Phenakite
Phenakite, Be2SiO4: hexagonal beryllium silicate, hardness 7.5 to 8, density 2.93 to 3.00, 16.37% beryllium. Cell and analysis checks, quartz test, localities.
Phenakite is beryllium silicate, Be2(SiO4) on the IMA list (G, 1833, Russia): hexagonal, colourless to yellow, pink or brown, hardness 7.5 to 8, density 2.93 to 3.00, 16.37% beryllium by mass. Its name is Greek for deceiver, for its likeness to quartz; a 20.00 g piece reads at least 0.72 g more than quartz in water.
In short
- The species. The Handbook of Mineralogy sheet gives a hexagonal beryllium silicate, point group -3, in rhombohedral, tabular to prismatic crystals to 20 cm, also columnar, spherulitic or granular; penetration twins about [0001]; cleavage distinct on {11-20}; conchoidal fracture; hardness 7.5 to 8; density 2.93 to 3.00; colourless, yellow, pink or brown; uniaxial (+). The September 2026 IMA-CNMNC Master List writes Be2(SiO4), status G, 1833, Russia.
- The 2.96 check. On standard atomic weights Be2SiO4 weighs 110.11: 16.37% beryllium, 45.43% BeO and 54.57% SiO2, the sheet’s ideal column exactly. Eighteen formula units in the sheet’s cell (1,111.6 Å3) give a density of 2.961, against the sheet’s calculated 2.960 and measured 2.93 to 3.00.
- The Ural analysis. The sheet’s analysis from the Ural Mountains, Russia, reads BeO 45.82 and SiO2 52.92 wt%, with 1.43 wt% of other oxides and loss on ignition. Worked, that is 2.08 beryllium per silicon against the formula’s 2: the analysed crystal is phenakite within a few per cent.
- The deceiver test. The name is for quartz. A 20.00 g phenakite reads 13.17 to 13.33 g in water; quartz, at 2.65, reads 12.45 g, 0.72 g lighter at the closest. Phenakite’s lowest index, ω 1.650, is 0.097 above quartz’s ε 1.553. Against beryl the balance fails; that question has its own page, is phenakite a beryl.
- Where. The sheet lists the Tokovaya River near Yekaterinburg and Miass in the Urals, Kragerø in Norway, Framont in Alsace, Reckingen in Switzerland, Habachtal in Austria, Florissant and Mt. Antero in Colorado, Bald Face Mountain in New Hampshire, Amelia in Virginia, São Miguel de Piracicaba and Catuqi in Minas Gerais, the Socotó mine in Bahia, Klein Spitzkopje in Namibia and Anjanabonoina in Madagascar.
| Quantity | Printed on the sheet | Worked here | Agreement |
|---|---|---|---|
| Formula | Be2SiO4 (sheet); Be2(SiO4) (IMA) | Formula mass 110.11 | Same formula |
| BeO, ideal | 45.43 wt% | 45.43 wt% | Exact |
| SiO2, ideal | 54.57 wt% | 54.57 wt% | Exact |
| Beryllium by mass | Not printed | 16.37% | — |
| Cell volume | a 12.472, c 8.252 Å, Z 18 | 1,111.6 Å3 | — |
| Density, calculated | 2.960 | 2.961 | Within 0.001 |
| Density, measured | 2.93–3.00 | Calculated value inside the range | Yes |
| Ural analysis, BeO | 45.82 wt% | 100.9% of the ideal 45.43 | Close |
| Ural analysis, SiO2 | 52.92 wt% | 97.0% of the ideal 54.57 | Close |
| Ural analysis, Be per Si | Not printed | 2.08 (formula 2) | Within 4% |
| Ural analysis, other oxides and LOI | MgO 0.13, CaO 0.20, Na2O 0.63, K2O 0.26, LOI 0.21; Al2O3 trace | 1.43 wt% in all | — |
| Birefringence | ω 1.650–1.656, ε 1.667–1.670 | 0.011–0.020 | — |
| 20.00 g piece in water | D(meas.) 2.93–3.00 | 13.17–13.33 g | — |
What is phenakite? The hexagonal beryllium silicate
Phenakite is a beryllium silicate with no other cation in its formula, and its sheet places it in pegmatites, greisens, and alpine and hydrothermal veins. The Handbook of Mineralogy phenakite sheet gives a hexagonal mineral, point group -3, space group R-3, a 12.472, c 8.252 Å, Z 18. Crystals are rhombohedral, tabular to prismatic, less commonly long prismatic to acicular, to 20 cm; it also forms columnar aggregates, spherulites and granular masses. Twins are penetration twins, by rotation about [0001]. Its associates are topaz, beryl, chrysoberyl, fluorite, muscovite and quartz.
Physical and optical properties. Cleavage distinct on {11-20}, imperfect on {10-11}; fracture conchoidal; brittle; hardness 7.5 to 8; measured density 2.93 to 3.00, calculated 2.960. Colourless, yellow, pink or brown, and it may be mottled; vitreous; transparent to translucent. Uniaxial (+), ω 1.650 to 1.656, ε 1.667 to 1.670. The only luminescence the sheet prints is bright blue cathodoluminescence, under an electron beam; a ligature-aware search finds the ultraviolet lines on the hemimorphite and willemite sheets, the positive controls, and none on phenakite. The International Gem Society phenakite article gives pale greenish or blue in UV light, occasionally pale rose, and sometimes blue fluorescence in X-rays.
Name and type material. The sheet derives the name from the Greek for deceiver, in allusion to its similarity to quartz when colourless. Type material is at the Mining Institute, St. Petersburg, Russia, 617/22; the IMA list gives 1833 and Russia. The sheet marks no type locality; what the term means is on what is a type locality. IGS says phenakite lends its name to a mineral group in which willemite is the zinc analogue; the IMA list writes willemite Zn2(SiO4) beside phenakite’s Be2(SiO4).
Phenakite density and formula: the 2.96 check
The sheet’s own numbers reproduce: the formula in the cell gives 2.961 against the printed 2.960. Worked on standard atomic weights, Be2SiO4 weighs 110.11: 18.02 of beryllium, 28.09 of silicon and 64.00 of oxygen. That is 16.37% beryllium by mass, or 45.43% BeO and 54.57% SiO2, the sheet’s ideal column (2) to the hundredth.
The cell. For a hexagonal cell the volume is a²c·sin 60°: 12.472² × 8.252 × 0.8660 = 1,111.6 Å3. Eighteen formula units of 110.11 in that volume give 2.961 g/cm3. The measured range, 2.93 to 3.00, brackets it, and a 20.00 g piece reads 13.17 to 13.33 g in water; the method is on how to measure specific gravity at home.
The Ural analysis. The sheet’s column (1), from the Ural Mountains, Russia, reads SiO2 52.92, BeO 45.82, Al2O3 trace, MgO 0.13, CaO 0.20, Na2O 0.63, K2O 0.26 and loss on ignition 0.21, total 100.17. Converted to moles, BeO 1.832 against SiO2 0.881, or 2.08 beryllium per silicon, 4% above the formula’s 2; the other oxides and LOI come to 1.43 wt%. Read together, the analysis, the cell and the ideal column all point to the same composition.
Phenakite or quartz? The deceiver test
Weight separates phenakite from quartz: a 20.00 g phenakite reads 13.17 to 13.33 g in water and quartz 12.45 g. The quartz sheet gives density 2.65, hardness 7, uniaxial (+), ω 1.544, ε 1.553. Phenakite is denser (2.93 to 3.00), at least as hard (7.5 to 8), and its lowest index, ω 1.650, sits 0.097 above quartz’s ε. Both are uniaxial positive and both can be colourless, so sign and colour do not separate them; index and weight do. The IGS article says quartz gems may be presented, erroneously or deliberately, as phenakites, and that phenakite has the higher refractive index, birefringence and specific gravity.
Documented failure: hardness. At 7.5 to 8 against 7, a scratch test separates them by half a step at best, and on a cut stone it does damage; see how to test mineral hardness properly. The weighing does the work without marking the piece. IGS also records near-colourless phenakite rough submitted as suspected diamond; the diamond and beryl comparisons, with their numbers, are on is phenakite a beryl.
Phenakite and bertrandite. The bertrandite sheet lists phenakite among its associates, and the USGS Professional Paper 1802-E, Beryllium (Foley and others, 2017) says phenakite is stable above 250 °C and hydrates to bertrandite at lower temperatures, citing Barton (1986). The formula arithmetic for that is on bertrandite. Beryllium-bearing dust is a hazard question this page does not answer; it is routed to toxic minerals in a collection, and nothing here is advice.
Where is phenakite found? The sheet’s localities
The sheet says numerous localities and names some for fine crystals. In Russia, along the Tokovaya River, near Yekaterinburg (Sverdlovsk), and at Miass, Ilmen Mountains, Southern Urals. Several places around Kragerø, Norway. Framont, Vosges Mountains, Alsace, France. Near Reckingen, Valais, and many other places in Switzerland. Leckbachrinne, Habachtal, Salzburg, Austria. In the USA, near Florissant, El Paso Co., and on Mt. Antero, Chaffee Co., Colorado; Bald Face Mountain, North Chatham, New Hampshire; Amelia, Virginia. In Brazil, exceptional crystals in the São Miguel de Piracicaba pegmatite, 50 km east of Belo Horizonte, and at Catuqi, Minas Gerais; the Socotó mine, northwest of Salvador, Bahia. Klein Spitzkopje, Namibia. Anjanabonoina, Madagascar. The sheet names no British locality.
As a gem. The IGS article calls phenakite a very hard gem material, usually colourless, with little fire; it says faceted stones normally range from one to five carats, that laboratories have confirmed cat’s-eye stones from Madagascar and Sri Lanka, and that radiation treatment can turn colourless phenakite yellow-brown. The Handbook says nothing on cutting. This page gives no buying advice.
Beryllium neighbours. euclase and bertrandite are the other beryllium silicates of the same pegmatites, chrysoberyl the oxide; the classification questions are on is euclase a beryl and is chrysoberyl a beryl. The reference section continues at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.