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Thomsonite-Ca
Current inventory:  1 gem
 

Chemistry:  NaCa₂Al₅Si₅O₂₀·6(H₂O)

Discovered in 1820;   IMA status: Valid (pre-IMA; Grandfathered).
Thomsonite is named after Dr. Thomas Thomson (1773-1852) the Scottish chemist and mineralogist who first analyzed the material. The Ca modifier was added by the Zeolite Committee due to its Calcium content.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/J.21-70

8 : Silicates
J : Tectosilicates (network) without anions unfamiliar to the tetraheders
21 : Zeolite group, Natrolite - Thomsonite series

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

Commonly prismatic, acicular, or bladed, flattened, elongated and striated, to 12 cm; in radiated spherical or columnar aggregates; smooth, globular, botryoidal; compact, massive. Twinning: On [110].

Physical Properties

Cleavage:

[101] Perfect, [100] Good

Fracture:

Uneven, Conchoidal, Subconchoidal

Tenacity:

Brittle

Moh's Hardness:

5.0 - 5.5

Density:

2.30 - 2.40 (g/cm³)

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, White, Greenish, Yellow, Pink; may be concentrically zoned

Transparency:

Transparent to Translucent

Luster:

Vitreous (Glassy), Pearly

Refractive Index:

1.511 - 1.545  Biaxial ( + )

Birefringence:

0.0050 - 0.0150

Dispersion:

Dispersion distinct to strong, r > v

Pleochroism:

Colorless

Occurances

Geological Setting:

Found in amygdules and fractures in maffic igneous rocks, typically basalts; in some alkalic igneous rocks, contact metamorphic zones, and hypabyssal rocks. As an authigenic cement in some sandstones.

Type Locality:

Old Kilpatrick, Strathclyde (Dunbartonshire), Scotland, UK

Year Discovered:

1820

View mineral photos:

Thomsonite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Thomsonite is a member of the Zeolite Group of minerals that includes over 40 minerals and these gem-type minerals: Analcime, Barrerite, Chabazite, Epistilbite, Gmelinite-Na, Goosecreekite, Mordenite, Natrolite, Pollucite, Scolecite, Stellerite, Stilbite, Thomsonite and Yugawaralite.

Thomsonite can be found in many locations worldwide but only few produce good crystal specimens and even fewer produce the beautiful and colorful concentric "eye" patterns like those found in the western Lake Superior area; Isle Royal and the Keweenaw Peninsula in Michigan and the Grand Marais, Minnesota area.

Pure Thomsonite is snow-white and sometimes translucent; very, very rarely suitable for faceting. Impurities such as ferric and/or ferrous iron or copper are responsible for the various colorations within the mineral. Commonly found colors are pink, tan, white, red and brown. Those with green, gray or black backgrounds or green eyes are the most highly prized but rarely found. These concentric, colorful eye patters make beautiful cabochons.
 

  
Thomsonite gems for sale: