Vesuvianite
Vesuvianite is hardness 6 to 7, density 3.32 to 3.43. Grossular garnet grows beside it at 3.594, and the two density ranges do not overlap at any point.
Vesuvianite is a calcium aluminium silicate of skarns, hardness 6 to 7, with a measured density of 3.32 to 3.43. Grossular garnet grows in the same rock, at the same hardness, in the same colours, and has a density of 3.594. The two ranges do not overlap at any point, so a density measurement on a clean specimen separates them outright.
In short
- The density ranges do not overlap. Vesuvianite tops out at 3.43 and grossular sits at 3.594 — a gap of 4.8% with clear air between. That is unusual: most species pairs a collector confuses overlap somewhere.
- Hardness does not separate them. Vesuvianite is 6 to 7, grossular 6.5 to 7. Both scratch glass, neither is marked by a steel point, and in the overlap region the test is silent.
- Shape separates them when crystals are sharp. Vesuvianite is tetragonal — short pyramidal to long prismatic, square in cross-section. Grossular is cubic, forming dodecahedra and trapezohedra. On broken or massive material that evidence is gone, which is when density earns its keep.
- The formula carries a variable boron content, 0 to 5 atoms per formula unit. That is unusually elastic even for a complex silicate, and it is one reason vesuvianite analyses vary so much between localities.
- Idocrase is the same species under an older name. The IMA-CNMNC Master List lists vesuvianite, approved by a 1962 special procedure, with Italy as the country of first description; idocrase is not a separate entry.
| Species | Formula | Hardness | Density | System | What separates it |
|---|---|---|---|---|---|
| Vesuvianite | Ca19(Al,Mg)13B0-5Si18O68(OH,O,F)10 | 6 to 7 | 3.32 to 3.43 | Tetragonal | — |
| Grossular | Ca3Al2(SiO4)3 | 6.5 to 7 | 3.594 | Cubic | Denser, with no overlap; dodecahedra not prisms |
| Epidote | Ca2Al2(Fe,Al)(SiO4)(Si2O7)O(OH) | 6 to 7 | 3.38 to 3.49 | Monoclinic | Perfect {001} cleavage, which vesuvianite lacks |
| Clinozoisite | Ca2Al3(SiO4)(Si2O7)O(OH) | 6.5 | 3.21 to 3.38 | Monoclinic | Perfect {001} cleavage; paler, iron-poor |
| Zoisite | Ca2Al3(SiO4)(Si2O7)O(OH) | 6 to 7 | 3.15 to 3.36 | Orthorhombic | Perfect {010} cleavage; lighter |
| Prehnite | Ca2Al2Si3O10(OH)2 | 6 to 6.5 | 2.80 to 2.95 | Orthorhombic | Far lighter; botryoidal green crusts |
Two species that grow together and separate on one number
Vesuvianite and grossular form in the same place under the same conditions: skarns, where a limestone has been recrystallised and chemically reworked against an intrusion. The Handbook of Mineralogy lists grossular first among vesuvianite's associates, ahead of diopside, wollastonite, epidote, scapolite, spinel and calcite.
They also share a colour range. Both run yellow, green and brown, both can be red or pink, and both are commonly zoned. Both are hard enough to scratch glass. A rounded, brown, glassy grain in a skarn could be either.
Density resolves it without ambiguity. Vesuvianite 3.32 to 3.43; grossular 3.594, a single value with no range. The highest vesuvianite is 4.8% lighter than the lowest grossular, and since grossular's figure carries no spread the gap is clean at both ends.
For a specific gravity near 3.5, the minimum-mass rule derived on how to measure specific gravity at home gives about 21 g of clean material on a 1 g scale, or 0.21 g on a 0.01 g scale. A single detached crystal is more than enough. The constraint here, as usual, is not the balance but the need for a clean single-species fragment, and a grain sitting in a calcite matrix cannot supply one without being freed.
Thirty forms on one crystal, and why the shape is worth reading
The Handbook records vesuvianite as morphologically complex, with up to 30 forms reported on one crystal. Crystals are short pyramidal to long prismatic, reaching 15 cm, and also occur columnar, granular and massive.
Thirty forms is a lot, and for a collector the practical consequence is that a good vesuvianite crystal is visually busy: a square prism whose terminations carry a stack of shallow pyramids and truncations rather than a simple point. That crowded, stepped termination on a square-sectioned prism is close to diagnostic on its own, and it is the reason a sharp vesuvianite reads as a more interesting object than its colour would suggest.
Cleavage is the negative evidence. Vesuvianite is poor on {110}, very poor on {100} and {001} — it is one of the fifteen species in this site's reference set with no usable cleavage, a list set out in full on the analcime page. Epidote, clinozoisite and zoisite, which share its colour and hardness, all cleave perfectly. A green prismatic crystal of hardness 6 to 7 that will not part along a plane is not epidote.
The boron problem, and what a vesuvianite analysis is worth
The formula the Handbook prints is Ca19(Al,Mg)13B0-5Si18O68(OH,O,F)10. The boron subscript is a range from zero to five atoms per formula unit.
Very few species are written that way. Substitutions are normal — the (Al,Mg) and (OH,O,F) sites here are ordinary — but a component that may be entirely absent or may occupy five sites is an unusually wide door. The two analyses the Handbook prints make the point: one has no boron reported at all, the other has 3.18% B2O3.
The practical consequence is that vesuvianite from one locality can differ substantially in composition from vesuvianite at another while both remain the same species. Physical properties shift with it: the density range 3.32 to 3.43 and the refractive index spread are wide for a single species, and both are real variation rather than poor measurement.
This is the same situation the chrysocolla page describes in a more extreme form, where a variable water content produces a density range from 1.93 to 2.4. A range in a reference book is sometimes a measurement problem and sometimes the species itself. Reading which is which is a skill worth having, and the way to tell is usually whether the formula contains something elastic.
Localities, the name, and the labels that say idocrase
Vesuvianite is named for its occurrence at Vesuvius, and Italian material remains the classic: Monte Somma and Vesuvius in Campania, Ariccia in Lazio, Val di Fassa in Trentino-Alto Adige, and Val d'Ala in Piedmont. Type material is at the Mining Academy in Freiberg, Germany.
Beyond Italy the Handbook lists Zermatt in Valais; Arendal, Stromskien and Egg in Norway; the Akhmatovsk mine near Zlatoust in the Urals and the Vilyui river area in Yakutia, Russia; Amity and Olmsteadville in New York, the Belvidere Mountain quarries in Vermont, Sanford and Auburn in Maine, Georgetown and Crestmore in California, and Ludwig in the Yerington district, Nevada; Litchfield, Asbestos and Mont Saint-Hilaire in Quebec; Xalostoc in Morelos and Lake Jaco in Chihuahua, Mexico; and Baluchistan in Pakistan.
Older labels frequently read idocrase, and it is worth being clear that this is not an error. The IMA-CNMNC Master List lists vesuvianite as the accepted name, approved by a 1962 special procedure. An idocrase label is simply an older determination of the same species, in the same way that an analcite label is an older analcime — see what ex coll means on a mineral label for why a superseded name on an old label adds to a specimen rather than detracting from it.
Nothing on this site is for sale and there is no catalogue; a Val d'Ala or Vilyui crystal is a wanted list matter, and the rest of the section is indexed under mineral species.