associated with diopside, hornblende, garnet and tourmaline.

At Loos, Sweden, scapolite occurs in pegmatite dykes, and at the contact between the dykes and limestone, associated with albite, plagioclase, diopside, wollastonite, pargasite, grossular and fluorite, commonly intergrown with diopside or quartz.

At French Creek, Pennsylvania, USA, scapolite is intergrown with prehnite and associated with heulandite, chlorite, titanite, apatite and epidote, an assemblage which may have formed under zeolite facies conditions.

Alteration

In many cases scapolite is derived by alteration from plagioclase feldspar.

anorthite and CO2 to meionite (scapolite series), corundum and quartz
4Ca(Al2Si2O8) + CO2 ⇌ Ca4Al6O24(CO3) + Al2O3 + 2SiO2

anorthite and calcite to meionite (scapolite series)
3Ca(Al2Si2O8) + CaCO3 ⇌ Ca4Al6O24(CO3)
This reaction occurs in the presence of a high CO2 pressure in an environment deficient in (Al+Na+K).

meionite (scapolite series) and H2O to clinozoisite and CO2
Ca4Al6O24(CO3) + H2O ⇌ 2Ca2Al3Si3O12(OH) + CO2

meionite (scapolite series) and augite to garnet, calcite and quartz
Ca4Al6O24(CO3) + 3Ca(Mg,Fe2+)Si2O6 ⇌ 3Ca2(Mg,Fe2+)Al2(SiO4)3 + CaCO3 + 3SiO2

meionite (scapolite series), calcite and quartz to grossular and CO2
Ca4Al6O24(CO3) + 5CaCO3 + 3SiO2 ⇌ 3Ca3Al2(SiO4)3 + 6CO2