Datolite
Datolite, CaB(SiO4)(OH), is monoclinic: hardness 5 to 5.5, density 2.96 to 3.00, no cleavage printed. Hardness plus weight part it from nine cavity-mates.
Datolite is calcium boron silicate, CaB(SiO4)(OH) on the IMA list (G, 1806, Norway): monoclinic, colorless or white with a greenish tinge, hardness 5 to 5.5, density 2.96 to 3.00, no cleavage printed. A 20.00 g piece reads 13.24 to 13.33 g in water. Of the nine associate sheets checked, none matches it on both hardness and weight.
In short
- The species. The Handbook of Mineralogy sheet gives CaBSiO4(OH), monoclinic, in short prismatic crystals to 12 cm or botryoidal, granular and cryptocrystalline masses; fracture conchoidal to uneven, no cleavage printed; hardness 5 to 5.5; density 2.96 to 3.00; colorless or white, commonly with a greenish tinge; biaxial (−). The September 2026 IMA-CNMNC Master List writes it CaB(SiO4)(OH), status G, 1806, Norway.
- The two-reading rule. The sheet names its associates as calcite, prehnite, zeolites, danburite, axinite and garnet. Against 9 sheets for them, hardness 5 to 5.5 and an in-water weight of 13.24 to 13.33 g for 20.00 g together part datolite from all 9: 4 match it on one reading, 5 on neither, none on both.
- Documented failure: either reading alone. Weight alone fails twice: prehnite reads 12.86 to 13.22 g, 0.02 g away, and danburite reads 13.17 to 13.38 g, containing datolite’s whole range. Hardness alone fails twice: the natrolite and analcime sheets print exactly 5 to 5.5. Each of the four is parted by the other reading.
- Boron, worked. Ideal CaB(SiO4)(OH) is 21.76% B2O3, 37.56% SiO2, 35.05% CaO and 5.63% H2O on standard atomic weights; boron alone is 6.76%. The sheet’s Silver Harbour, Lake Superior, Ontario, analysis prints 19.38% B2O3, 89.1% of ideal, which the sheet writes as B0.90.
- Light and magnet. The sheet says datolite commonly fluoresces blue under SW UV; a ligature-aware search finds no such line on the prehnite, danburite or analcime sheets. USGS Open-File Report 99-529, Table 1 left its one datolite sample behind at 1.70 amp, the separator’s top. Exceptional crystals are named from Dal’negorsk.
| Name | Formula (IMA), status | Hardness | Density | 20.00 g piece in water | Cleavage (sheet) | UV (sheet) | Matches datolite on |
|---|---|---|---|---|---|---|---|
| Datolite | CaB(SiO4)(OH); G, 1806 | 5–5.5 | 2.96–3.00 | 13.24–13.33 g | None printed | Commonly fluoresces blue, SW | — |
| Calcite | Ca(CO3); G, 1836 | 3 | 2.7102 | 12.62 g | Perfect, {10-11} | May fluoresce red, blue, yellow and other colors, SW and LW | Neither |
| Prehnite | Ca2Al(Si3Al)O10(OH)2; G, 1788 | 6–6.5 | 2.80–2.95 | 12.86–13.22 g | Good | None printed | Weight only (0.02 g) |
| Natrolite (zeolite) | Na2(Si3Al2)O10·2H2O; A, 1997 s.p. | 5–5.5 | 2.20–2.26 | 10.91–11.15 g | Perfect | Commonly fluoresces orange to yellow | Hardness only |
| Analcime (zeolite) | Na(AlSi2O6)·H2O; A, 1997 s.p. | 5–5.5 | 2.24–2.29 | 11.07–11.27 g | Very poor | None printed | Hardness only |
| Danburite | CaB2Si2O8; G, 1839 | 7–7.25 | 2.93–3.02 | 13.17–13.38 g | Indistinct | None printed | Weight only (contains it) |
| Axinite-(Fe) | Ca4Fe2+2Al4[B2Si8O30](OH)2; Rn, 1968 s.p. | 6.5–7 | 3.25–3.28 | 13.85–13.90 g | Good | None printed | Neither |
| Axinite-(Mn) | Ca4Mn2+2Al4[B2Si8O30](OH)2; Rn, 2004 s.p. | 6.5–7 | 3.314 | 13.96 g | Good | None printed | Neither |
| Grossular (garnet) | Ca3Al2(SiO4)3; A, 1962 s.p. | 6.5–7 | 3.594 | 14.44 g | Parting, rarely observed | May fluoresce weak golden yellow, LW or SW | Neither |
| Andradite (garnet) | Ca3Fe3+2(SiO4)3; G, 1868 | 6.5–7 | 3.8–3.9 | 14.74–14.87 g | None printed | None printed | Neither |
What is datolite? A calcium borosilicate of cavities and skarns
Datolite is CaB(SiO4)(OH), a monoclinic calcium boron silicate, colorless to white with a greenish tinge. The Handbook of Mineralogy datolite sheet gives point group 2/m, space group P21/c: crystals typically short prismatic, with prominent {110} and {011}, or tabular on {010}, to 12 cm; also botryoidal or globular with a columnar structure, granular to compact, and cryptocrystalline. Fracture conchoidal to uneven, brittle, hardness 5 to 5.5, measured density 2.96 to 3.00 against a calculated 3.00. The sheet prints no cleavage and no streak.
Colour and optics. Colorless or white, commonly with a greenish tinge; may be grayish, yellow, green, red or pink; vitreous, rarely subresinous on a fracture surface. Biaxial (−), α 1.622 to 1.626, β 1.649 to 1.654, γ 1.666 to 1.670, 2V 72° to 75°, read from the rendered sheet; the birefringence, worked, is 0.040 to 0.048.
What the lists record. The September 2026 IMA list writes CaB(SiO4)(OH), status G, 1806, Norway; the sheet writes CaBSiO4(OH), the same atoms, and marks no type locality. The sheet places datolite in the gadolinite group. The IMA list has no group column; a field scan and a token scan each find exactly 1 row written with B(SiO4)(OH), datolite’s own, and 4 rows built only of calcium, boron, silicon, oxygen and hydrogen: datolite, howlite, oyelite and shinichengite. The name is from the Greek to divide, for the granular structure of a massive variety.
Datolite or prehnite, calcite or a zeolite? The two-reading rule
Hardness and weight, taken together, part datolite from every associate sheet checked; either one alone fails twice. Method: the sheet’s Association line is calcite, prehnite, zeolites, danburite, axinite, garnet. Each was represented by a cached Handbook sheet: calcite; prehnite; the zeolites natrolite and analcime, chosen because their sheets print datolite’s own 5 to 5.5; danburite; axinite-(Fe) and axinite-(Mn) on the ferroaxinite and manganaxinite sheets (see axinite); and the calcium garnets grossular and andradite. A hardness match is any overlap of the printed ranges; a weight match is an in-water reading for a 20.00 g piece that overlaps datolite’s 13.24 to 13.33 g or lies within 0.05 g of it.
Result. Of 9 sheets, 0 match datolite on both readings, 4 on one and 5 on neither. Calcite, at hardness 3 and 12.62 g, fails both and has perfect {10-11} cleavage where datolite prints none. The axinites read 13.85 to 13.96 g and the garnets 14.44 to 14.87 g, all at hardness 6.5 to 7. The methods are on how to test mineral hardness properly and how to measure specific gravity at home.
Documented failure: either reading alone. On weight, prehnite reads 12.86 to 13.22 g, 0.02 g under datolite, the figure is prehnite a zeolite also gives; danburite, at 2.93 to 3.02, reads 13.17 to 13.38 g and contains datolite’s whole range. On hardness, natrolite and analcime both print 5 to 5.5. Hardness then parts prehnite (6 to 6.5) and danburite (7 to 7.25), and weight parts natrolite (10.91 to 11.15 g) and analcime (11.07 to 11.27 g) by about 2 g. The zeolite question itself is on which minerals are zeolites.
How much boron is in datolite?
Ideal CaB(SiO4)(OH) is 21.76% B2O3, or 6.76% boron, by mass. Worked on standard atomic weights (Ca 40.078, B 10.81, Si 28.085, O 15.999, H 1.008), formula mass 159.98: as oxides, B2O3 21.76%, SiO2 37.56%, CaO 35.05% and H2O 5.63%.
Worked on the sheet’s analysis. The Silver Harbour, Lake Superior, Ontario, analysis prints SiO2 38.00, B2O3 19.38, CaO 35.39, H2O+ 5.71, with small Al2O3, Fe2O3, MnO and MgO, total 100.15. Its B2O3 is 89.1% of the ideal 21.76, and the sheet’s formula for it carries B0.90 against Ca1.02, Mg0.02 and Mn0.01. The sheet does not say where the shortfall goes, and this page does not guess.
Its neighbours in boron. Danburite, on its own page, is CaB2Si2O8, 28.32% B2O3; howlite is 13.81% boron. Both figures are worked on those pages and not repeated here. The sheet calls datolite a secondary mineral in mafic igneous rocks, in geodes in tuffs, in skarns in limestones, in serpentinites and hornblende schists, and from some ore veins.
Where is datolite found? Does it glow or pull a magnet?
Where it is found. The sheet calls datolite widespread and names some localities for fine specimens: Arendal and north of Hardangerfjord, Norway; St. Andreasberg, Harz Mountains, Germany; Serra dei Zanchetti near Bologna and Alpe di Siusi, Italy; the Kratzenberg, Habachtal, Austria; the Iwato copper mine, Miyazaki Prefecture, Japan; exceptional crystals from Dal’negorsk, Primorskiy Kray, Russia; large crystals from the Lane quarry, Westfield, Massachusetts; the Roncari quarry, East Granby, Connecticut; Bergen Hill, Great Notch, Paterson and Prospect Park, New Jersey; the Goose Creek quarry near Leesburg, Virginia; and very large crystals from Charcas, San Luis Potosí, Mexico. This page names no locality the sheet does not.
Under UV. The sheet says datolite commonly fluoresces blue under SW UV. A ligature-aware search of the nine associate sheets finds fluorescence wording on calcite, natrolite and grossular, and none on prehnite, danburite, analcime, either axinite or andradite; the hemimorphite and willemite sheets, controls, match. The sheet names no cause, and this page offers none. The general subject is on fluorescent minerals explained.
Magnet. USGS Open-File Report 99-529 (Rosenblum and Brownfield) lists datolite with a total and best range above 1.70 amp, on 1 sample: nothing was pulled at the separator’s maximum. The sheet prints no magnetic property. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.