Olivine
Olivine is a group name, not a species: forsterite Mg2SiO4 and fayalite Fe2SiO4 are its common ends, hardness 6.5 to 7. How to read an olivine or peridot label.
Olivine is a group name, not a species: no row on the September 2026 IMA list is called olivine. A label reading olivine usually means the forsterite-fayalite series, Mg2SiO4 to Fe2SiO4: orthorhombic, hardness 6.5 to 7, density 3.275 to 4.392. Which species it is depends on whether magnesium or iron dominates; peridot is the gem name.
In short
- No mineral called plain olivine is on the September 2026 IMA-CNMNC Master List. A search of the list for the word returns one row, calcio-olivine, Ca2(SiO4), status Rd, 2007 s.p.: a separate calcium species that shares its formula with larnite. A label reading olivine is a group determination; the species are forsterite, fayalite, tephroite and liebenbergite.
- Four Handbook sheets file themselves under Olivine group. We read the Mineral Group field on the seven Handbook of Mineralogy sheets with an orthorhombic Pbnm cell and a two-cation SiO4 formula. Forsterite, fayalite, tephroite and liebenbergite print Olivine group. Monticellite, kirschsteinite and glaucochroite print no group, and we add none.
- The 50% line, worked. Half-and-half MgFeSiO4 is 23.40% MgO and 41.71% FeO by mass on standard atomic weights. A magnesium-iron olivine analysis above 23.40% MgO is forsterite; below it, fayalite. The species is named for the dominant cation, which is the largest, not necessarily a majority.
- Density separates the ends, not the middle. A 20.00 g piece weighs 13.89 g in water as forsterite (3.275) and 15.45 g as fayalite (4.392), the same method as the live amethyst and fluorite pages. A reading between the two does not convert to a composition without a calibration this page does not have.
- Peridot is the gem name. The USGS gemstone list gives Peridot [Olivine]: hardness 7, specific gravity 3.22 to 3.45, olive to lime green, magnesium iron silicate. The naming question is set out on is olivine the same as peridot and is not repeated here.
| Species | IMA formula | IMA status, year | Group on the Handbook sheet | Hardness | Density (measured) | 20.00 g piece in water | Series on the sheet | Dominant cation in the sheet's analysis |
|---|---|---|---|---|---|---|---|---|
| Forsterite | Mg2(SiO4) | G; 1824 | Olivine group | 7 | 3.275 | 13.89 g | With fayalite; with tephroite | Mg 98.9% (Mogok, Myanmar) |
| Fayalite | Fe2+2(SiO4) | G; 1840 | Olivine group | 6.5 to 7 | 4.392 | 15.45 g | With forsterite; with tephroite | Fe 99.6% (Brocken massif, Germany) |
| Tephroite | Mn2+2(SiO4) | G; 1823 | Olivine group | 6 | 3.87 to 4.12 | 14.83 to 15.15 g | With fayalite; with forsterite | Mn 89.6% (Benallt mine, Wales) |
| Liebenbergite | Ni2(SiO4) | A; 1972-033 | Olivine group | 6 to 6.5 | 4.60 | 15.65 g | None printed | Ni 75.4% (Bon Accord, South Africa) |
| Monticellite | CaMg(SiO4) | G; 1831 | None printed | 5 to 5.5 | 3.03 to 3.27 | 13.40 to 13.88 g | With kirschsteinite | Not applicable: Ca and Mg are both essential |
| Kirschsteinite | CaFe2+(SiO4) | G; 1957 | None printed; space group given by analogy to the olivine group | Not determined | 3.434 | 14.18 g | With monticellite | Not applicable: Ca and Fe are both essential |
| Glaucochroite | CaMn2+(SiO4) | G; 1899 | None printed | About 6 | 3.407 | 14.13 g | None printed | Not applicable: Ca and Mn are both essential |
Is olivine a mineral? A group name with four Handbook members
First check: the IMA list. The September 2026 IMA-CNMNC Master List has no row for olivine. A case-insensitive search of the list finds the word once, in calcio-olivine, Ca2(SiO4), status Rd, 2007 s.p. Larnite, also Ca2(SiO4), is a separate row with the same formula, so the formula alone does not name a species. The rows that matter for an olivine label are forsterite, Mg2(SiO4), G, 1824, Italy; fayalite, Fe2+2(SiO4), G, 1840, Portugal; tephroite, Mn2+2(SiO4), G, 1823, USA; and liebenbergite, Ni2(SiO4), A, IMA 1972-033, South Africa.
Second check: what the Handbook sheets say about themselves. We fetched the seven Handbook sheets for species with an orthorhombic Pbnm cell and a two-cation SiO4 formula, and read each Mineral Group field by script. Four print Olivine group: forsterite, fayalite, tephroite and liebenbergite. Monticellite, kirschsteinite and glaucochroite print none. The kirschsteinite sheet gives its space group as Pbnm by analogy to the olivine group, which is a statement about its structure, not its group. This page does not add them to the group.
This page is filed under olivine deliberately, as the reference page for that label name. The apatite, tourmaline and axinite pages treat their group names the same way. The magnesium end member has its own page, forsterite, with the Mogok Fo-number calculation and the hardness comparison against green look-alikes.
Forsterite or fayalite? The 50 per cent line, worked
The rule: name the specimen for the dominant cation at the M sites. The IMA-CNMNC’s rule of the dominant constituent is the general principle. For a series with only magnesium and iron, dominance flips at the midpoint, MgFeSiO4. On standard atomic weights that composition has a formula mass of 172.23 and is 23.40% MgO, 41.71% FeO and 34.89% SiO2. Above 23.40% MgO the olivine is forsterite; below it, fayalite. The end members are 57.29% MgO (forsterite) and 70.51% FeO (fayalite).
Worked on the Handbook’s own analyses, dividing each oxide percentage by its formula mass and comparing the cations:
- Mogok district, Myanmar: MgO 57.83, FeO 1.11. Mg is 98.9% of the cations: forsterite. The forsterite page shows this one in full.
- Brocken massif, Germany: FeO 69.48, MnO 0.28. Fe is 99.6%: fayalite.
- Benallt mine, Wales: MnO 65.23, CaO 2.32, MgO 1.98, FeO 1.20. Mn is 89.6%: tephroite, the manganese member.
- Bon Accord, South Africa: NiO 56.32, MgO 6.50, FeO 4.37, CoO 1.80. Ni is 75.4%: liebenbergite, the nickel member.
The 23.40% line holds only for a binary. Once manganese, nickel or calcium enter, compare cation proportions, not one oxide. Pure end members by mass: forsterite is 34.55% magnesium, fayalite 54.81% iron, tephroite 54.41% manganese and liebenbergite 56.04% nickel. A label reading olivine with no analysis should stay a group label, and the form is on how to label and catalogue a mineral collection.
Can you tell olivines apart by weighing them? Only the ends
Weigh in air, then suspended in water. A 20.00 g piece weighs 20 − 20/D in water. Forsterite gives 13.89 g and fayalite 15.45 g, 1.56 g apart. Tephroite, 14.83 to 15.15 g, and liebenbergite, 15.65 g, fall near the iron end. The method is on how to measure specific gravity at home.
What the scale cannot do. It cannot convert an in-between reading into a Fo number; we have no published calibration to quote. Nor can it separate forsterite from monticellite: monticellite’s 3.03 to 3.27 gives 13.40 to 13.88 g, against forsterite’s 13.89 g. Hardness does that, 5 to 5.5 against 7. The USGS gives peridot a specific gravity of 3.22 to 3.45, which is 13.79 to 14.20 g, at the magnesium end.
Same atoms, different minerals. The forsterite sheet records it as trimorphous with ringwoodite and wadsleyite. The IMA list prints the three formulas differently: Mg2(SiO4) for forsterite, Mg2SiO4 for wadsleyite (IMA 1982-012), SiMg2O4 for ringwoodite (IMA 1968-036, Australia). Their sheets describe ringwoodite as rounded grains to 100 µm and wadsleyite as aggregates with grains to 5.0 µm. Neither is a hand-specimen mineral. Polymorphs in general are on crystal vs mineral.
Where olivine-group minerals are found, including Wales, Devon and Cornwall
Forsterite is in mafic and ultramafic igneous rocks and thermally metamorphosed impure dolomitic limestones. Its sheet lists Monte Somma and Vesuvius; Ojamo, Finland; the Kovdor massif; Peridot Mesa, Arizona; Mogok; Sapat, Pakistan; and Zabargad in the Red Sea. Fayalite is in ultramafic and, less commonly, felsic rocks and lithophysae in obsidian. Its sheet lists Obsidian Cliff, Yellowstone; the St. Peters Dome area near Pikes Peak; Rockport, Massachusetts; the Brocken massif; and the Skaergaard intrusion, Greenland.
Neither the forsterite nor the fayalite sheet names a British locality. Tephroite does. It forms in iron-manganese ore deposits, skarns and metamorphosed manganese-rich sediments. Its sheet lists the Benallt mine, Rhiw, Lleyn Peninsula, Wales; the Meldon quarry, Okehampton, Devon; and the Treburland mine, Altarnun, Cornwall. It also lists Franklin and Sterling Hill, New Jersey; Långban, Sweden; and the Wessels mine, South Africa.
Liebenbergite has one locality on its sheet: three km west of the Scotia Talc mine, Bon Accord, Barberton, South Africa, as grains to 1 mm. Monticellite, which has no group on its sheet, lists Camas Mòr on the Isle of Muck, Scotland, and Scawt Hill, Co. Antrim. 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.