A name used for the solid-solution mineral pair of marialite and meionite.
Article by: Hobart M. King, Ph.D., RPG
What is Scapolite?
Scapolite is a name used for a group of aluminosilicate minerals that includes meionite, marialite, and silvialite. Meionite and marialite are end members of a solid solution series. Silvialite is a mineral that is very similar to meionite.
These minerals have very similar compositions, crystal structures, and physical properties. They cannot be easily distinguished from one another in the field or during hand specimen examination in a laboratory. The name "scapolite" is a term used for convenient communication. These minerals are found in small quantities in some metamorphic and igneous rocks. Their compositions are compared in the table below.
Physical Properties of Scapolite
Scapolite has an appearance that is very similar to many feldspars. As a result, it can easily be overlooked in the field and during hand specimen examination in a laboratory.
Massive scapolite is found in regionally metamorphosed rocks such as marble, gneiss, and schist. These massive specimens often exhibit a wood-grain or fibrous texture which facilitates their identification. Well-formed, gem-quality, prismatic crystals with a square cross-section are sometimes found in marbles.
In metamorphosed igneous rocks, especially gabbro and basalt, scapolite often occurs as complete or partial replacements of the feldspar grains. Crystals of scapolite are sometimes found in pegmatites and rocks altered by contact metamorphism.
Scapolite minerals are easily attacked by weathering. They are some of the first minerals attacked in their host rocks and easily alter to micas and clay minerals. As weathering begins, the mineral grains lose their transparency, become opaque, and have a reduced hardness.
Physical Properties of Scapolite
|Color||Colorless, white, gray, yellow, orange, pink, purple|
|Diaphaneity||Transparent to translucent|
|Mohs Hardness||5 to 6|
|Specific Gravity||2.5 to 2.7|
|Diagnostic Properties||Luster, specific gravity, massive specimens often have a wood-grain or fibrous appearance, prismatic crystals have a square cross-section|
|Chemical Composition||A solid solution between marialite (Na4(AlSi3O8)3Cl) and meionite (Ca4(Al2Si2O8)3(CO3,SO4))|
|Uses||Faceted gemstones and cat's-eye cabochons|
Uses of Scapolite
Scapolite does not have a role as an industrial mineral. It is rarely found in minable quantities and does not have a composition or physical properties that make it of industrial use.
The only use of scapolite is as a minor gemstone; however, in that use it can be beautiful and interesting. Yellow and pink transparent scapolite can be cut into very attractive gems like the yellow scapolite shown on this page. Some specimens contain tiny fibrous inclusions that produce a "silk" within the stone that reflects light to form a cat's-eye. A specimen with a coarse silk that forms both a cat's-eye and a diffraction grating is shown in the photo at the top of this page.
Scapolite has a Mohs hardness of between 5 and 6, which is too soft to serve as a ring stone. Its use is therefore limited to being a collector's stone and being mounted in jewelry such as earrings and pendants that have a low risk of impact or abrasion.
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