DATOLITE, a mineral species consisting of basic calcium and boron orthosilicate, Ca(BOH)SiO4. It was first observed by J. Esmark in 1806, and named by him from δατεῖσθαι, “to divide,” and λίθος, “stone,” in allusion to the granular structure of the massive mineral. It usually occurs as well-developed glassy crystals bounded by numerous bright faces, many of which often have a more or less pentagonal outline. The crystals were for a long time considered to be orthorhombic, and indeed they approach closely to this system in habit, interfacial angles and optical orientation; humboldtite was the name given by A. Lévy in 1823 to monoclinic crystals supposed to be distinct from datolite, but the two were afterwards proved to be identical. The mineral also occurs as masses with a granular to compact texture; when compact the fractured surfaces have the appearance of porcelain. A fibrous variety with a botryoidal or globular surface is known as botryolite. Datolite is white or colourless, often with a greenish tinge; it is transparent or opaque. Hardness 5-512; specific gravity 3.0.
Datolite is a mineral of secondary origin, and in its mode of occurrence it resembles the zeolites, being found with them in the amygdaloidal cavities of basic igneous rocks such as basalt; it is also found in gneiss and serpentine, and in metalliferous veins and in beds of iron ore. At Arendal in Norway, the original locality for both the crystallized and botryoidal varieties, it is found in a bed of magnetite. In amygdaloidal basaltic rocks it is found at Bishopton in Renfrewshire and near Edinburgh; and as excellent crystallized specimens at several localities in the United States, e.g. at Westfield in Massachusetts, Bergen and Paterson in New Jersey, and in the copper-mining region of Lake Superior. At St Andreasberg in the Harz it occurs both in diabase and in the veins of silver ore. Fine specimens have recently been obtained from Tasmania.
Large crystals of datolite completely altered to chalcedony were formerly found with magnetite in the Haytor iron mine on Dartmoor in Devonshire; to these pseudomorphs the name haytorite has been applied. (L. J. S.)