Thomsonite-Ca

The name on this page is a Scottish chemist's name — passed first to a mineral he never named himself, then split, almost two centuries later, in two.

In 1820 the English crystallographer Henry James Brooke described a new zeolite-group mineral — a member of the family of hydrated framework silicates that release water when heated — from material collected in Scotland. He called it thomsonite, in honour of Thomas Thomson. Thomson, born in Crieff, Perthshire on 12 April 1773, held the chair of chemistry at the University of Glasgow and was one of the busiest mineral analysts of his generation. He worked through enormous numbers of accurate chemical analyses and described several new species himself. The honorific suited him: a chemist's name, fastened to a mineral whose interest is largely chemical.

Through the 19th century collectors found thomsonite in basalt cavities on every continent — Scotland, Nova Scotia, the Faroe Islands, New Jersey, Oregon, the Deccan Trap basalts of Gujarat in western India, the Ural Mountains. One of those occurrences turned the mineral into a small American industry. The first documented report of thomsonite from the north shore of Lake Superior was published in 1888. The pebbles were small — most less than a quarter of an inch across — but their concentric pink, green and white banding made them unlike any other Great Lakes stone. After the completion of the North Shore Highway (state Route 61) in the 1920s the locality became accessible to weekend visitors from Duluth and the Twin Cities, and a rockhound trade grew up around Grand Marais in Cook County, Minnesota. In 1962 Tania and Maurice Feigal opened the Thomsonite Beach Motel and Jewelry Shop six miles south of Grand Marais, beside their own gem-mining claim, and the place gave the beach its modern name.

The mineral kept Brooke's name straight through the 19th and 20th centuries. The current form Thomsonite-Ca dates only from 1997. That year the International Mineralogical Association adopted a new zeolite nomenclature. It split what had been called thomsonite into a series of two species, distinguished by their dominant exchangeable cation. Thomsonite-Ca, the calcium-dominant end-member, is by far the more common of the two; Thomsonite-Sr, the strontium-dominant one, is rare. The suffix -Ca on the page header reads as the chemist's shorthand: calcium dominates the cation site.

Thomsonite-Ca has almost no direct industrial use today. It is a zeolite-group mineral — built around an open framework of silicon, aluminium and oxygen with water and exchangeable cations trapped in the channels — but the commercial zeolite trade runs on other species. Detergent water-softening, agricultural soil treatment, ion-exchange water purification and catalysis in petroleum refining all rely on synthetic zeolites grown in factories, or on the natural species clinoptilolite, chabazite and mordenite. Thomsonite-Ca does not appear on that short list.

What demand exists is lapidary and decorative. Lake Superior beach nodules are worked into cabochons — domed, polished gemstones cut from radial pebbles with concentric pink, green and white banding. The pieces are small (most less than a quarter of an inch across) and opaque, but their banding is unlike any other Great Lakes stone. A specialist trade still operates from Grand Marais, Minnesota, where the Thomsonite Beach Inn and a handful of nearby jewellers buy, cut and sell the local material. Radial Indian specimens from the Deccan Trap basalts of Gujarat and Faroese spherules also reach museum cabinets and high-end specimen collections.

The remaining demand is scientific. Thomsonite-Ca is a classic low-temperature alteration mineral in the gas cavities — amygdales, almond-shaped holes left by trapped vapour — of weathered basaltic lava flows. Its presence, alongside other zeolites in the same cavities, helps petrologists read the burial-and-heating history of a basalt province. Each zeolite has its own narrow stability window, and the assemblage that survives tells the petrologist how hot the rock got.