Chemistry

One formula, thirty-odd minerals

Tourmaline does not have a composition. It has a range of them. The structure is a set of sockets that will accept almost any metal ion present in the melt around it, which is why one mineral group covers a colour range no other gem family approaches — and why its chemistry has been called a prescription rather than a formula.

General formula

XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W

A cyclosilicate with borate groups: six silicon tetrahedra joined in a ring, three planar BO₃ triangles, and two sets of octahedral sites around them. Trigonal, space group R3m.

The sites

What can sit where

The Y and Z sites are the reason for everything else on this site. They accept lithium, iron, magnesium, manganese, aluminium, chromium, vanadium and copper more or less interchangeably — so the crystal records whatever the pegmatite had left over when it cooled.

Na⁺, Ca²⁺, K⁺, or vacant (□)

Sits in the channel at the centre of the silicate ring. A vacancy here defines its own species — rossmanite, foitite.

Li⁺, Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Fe³⁺, Cr³⁺, V³⁺

Three octahedral positions. This is where most of the colour comes from, and where the species is decided.

Al³⁺, Mg²⁺, Fe³⁺, Cr³⁺, V³⁺

Six octahedral positions, usually aluminium. Chromium and vanadium here give the intense greens.

Si⁴⁺, occasionally Al³⁺ or B³⁺

The six-membered ring of tetrahedra, Si₆O₁₈, that makes tourmaline a cyclosilicate.

B³⁺ — always

Three planar BO₃ groups. Boron is never absent: it is what makes a tourmaline a tourmaline.

OH⁻, O²⁻

Three anion positions bridging the octahedra.

OH⁻, F⁻, O²⁻

A single anion position on the threefold axis. Fluorine here produces the fluor- species.

The species

Six that matter out of more than thirty

Variety names describe colour. Species names describe chemistry. Confusing the two is the root of most bad descriptions in this trade — see the colour range for the variety side of it.

Elbaite

Na(Li₁.₅Al₁.₅)Al₆(Si₆O₁₈)(BO₃)₃(OH)₄

The lithium species, and effectively all gem tourmaline. Rubellite, indicolite, verdelite and Paraíba are one species with different trace elements.

Schorl

NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₄

The iron species. Black, opaque, and by a wide margin the most abundant tourmaline on earth.

Dravite

NaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₄

The magnesium species. Brown to warm amber, occasionally gem quality.

Uvite

CaMg₃(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₄

Calcium-magnesium. Typically brown to green, often from metamorphosed limestone rather than pegmatite.

Liddicoatite

Ca(Li₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃F

Calcium-lithium, chiefly Madagascar. The famous triangular colour-zoned slices are usually this, not elbaite.

Rossmanite · Foitite

□(LiAl₂)Al₆… · □(Fe²⁺₂Al)Al₆…

Species defined by an empty X site. Rarely cut, but they show how far the formula stretches.

Chromophores

Which ion makes which colour

Fe²⁺ / Fe³⁺

Green, blue, black

The default colouring agent, and the source of the grey component that dulls ordinary material.

Mn³⁺

Pink, red, purple

Absorbs near 515 nm. Low-temperature heating converts it to Mn²⁺ and the absorption disappears.

Mn²⁺

Yellow

A weak absorber. Responsible for canary tourmaline, and it brightens copper blue rather than muddying it.

Cu²⁺

Neon blue to green

Broad absorption near 700 and 900 nm. The chromophore that defines Paraíba, at roughly 0.3–1.5% by weight.

Cr³⁺ / V³⁺

Intense green

Chrome tourmaline. The same ions that colour emerald and tsavorite.

What the structure does

Two consequences you can feel

No cleavage

The silicate rings bound by borate groups leave no plane of weakness. Tourmaline fractures conchoidally rather than parting, which is why it cuts well at 7–7.5 on the Mohs scale despite being brittle when heavily included. See the full property table.

Piezoelectric and pyroelectric

The structure is polar and hemimorphic — no centre of symmetry, and the two ends of the prism are not the same. Compress it or change its temperature and a charge develops at the poles. Dutch traders in the 1700s knew it as the stone that pulls ash out of a fire; the effect had no name yet. That story is on the history page.

Why this is not academic

Every gem tourmaline shares the same optical constants: refractive index 1.619–1.655, birefringence around 0.018–0.020, specific gravity near 3.06. A refractometer cannot tell a rubellite from a Paraíba, because they are the same species. Only chemistry separates them.

That is why a laboratory reaches for energy-dispersive X-ray fluorescence or LA-ICP-MS: to read the Y and Z sites directly. Copper found there makes a stone Paraíba. Copper absent means the name cannot be used, whatever the colour suggests. The formula at the top of this page is, in the end, the reason we insist on naming things correctly.