THE MINERAL THOMSONITE Thomsonite is an uncommon and desirable member of the zeolite group, forming in unique and interesting crystal aggregates. In 1997, the Zeolite Subcommittee of the IMA divided this mineral into two individual sub-species, thereby regarding Thomsonite as a series of two members. The series is defined by calcium and strontium end members. Common Thomsonite, which is calcium-dominating, is called Thomsonite-Ca, and is usually pure without any strontium. Strontium-dominating Thomsonite is known as Thomsonite-Sr, and is extremely rare. Almost all Thomsonite specimens in collections are Thomsonite-Ca. A distinction among the two Thomsonite types is rarely made, and the term Thomsonite is generally used without any further breakdown. An interesting habit of Thomsonite is as rounded, concentric bands, which are polished into aesthetic rounded stones. This form is most well-known at Thomsonite Beach at Grand Marais, Minnesota, on the shoreline of Lake Superior. It usually forms in association with other zeolites and green Chlorite. Thomsonite is named in honor of Scottish mineralogist and chemist Thomas Thomson.