Euclase
Euclase, BeAlSiO4(OH): monoclinic beryllium aluminium silicate, hardness 7.5, density 2.99 to 3.10, perfect {010} cleavage, a breakdown product of beryl.
Euclase is a beryllium aluminium silicate, BeAlSiO4(OH) on the IMA list (G, 1792, Brazil). It forms monoclinic prisms to 12 cm, colourless to deep blue or green, hardness 7.5, density 2.99 to 3.10, with a perfect {010} cleavage that gives the name. The sheet calls it a product of the decomposition of beryl in pegmatites.
In short
- One beryllium, one aluminium, one silicon. The September 2026 IMA-CNMNC Master List writes BeAlSiO4(OH), status G, 1792, country Brazil, and the Handbook of Mineralogy sheet writes the same. On standard atomic weights that is 6.21% beryllium by mass, against 5.03% in beryl.
- The 3.115 check. The sheet’s cell (a 4.763, b 14.29, c 4.618 Å, β 100°15′, Z = 4) has a volume of 309.30 ų. With the ideal formula (145.08 g/mol) that gives 3.116, the sheet’s printed D(calc.) 3.115. The measured 2.99 to 3.10 sits just below it, and a 20.00 g piece reads 13.31 to 13.55 g in water.
- Both analyses reproduce. Worked from the printed oxide weights, the Ottré, Belgium analysis gives Be 1.00, Al 0.95, Si 1.00 and OH 1.01 per silicon, exactly as the sheet prints. The Santo do Encoberto, Brazil microprobe analysis gives Be 0.99, Al 1.00 and Si 1.01 on five oxygens, as printed, and hydroxyl 0.965 against the printed 0.96.
- Documented failure: the balance and the hardness. Phenakite, Be2SiO4, reads 13.17 to 13.33 g for 20.00 g on its sheet, overlapping euclase’s 13.31 g end, and its ω 1.650 to 1.656 straddles euclase’s α. Optic class separates them: phenakite is uniaxial (+), euclase biaxial (+). The International Gem Society euclase article gives hardness 6.5 to 7.5, variable within a single crystal; the Handbook prints 7.5.
- Where. The sheet’s Distribution names the Sanarka and Kamenka rivers in the southern Urals, Epprechtstein in Bavaria, Alto Ligonha in Mozambique, Morogoro in Tanzania, Karoi in Zimbabwe, several places in Minas Gerais, and large crystals from the Gachalá mine near Chivor, Colombia. It marks no type locality.
| Line | SiO2 wt% | Al2O3 wt% | BeO wt% | H2O wt% | Worked from the numbers | Printed on the sheet |
|---|---|---|---|---|---|---|
| Ideal BeAlSiO4(OH) | 41.41 | 35.14 | 17.24 | 6.21 | Be : Al : Si = 1 : 1 : 1; 145.08 g/mol | — |
| Ottré, Belgium, analysis (1) | 42.05 | 33.97 | 17.51 | 6.35 | Be 1.00, Al 0.95, Si 1.00, OH 1.01 | Be1.00 Al0.95 Si1.00 O4 (OH)1.01 |
| Santo do Encoberto, Brazil, analysis (2) | 41.60 | 34.76 | 16.95 | 5.95 | Be 0.99, Al 1.00, Si 1.01, OH 0.965 | (Be0.99 Fe0.01 Na0.01) Al1.00 Si1.01 O4 (OH)0.96 |
| Cell density | — | — | — | — | 3.116 (V 309.30 ų, Z = 4) | D(calc.) 3.115 |
| Measured density | — | — | — | — | 20.00 g reads 13.31 to 13.55 g in water | D(meas.) 2.99–3.10 |
What is euclase? The formula and the 3.115 check
A hydrous beryllium aluminium silicate. The September 2026 IMA list gives BeAlSiO4(OH), status G, year 1792, country Brazil, with a first reference in Observations sur la Physique, sur l’Histoire Naturelle et sur les Arts 41 (1792) and a second in Canadian Mineralogist 55 (2017). The Handbook of Mineralogy euclase sheet writes the same formula. On standard atomic weights it is 145.08 g/mol, 6.21% beryllium and 17.24% BeO by mass, and the ideal oxide column is SiO2 41.41, Al2O3 35.14, BeO 17.24, H2O 6.21.
The check. The sheet prints a monoclinic cell, space group P21/a, a 4.763, b 14.29, c 4.618 Å, β 100°15′, Z = 4. Its volume is 309.30 ų, and four formula units in it give a density of 3.116, the printed D(calc.) 3.115 to the third decimal. The measured range, 2.99 to 3.10, is lower; the sheet does not say why, and this page weighs against the measured range only.
The analyses agree with the formula. The Ottré analysis, recalculated by the sheet to 100.00% after deducting 3.5% quartz, works out at Be 1.00, Al 0.95, Si 1.00, OH 1.01 per silicon, as printed. The Santo do Encoberto microprobe analysis gives Be 0.99, Al 1.00, Si 1.01 on five oxygens, with traces of Fe, Na and K. Whether euclase is a beryl is answered on is euclase a beryl, which uses a different check.
Why is it called euclase? The perfect cleavage
Named for breaking easily. The sheet derives the name from the Greek for easily and fracture, regarding its perfect cleavage. The cleavage is perfect on {010}, with imperfect cleavages on {110} and {001}; the fracture is conchoidal and the tenacity brittle. Hardness is 7.5, VHN 1310, so euclase is hard but splits readily along one plane; see cleavage or fracture: how to tell.
What the hardness hides. The International Gem Society euclase article gives hardness 6.5 to 7.5 and says it may be variable within a single crystal. It also says the perfect cleavage in one direction makes faceting tricky. The Handbook prints the single figure 7.5. A scratch test on a crystal with a perfect cleavage risks splitting it; method on how to test mineral hardness properly.
How do you tell euclase from phenakite, topaz and beryl?
Optics first. Euclase is biaxial (+), α 1.651 to 1.653, γ 1.669 to 1.675. Phenakite, Be2SiO4, is uniaxial (+), ω 1.650 to 1.656, ε 1.667 to 1.670, so its ordinary index overlaps euclase’s lowest. Beryl is uniaxial (−) with no index above 1.610. Topaz is biaxial (+) too, but denser: 3.49 to 3.57 on its sheet, so 20.00 g reads 14.27 to 14.40 g in water.
Documented failure: the balance. A 20.00 g piece reads 13.31 to 13.55 g in water as euclase (2.99 to 3.10), 13.17 to 13.33 g as phenakite (2.93 to 3.00) and 12.40 to 13.27 g as beryl (2.63 to 2.97). Euclase and phenakite overlap between 13.31 and 13.33 g, and euclase clears beryl by 0.045 g. Weighing is set out on how to measure specific gravity at home; on these numbers it sorts beryl from euclase only at the margin and cannot sort euclase from phenakite.
Colour does not help. The euclase sheet runs colourless, white, pale to deep yellowish green, greenish blue and pale to deep blue; the beryl sheet covers the same blues and greens. The IGS says some aquamarines have been misrepresented as euclase; that question, beryl or not, has its own page.
Where is euclase found? Brazil, Colombia, Zimbabwe and the Urals
As the sheet prints it. Fine crystals occur from the Sanarka River, Orenburg district, and the Kamenka River, Southern Ural Mountains, Russia; at Epprechtstein, Bavaria, Germany; in the Muiâne pegmatite, Alto Ligonha district, Mozambique; near Mikese Station, Morogoro district, Tanzania; and from Karoi, Miami district, Zimbabwe. In Brazil, in Minas Gerais, around Villa Rica, Boa Vista, Santa do Encoberto, Dom Bosco and Ouro Prêto, at Mãe dos Homens in Capelinha, and many other localities. Large crystals come from the Gachalá mine, Las Cruces, near Chivor, Colombia. The sheet marks no type locality; see what is a type locality.
Where it forms. The sheet calls euclase the product of decomposition of beryl in pegmatites, and also of low-temperature alpine veins. Its associates are feldspar, quartz, topaz, beryl, mica, calcite, ankerite and chlorite. Its Belgian analysis comes from Ottré, where reference (6) reports euclase in a quartz vein. Chrysoberyl is another beryllium mineral on this site; the rest are in the species index.
Is euclase a gemstone? The International Gem Society treats it as one: it says Brazil produces most cuttable, gem-quality euclase and that radiation treatments can turn colourless euclase green or blue. The Handbook says nothing about cutting or treatment. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.