DATOLITE  , a

mineral species consisting of basic calcium and boron orthosilicate, Ca(BOH)SiO4 . It was first observed by J . Esmark in ,8o6, and named by him from Sareia-Bau, " to divide," and ALOoc, " 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 . Levy 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 5-52; 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 .