THOMSONITE

( Fr- thomsonite; Ger- Thomsonit; Nor- thomsonitt; Rus )

THOMSONITE, NaCa2Al5Si5O20·6H2O.

A. Thomsonite. Polished piece ("diameter" - 2.6 cm) extacted from a basalt outcrop about five miles south of Grand Marias, Minnesota. (© photo by John Truax, www.mylittlerockshop.com)

B. Thomsonite. Polished pieces ("diameter" of spheroid on upper right - 1.4 cm) extacted from a basalt outcrop about five miles south of Grand Marias, Minnesota. (© photo by John Truax, www.mylittlerockshop.com)

C. Thomsonite. Polished piece (greater axis -1.4 cm) extacted from a basalt outcrop about five miles south of Grand Marias, Minnesota. (© photo by John Truax, www.mylittlerockshop.com)

DESCRIPTION: Thomsonite is one of the zeolite minerals. The following properties are for the variety of thomsonite that is most often utilized in the fashioning of gemstones.   
    Colors - pink, reddish, purplish, yellowish, green, brown, tan, off-white, and nearly black, typically comprising concentric rings, sometimes referred to as eyelike masses
    H. 5-5½
    S.G. 2.10 - 2.40
    Light transmission - translucent
    Luster -
pearly  to subvitreous
    Miscellany - rarely chatoyant;
consists of compact spheroidal masses made up of radiating fibers.

OTHER NAMES:   The following names (listed alphabetically) are given as discredited synonyms for thomsonite by the Commission on New Minerals and Mineral Names of the International Mineralogical Associationbagotite, carphostilbite, comptonite, faröelite, gibsonite, karphostilbite, koodilite, lintonite, mesole, mesolitine, metathomsonite, ozarkite, picrothomsonite, scoulerite, sphaerodesmine, sphaerostilbite, strontium thomsonite, tonsonite, triploclase, triploklase and winchellite.  All are noted here even though several of them have never been applied to the gemrock variety described in this entry.  Those I have seen applied to the gemrock variety are underlined in the just listed discredited synonyms and are also on the following list:     

In addition to the above,  thomsonite-Sr, in which strontium is coupled with calcium (in parentheses) and the water content is indicated to be 6-7, may constitute some massive thomsonite.  Also, I have found four other names given to thomsonite:   harringtonite, which has been discredited as a mineral species because it is a mixture or thomsonite and mesolite (another zeolite);  Kalkkreuzstein (German ~ lime cross stone); sarbossa (for which I have been unable to find an origin);  and "fire rock" (attributed to the Inuit).  The last two terms  are listed on web sites dealing with "Gemstones of the Realms."  One can only hope that these terms, especially the "johnny come lately" sarbossa, do not receive widespread attention and further clutter the nomenclature for this mineral. 

USES: Cabochons for rings (etc.) and beads.

OCCURRENCES:
Filling preexisting cavities, especially in basaltic rocks, and as small pebbles weathered out of those rocks -- e.g. in beach  gravels.

NOTEWORTHY LOCALITIES: near Grand Marais, Cook County, Minnesota;   on Isle Royale National Park, and nearby Keweenawan Peninsula, Michigan;  on Michipicotin Island, Ontario;  and sporadically at other localities along or near the western part of Lake Superior.  Although several non North American localities have been alluded to in the literature -- e.g., Breiddalsheidi, Iceland;  Saxony, Germany;  Italy; and the Faroe Islands -- I have been unable to determine whether the thomsonite from these localities was or was not of the gemrock variety.  

REMARKS: Thomsonite was named for the Scottish chemist Thomas Thomson (1773 -1852), who analyzed the mineral from the type locality,  Old Kilpatrick, Strathclyde, Dumbaronshire (a former county of west central Scotland).

Most thomsonite nodules and their derived pebbles are less than 0.6 cm (~ 1/4 inch) in greatest dimension -- i.e., those illustrated are larger than the average.  And, those enclosed in basalt are extremely difficult to remove without breaking them.  Consequently, a very large percentage of those used as gemstones have been, and continue to be, pebbles collected from gravels.  These days, many are recovered off-shore by scuba divers.

Thomsonite nodules have been found in Alaskan Eskimo burial sites over 2,000 years old.  Russians are said to have made jewelry including  thomsonite during the 8th century.  Queen Victoria commissioned the Chippewa Indians to mine Grand Marais (Minnesota) thomsonites because the nodules were becoming scarce in Scotland. (Weaver, 2002)

Many specimens and gemstones with well defined "eyes" and originally thought to be thomsonite have been shown to be prehnite that appears pink because of internal reflections from very fine, evenly disseminated grains of native copper.  I once examined almost 100 specimens labeled "thomsonite" from the Keweenaw Peninsula of Michigan's Upper Peninsula -- kindly supplied by members of the Michigan Mineralogical Society -- and found more than 50 percent (in fact, nearly 60 percent) of them to be prehnite.   See also Huber (1983).

SIMULANT:  Prehnite with native copper inclusions -- see last paragraph under the REMARKS subheading. [ I used a petrographic microscope and/or x-ray diffractometer to distinguish the prehnite from thomsonite.].

REFERENCE: Huber, 1983.

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Last update:  14 July 2005
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