- Stilbite with Thomsonite
- Columbia County, Oregon
- Specimen measures approx. 1 3/4″ wide
Stilbite is a common member of the zeolite group, and is found in most of the world’s important zeolite deposits. Since its original classification, Stilbite was always regarded as a single mineral species with a slightly variable elemental makeup. In 1997, the Zeolite Subcommittee of the IMA divided this mineral into individual sub-species, thereby regarding Stilbite as a series of two members: Stilbite-Ca and Stilbite-Na. Stilbite-Ca is the calcium-dominating member, and Stilbite-Na is the sodium-dominating member. Stilbite-Ca is the more common type, and almost all Stilbite in collections are of the Stilbite-Ca type. A distinction between the two Stilbite types is rarely made, and most collectors simply refer to this mineral as Stilbite. Stilbite is named for the Greek word stilbein, which means “to glitter”, in reference to the luster of this mineral which is pearly on cleavage surfaces.
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.
A mineral is a naturally occurring chemical compound. Most often, they are crystalline and abiogenic in origin. A mineral is different from a rock, which can be an aggregate of minerals or non-minerals and does not have one specific chemical composition, as a mineral does. The exact definition of a mineral is under debate, especially with respect to the requirement that a valid species be abiogenic, and to a lesser extent with regard to it having an ordered atomic structure.
The study of minerals is called mineralogy. There are over 5,300 known mineral species; over 5,070 of these have been approved by the International Mineralogical Association (IMA). The silicate minerals compose over 90% of the Earth’s crust. The diversity and abundance of mineral species is controlled by the Earth’s chemistry. Silicon and oxygen constitute approximately 75% of the Earth’s crust, which translates directly into the predominance of silicate minerals.
Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral’s geological environment when formed. Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals.
Minerals can be described by their various physical properties, which are related to their chemical structure and composition. Common distinguishing characteristics include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, and specific gravity. More specific tests for describing minerals include magnetism, taste or smell, radioactivity and reaction to acid.
Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. The silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure. All silicate minerals have a base unit of a [SiO4]4− silica tetrahedron—that is, a silicon cation coordinated by four oxygen anions, which gives the shape of a tetrahedron. These tetrahedra can be polymerized to give the subclasses: orthosilicates (no polymerization, thus single tetrahedra), disilicates (two tetrahedra bonded together), cyclosilicates (rings of tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), and tectosilicates (three-dimensional network of tetrahedra). Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates.