Vermiculite from Idaho, USA.
A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.
The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).
Vermiculite is one of many types of mica, a group of phyllosilicate minerals. The most common micas are muscovite, which consists of clear, thin, flexible sheets, and biotite, which consists of black sheets. (See:
www.flickr.com/photos/jsjgeology/albums/72157664164534424 and
www.flickr.com/photos/jsjgeology/albums/72157678659640066 ) Micas easily weather - they take on water (H2O) and potassium ions escape. When that happens to muscovite, the breakdown products are illite or sericite, If this happens to biotite mica or phlogopite mica, the breakdown product is vermiculite.
Vermiculite is a hydrous magnesium hydroxy-aluminosilicate mineral, (Mg,Fe,Al)3(Al,Si)4O10(OH)2ยท4H2O. It has a nonmetallic luster, varies in color, is soft (H = 1.5 to 2 on the Mohs Hardness Scale), and has one perfect cleavage. Vermiculite is famous for expanding into worm-like masses when heated as it takes on extra water between layers. Expanded vermiculite is fireproof and chemically inert - it is used to package chemicals.
Locality: unrecorded / undisclosed site in Idaho, USA
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Photo gallery of vermiculite:
www.mindat.org/gallery.php?min=4170
