Datolite

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Formula: CaB(SiO₄)(OH)
Nesosilicate (insular SiO₄ groups), gadolinite group, boron-bearing mineral
Bakerite is a microcrystalline, boron-rich variety of datolite (Mindat)
Crystal System: Monoclinic
Specific gravity: 2.9 to 3.0
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, yellow, greenish, seldom grey, reddish
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Mn,Mg,Al,Fe
Environments:

Metamorphic environments
Hydrothermal environments
Basaltic cavities

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 zeolites, prehnite, apophyllite and calcite.
Common associates include axinite, calcite, danburite, garnet, prehnite and zeolites (HOM, Mindat).

Localities

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).
Datolite from Huangshi - Image

At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Datolite occurs as yellow-greenish, monoclinic crystals rarely reaching more than 1 cm. Attractive specimens were found on the Shalonggou dumps. It is not associated with other borate species. Datolite is also confirmed at Shijiangshan from a find in 2019 and identified as variety bakerite (R&M 96.5.401).
Datolite from Shijiang Shan - Image

At Charcas, Charcas Municipality, San Luis Potosí, Mexico, the primary minerals are sphalerite, galena, chalcopyrite, bornite, tetrahedrite, arsenopyrite, pyrite and silver minerals such as jalpaite, diaphorite and acanthite. In the host rock, as metamorphic or alteration minerals, danburite, datolite, hedenbergite, epidote, chlorite, andradite, actinolite and wollastonite have been reported.
Quartz, calcite and danburite crystallised during the entire life of the systems, throughout the intrusive emplacement, metamorphism, and mineralising events. With depth, both sphalerite and galena decrease while chalcopyrite increases.
Secondary sulphides formed include bornite, covellite, digenite and chalcocite. Native silver, native gold, hematite and goethite were deposited after the sulphides (Minrec 55.6.727-728).
Datolite is abundant in all of the Charcas mines; it comes in a range of colours, with shades of seafoam green being the most common, but it can also be grey, blue, and pink, and also ochre-coloured due to inclusions of iron oxides. It is more common in massive form than in crystals, but where crystals occur they are usually short-prismatic, but botryoidal to granular or cryptocrystalline masses are also known. Some of the best specimens are crystal clusters up to 15 cm across. Specimens of amethyst crystals on green or pink datolite crystals to 5 mm coating white quartz make for attractive contrasts.
Between 2010 and 2015, several finds in the San Bartolo mine produced an abundance of seafoam-green datolite crystals up to 10 cm on edge (Minrec 55.6.756).
Associated minerals include danburite, pyrite, sphalerite and chalcopyrite (R&M 85.4.338-345).
Datolite from the San Bartolo Mine, Charcas - Image

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).
Datolite from the Pervomaiskii quarry - IMage

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).

The Central Mine, Central, Keweenaw county, Michigan, USA, initially targeted a series of sub-parallel mineralised fissure veins where the most copper-rich portion of the vein was close to the base of the main greenstone flow.
The Central mine has produced outstanding datolite specimens over its entire lifetime. It is common to find excellent examples of polished datolite nodules from the Central mine in early collections, pre-1900. Typically, these datolite nodules are opaque white with brown to tan mottling. In 1995, when crushing of the Central No. 2 dump for roadfill was occurring, local collectors recovered many excellent 2 to 10-cm datolite nodules that are a translucent cream to brownish colour, as well as porcelain-white. Additionally, the dumps of the nearby Central Exploration mine have produced attractive white to pink datolite nodules to 10 cm, many showing a sugary texture (MinRec 54.1.53-81).

At the Copper Falls Mine, Copper Falls, Keweenaw county, Michigan, USA, mineralisation occurs primarily in hydrothermal veins cutting preexisting lavas and as amygdules in the Ashbed flow.
Datolite occurs commonly as rounded, cryptocrystalline masses which show colours ranging from pink to white and yellow, with occasional inclusions of native copper. The mine has also produced significant numbers of otherwise rare (in the Lake Superior District) crystals of datolite. These may reach 5 mm or more and are typically colourless but commonly carry inclusions of native copper, sometimes so dense as to make them look pink (MinRec 54.1.107).

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).

The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre basalt cliff. A curious feature of the impressive thickness of the greenstone flow here is that it contains zones of “pegmatoid”: areas where slow cooling in the core of the lava flow allowed for large feldspar crystals exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff fissure (vein). Although mineralised with copper to some extent along its entire length, the part of the vein just below the greenstone flow carried the richest copper mineralisation by far. A significant amount of the copper recovered at the Cliff mine came from amygdaloids in the tops of 13 basalt flows which were cut by the Cliff vein. The discovery and mining of this vein proved that the veins were the source of the large masses of float copper that were already well known, and proved that the primary ore mineral in the district was native copper, not sulphides, as had been suspected earlier.
Datolite occurs here primarily as nodules of massive material, typically coloured reddish by minute copper inclusions. It also occurs as small, colourless crystals in the Cliff vein. In some places, a significant portion of the vein filling material is datolite (MinRec 54.1.25-49).

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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