Nesosilicate (insular SiO4 groups), gadolinite group, boron-bearing mineral
Specific gravity: 2.9 to 3.0
Hardness: 5 to 5½
Colour: Colourless, white, yellow, greenish, seldom grey, reddish
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Mn,Mg,Al,Fe
Datolite is a secondary mineral in mafic
igneous rocks, as geodes in tuff, in limestone
skarn, in serpentinite and hornblende
schist and in ore veins (HOM, Mindat). It is usually found in cavities in
basalt lavas and similar rocks, associated with
apophyllite and calcite.
Common associates include axinite, calcite, danburite, garnet, prehnite and zeolites (HOM, Mindat).
At the Cleary Brothers quarry, Albion Park, New South Wales, Australia, datolite crystals to 5.6 cm occur on a quartz matrix which overlies laumontite. Asociated minerals include prehnite, calcite and pyrite (AJM 19.1.38).
Near Allendale, Hunter valley, New South Wales, Australia, datolite occurs as a secondary minerals in andesite. It is found as crusts coating calcite, and as crystals associated with later stellerite. Datolite has also been found as small crystals associated with laumontite crystals to 1 cm in cavities between andesite pillows (AJM 18.2.32).
At Huangshi Prefecture, Hubei Province, China, datolite occurs as crystals to 7 cm with apophyllite (R&M 85.4.338-345).
At Charcas, San Luis Potosí, Mexico, datolite occurs as crystals to 8 cm associated with danburite, pyrite, sphalerite, and chalcopyrite (R&M 85.4.338-345).
The type locality is the Nødebro mine, Øyestad, Arendal, Aust-Agder, Norway.
Pseudomorphs of datolite after quartz have been reported from Russia (R&M 85.4.338-345).
At the Pervomaiskii quarry, Trudolyubovka Village, Bakhchysarai, Crimea, Russia, datolite is associated with babingtonite (R&M 85.4.338-345).
At Dalnegorsk, Primorskiy Kray, Russia, some of the finest datolite specimens known have been recovered from the contact metamorphic boron deposits (R&M 85.4.338-345). Twinned crystals of datolite are also found here. Datolite is at least partially derived from the alteration of earlier danburite. Associated minerals include quartz, calcite, wollastonite, hedenbergite, andradite-grossular, fluorapophyllite and axinite-(Mn). Datolite mineralisation completely replaces the limestone matrix in the central portion of the deposit (MinRec 40.2.127-129).
At the Palabora mine, Loolekop, Phalaborwa, Limpopo, South Africa, datolite occurs only in the upper portion of the Main Dike fracture zone, as specks on the altered dolerite of the cavity walls, and as spherulitic aggregates to 1 mm with calcite. Later fluorapophyllite sometimes overgrows the datolite. It is suggested that datolite can form from the interaction of boron-enriched hydrothermal fluids with calcite (R&M 92.5.438). Datolite has also been found with zeolites (R&M 85.4.338-345).
At the Wessels Mine, Hotazel, Kalahari manganese field, Northern Cape, South Africa, crystals of datolite occur with hydroxyapophyllite, pectolite and baryte (R&M 85.4.338-345).
At Graubünden, Switzerland, datolite occurs with danburite and axinite (R&M 85.4.338-345).
At Barrasford Quarry, Chollerton, Northumberland, England, UK, datolite occurs both as a partial filling of pectolite- and stevensite- bearing vesicles, and as a constituent of small concentrations of pegmatitic dolerite beneath these vesicles. A single specimen has been found of crystals of glassy datolite up to 5 mm across lining a vesicle. The datolite overgrows a few quartz crystals and is overgrown by radiating clusters of crystalline pectolite up to 20 mm long (JRS 21.8).
At the Merehead Quarry, Cranmore, Mendip, Somerset, England, UK, datolite and apophyllite were found encrusting botryoidal goethite and associated with baryte and calcite (JRS 15.25). The datolite occurs as crusts on radiating aggregates of baryte, apophyllite and calcite. On these crusts individual crystals of apophyllite, baryte or calcite up to 10 mm across can sometimes be found (JRS 13.10-11).
At Glenfarg, Perth and Kinross, Scotland, UK, datolite is associated with analcime (R&M 85.4.338-345).
At Bishopton, Renfrewshire, Scotland, UK, datolite is associated with analcime (R&M 85.4.338-345).
At the San Carlos mine at Kearsarge, Inyo County, California, USA, massive datolite occurs with vesuvianite and garnet (R&M 85.4.338-345).
Near Cloverdale, Mendocino county, California, USA, datolite replaces vesuvianite (R&M 85.4.338-345).
At the Roncari quarry at East Granby, Hartford county, Connecticut, USA, datolite is asociated with apophyllite and prehnite (R&M 85.4.338-345).
In the northeastern United States datolite is widespread, associated with natrolite, analcime, stilbite, heulandite, pectolite, apophyllite and calcite (R&M 85.4.338-345).
At the Cheapside quarry at Deerfield, Franklin County, Massachussetts, USA, datolite occurs with prehnite (R&M 85.4.338-345).
At the Upper New Street quarry, Paterson, Passaic county, New Jersey, USA, a datolite pseudomorph after anhydrite has been found, with dozens of datolite crystals forming a flat cast where a blade of anhydrite once existed, but has since dissolved away (KL p223). Datolite pseudomorphs after glauberite are also found in New Jersey (R&M 85.4.338-345).
At the Mesnard mine, Hancock, Houghton county, Michigan, USA, fine nodules of yellow datolite to 12 cm have been found embedded in dense basalt (R&M 81.4.272-273).
At the Delaware Mine, Keweenaw county, Michigan, USA, a boulder of altered basalt was broken up to extract nodules of datolite that were in vein zones and in the basalt itself. The nodules had pale red centres, due to hematite inclusions (R&M 81.4.274-275).
At the Ojibway Mine, Ojibway, Keweenaw county, Michigan, USA, datolite nodules are found containing densely concentrated, minute, copper inclusions that can fill up to 80 percent of the nodule. The smaller nodules generally have higher concentrations of copper and may show partial oxidation, which results in dispersion halos of tenorite around the minute copper inclusions (R&M 86.3.220-225).
At Point Prospect, Keweenaw county, Michigan, USA, a pocket of datolite nodules to 7.6 cm across was discovered in a copper-bearing fissure vein (R&M 81.4.275-276).
At the Ridge Mine, Mass Consolidated Mine, Mass City, Ontonagon county, Michigan, USA, datolite nodules to 6 cm occur containing heavy copper inclusions (R&M 86.3.220-225).
At the Wright pocket, on the north shore of Lake Superior between Duluth and Two Harbors, St Louis county, Minnesota, USA, good quality nodules of porcelaneous datolite were found, with very little, if any, visible native copper in the datolite. The pocket is in a shear zone within basalt lava flows (R&M 95.3.276-278).
At the Braen quarry, Haledon, Passaic county, New Jersey, USA, datolite is the most common mineral after calcite. It occurs as drusy plates of crystals to 1.3 cm lining cavities. Minerals that form on the crystal faces include prehnite, apophyllite, calcite, hematite, heulandite and sulphides. Datolite epimorphs after anhydrite are often found (MinRec 40.6.517).
At the Fanwood quarry, Somerset county, New Jersey, USA, datolite occurs as compact masses of microcrystals with prehnite, calcite, albite and zeolites (MinRec 41.2.167).
At the danburite locality near Russell, St Lawrence county, New York, USA, danburite occurs in irregular pods in a layered diopside-danburite metasedimentary rock. A late-stage mineral assemblage, probably a low-temperature hydrothermal phase, includes datolite, quartz variety amethyst, quartz partially replacing danburite, dravite, pyrite and tremolite. The datolite occurs as fine-grained crystals in fractures and as patches on danburite crystals (R&M 86.2.175). It is a retrograde alteration product of danburite (R&M 90.3.217).
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