Forsterite
Forsterite is Mg2SiO4, the magnesium end member of the olivine group: hardness 7, density 3.275. Gem peridot is forsteritic olivine; fayalite is the iron end.
Yes: forsterite is olivine. It is the magnesium end member, Mg2SiO4, of the olivine group, and gem peridot is forsteritic olivine. Forsterite is orthorhombic, hardness 7, measured density 3.275, with imperfect {010} and {100} cleavages and a conchoidal fracture. The Handbook records it forming two series: with fayalite, Fe2SiO4, and with tephroite, Mn2SiO4.
In short
- Is forsterite olivine? Yes — it is the magnesium end of it. The Handbook of Mineralogy files forsterite, fayalite and tephroite under olivine group, and they are the only three of the 176 sheets in this site's reference set that do. Forsterite is a valid species on the September 2026 IMA list; olivine has no entry of its own.
- Is forsterite peridot? Peridot is the gem name for forsteritic olivine, not a separate mineral. The naming question is set out in full on is olivine the same as peridot; this page is about the species behind the gem.
- Forsterite is one of six sheets in the 176-sheet set whose hardness never falls below 7. The others are corundum 9, topaz 8, uvite 7.5, schorl 7 and quartz 7. Fayalite, the iron end, reads 6.5 to 7 and tephroite 6.
- The Handbook's analysed forsterite is Fo98.9. Its Mogok analysis, 57.83% MgO against 1.11% FeO, works out at 98.9 mol% of the magnesium end member — the calculation is below. Pure Mg2SiO4 is 34.55% magnesium by mass.
- Not all forsterite is green. The sheet records green, yellowish, lemon-yellow, white, greyish and blue-grey, and it names two quite different settings: mafic and ultramafic igneous rocks, and thermally metamorphosed impure dolomitic limestones.
| Species | Formula | Hardness | Density | Cleavage | The separating feature |
|---|---|---|---|---|---|
| Forsterite | Mg2SiO4 | 7 | 3.275 | {010}, {100} imperfect | Hardness 7 with no good cleavage; conchoidal fracture |
| Fayalite | Fe2SiO4 | 6.5 to 7 | 4.392 | {010}, {100} imperfect | 34.1% denser; greenish yellow to brown, resinous on fractures |
| Tephroite | Mn2SiO4 | 6 | 3.87 to 4.12 | {010} distinct; {001} imperfect | Olive-green to flesh-red or grey; Fe-Mn ore deposits and skarns |
| Monticellite | CaMgSiO4 | 5 to 5.5 | 3.03 to 3.27 | Poor on {010} | Density almost meets forsterite's; hardness is 1.5 to 2 points lower |
| Diopside | CaMgSi2O6 | 5.5 to 6.5 | 3.22 to 3.38 | Distinct {110}, planes at about 87° | Two cleavages near right angles, where forsterite fractures |
| Enstatite | Mg2Si2O6 | 5 to 6 | 3.2 to 3.9 | Good {210}, planes at about 88° | Shares forsterite's ultramafic company; cleaves, and is softer |
| Epidote | Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH) | 6 to 7 | 3.38 to 3.49 | Perfect {001} | Pistachio-green; one perfect cleavage |
| Vesuvianite | Ca19(Al,Mg)13B0–5Si18O68(OH,O,F)10 | 6 to 7 | 3.32 to 3.43 | {110} poor | Tetragonal; yellow, green, brown; also on Monte Somma |
Is forsterite olivine?
Yes. Every forsterite is an olivine; not every olivine is forsterite. Olivine is a group name, and forsterite, Mg2SiO4, is its magnesium end member. The Handbook of Mineralogy sheet files it under olivine group and records that it forms two series, with fayalite, and with tephroite — the iron and manganese end members, Fe2SiO4 and Mn2SiO4.
Counted across the 176 Handbook sheets in this site's reference set (method: each sheet converted with pdftotext -layout, the Mineral Group field read by regular expression), exactly three name the olivine group: forsterite, fayalite and tephroite. Monticellite, CaMgSiO4, has the same orthorhombic Pbnm cell type and is often discussed alongside them, but its sheet names no group, so it is not counted here.
Is forsterite the same as olivine? Not quite, and the difference matters on a label. The September 2026 IMA-CNMNC Master List lists forsterite as a valid, grandfathered species, first described in 1824 from Italy. It has no entry for olivine; the only name on it containing the word is calcio-olivine, Ca2SiO4, which is a different species. Olivine on a label is therefore a group determination, and forsterite is a species determination. Label practice is on how to label and catalogue a mineral collection, and the same group-name problem in a mica is on lepidolite.
Is forsterite the same as peridot?
Peridot is gem-quality forsteritic olivine: forsterite is the species, peridot the gem name. Peridot fixes quality and colour; forsterite fixes composition. The full comparison, with GIA's description of the gem, is on is olivine the same as peridot, and it is not repeated here.
What the species sheet adds is where the gem-grade crystals actually are. The Handbook's forsterite distribution list singles out three: exceptional gemmy crystals from Sapat, 24 km northeast of Naran, in what the sheet calls the Northwest Frontier Province of Pakistan; large gem crystals from Zabargad (Zabirget or St Johns Island) in the Red Sea, Egypt; and Peridot Mesa, four kilometres southwest of San Carlos, Gila County, Arizona. Kingiti in Tanzania and the Mogok district and Pyaung Gaung in Myanmar are on the same list.
A green, transparent, well-terminated crystal from one of those localities is correctly labelled forsterite. A label reading peridot is not wrong, but it describes the crystal as a gem rough rather than as a mineral.
What is forsterite and fayalite? The two ends of one series
Forsterite is Mg2SiO4 and fayalite is Fe2SiO4: the same orthorhombic structure, the same imperfect {010} and {100} cleavages, and iron in place of magnesium. Everything between them is olivine, and the composition is written as Fo number, the molar percentage of the forsterite end member.
A worked calculation from the Handbook's own analysis. The sheet prints one forsterite analysis, from the Mogok district: SiO2 41.72%, FeO 1.11%, MgO 57.83%. Divide each oxide by its formula mass (MgO 40.304, FeO 71.844, from standard atomic weights): MgO gives 1.4348 mol per 100 g, FeO gives 0.0155. Fo = 1.4348 / (1.4348 + 0.0155) × 100 = 98.9. The sheet's own formula for the same analysis, (Mg2.02Fe0.02)Si0.98O4, gives 99.0; the difference is rounding in the printed formula. Substitute any analysis with MgO and FeO and the method is identical.
By mass, pure forsterite is 34.55% magnesium, 19.96% silicon and the rest oxygen (formula mass 140.691); pure fayalite is 54.81% iron. That iron is why fayalite's measured density, 4.392, is 34.1% above forsterite's 3.275, and why its refractive indices run from 1.827 to 1.879 against forsterite's 1.635 to 1.670.
The two ends also come from different geology. Fayalite's sheet puts it in ultramafic volcanic and plutonic rocks, less commonly felsic plutonic rocks, lithophysae in obsidian and metamorphosed iron-rich sediments; forsterite's in mafic and ultramafic igneous rocks and in thermally metamorphosed impure dolomitic limestones.
Hardness 7 separates forsterite from most green look-alikes
Across the 176 sheets in the reference set, 168 print a numerical hardness. Six have a hardness that never falls below 7: corundum 9, topaz 8, uvite 7.5, and schorl, quartz and forsterite at 7. Method: the Hardness field parsed by regular expression, the lower bound of any range taken, and the 13 sheets the expression missed checked by hand: six print n.d., two soft, three an approximate value and two a doubled equals sign. Grossular, fayalite, zoisite, vesuvianite, epidote and cassiterite reach 7 only at the top of a range.
That makes the steel-and-quartz test unusually decisive for a green silicate. Diopside at 5.5 to 6.5, enstatite at 5 to 6 and monticellite at 5 to 5.5 all yield to quartz; forsterite does not. The method and its pitfalls are on how to test mineral hardness properly.
Monticellite is the trap density cannot catch. Its measured density, 3.03 to 3.27, runs up to within 0.005 of forsterite's 3.275, it is colourless to pale greenish grey, and it forms in the same metamorphosed dolomitic rocks — Monte Somma, Vesuvius and the Kovdor massif are on both sheets. Hardness separates them by 1.5 to 2 points. Cleavage then separates forsterite from the pyroxenes: diopside and enstatite have two good cleavages meeting at about 87° and 88°, where forsterite shows conchoidal fracture. See cleavage or fracture: how to tell.
Where forsterite forms, and where the good crystals come from
The Handbook gives two settings: mafic and ultramafic igneous rocks, and thermally metamorphosed impure dolomitic limestones. The sheet's association list reads like both rocks at once: enstatite, plagioclase, phlogopite, magnetite, chromite, antigorite, dolomite, brucite, diopside, corundum, amphiboles, calcite, spinel and augite. A white or grey grain in a thermally metamorphosed dolomitic marble can be forsterite just as much as a green crystal from an ultramafic rock.
The sheet's localities for near-end-member material or good crystals: Monte Somma and Vesuvius in Campania; Ojamo, Lohja, Finland; Møre og Romsdal, Norway; large crystals from the Kovdor massif on the Kola Peninsula, and Zlatoust and Mt Itkul in the Urals; Forstberg in the Eifel; Peridot Mesa and Buell Park in Arizona; Mogok and Pyaung Gaung in Myanmar; Sapat in Pakistan; Kingiti in Tanzania; and Zabargad in the Red Sea. The largest crystals the sheet records reach 17 cm.
No British locality appears on the forsterite or fayalite sheets. Tephroite, the manganese member, is the one of the three with British entries: the Benallt mine near Rhiw on the Lleyn Peninsula, the Meldon quarry at Okehampton in Devon, and the Treburland mine at Altarnun in Cornwall. The Handbook's tephroite analysis is of Benallt material.
The rest of this reference section is indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route, and sharp Sapat or Kovdor forsterite on matrix is on it.