Appendix 5: Simplified Tourmaline Species Classification for LA-ICP-MS Data

Abbreviations used below:

^TSi: T-site Si atoms per formula.

^YZR²⁺: Y+Z-site total divalent cation.

Vac: Vacancy

Decide which primary group the tourmaline belongs to:

If Na+K(+Rb+Cs) is greater than both Ca(+Pb) and the X-site vacancy (^X□_apfu), the tourmaline belongs to the alkali primary group. If Ca(+Pb) is greater than both Na+K(+Rb+Cs) and ^X□_apfu, the tourmaline belongs to the calcic primary group. If ^X□_apfu is greater than both Na+K(+Rb+Cs) and Ca(+Pb), the tourmaline belongs to the vacant primary group (Henry et al., 2011).

Alkali-group tourmaline – classify alkali subgroups 1 and 2.

If ^TSi is greater than 4.5 and ^YZR²⁺/(2Li+ ^YZR²⁺) is greater than 0.5, the tourmaline belongs to alkali subgroup 1.

If ^TSi is greater than 4.5 and ^YZR²⁺/(2Li + ^YZR²⁺) is less than or equal to 0.5, the tourmaline belongs to alkali subgroup 2.

Otherwise the tourmaline belongs to alkali subgroups 3, 4, or 5, which are not considered here.

Alkali subgroup 1

If Al³⁺ is the dominant trivalent cation on the Z-site and Mg²⁺ is the dominant divalent cation on the Y-site, the tourmaline is dravite.

If V³⁺ is the dominant trivalent cation on the Z-site and Mg²⁺ is the dominant divalent cation on the Y-site, the tourmaline is vanadium-dravite, now redefined as oxy-vanadium-dravite (Bosi et al., 2013). 

If Cr³⁺ is the dominant trivalent cation on the Z-site and Mg²⁺ is the dominant divalent cation on the Y-site, the tourmaline is chromium-dravite.

If Al³⁺ is the dominant trivalent cation on the Z-site and Fe²⁺ is the dominant divalent cation on the Y-site, the tourmaline is schorl.

Alkali subgroup 2

If the tourmaline belongs to alkali subgroup 2, it is elbaite.

Calcic-group tourmaline – classify calcic subgroups 1 and 2.

If ^TSi is greater than 4.5 and ^YZR²⁺/(1.5Li+ ^YZR²⁺) is greater than 0.5, the tourmaline belongs to calcic subgroup 1.

If ^TSi is greater than 4.5 and ^YZR²⁺/(1.5Li+ ^YZR²⁺) is less than or equal to 0.5, the tourmaline belongs to calcic subgroup 2.

Calcic subgroup 1

If Al³⁺ is the dominant Z-site cation and Mg²⁺ is the dominant Y-site cation, the tourmaline is uvite.

If Al³⁺ is the dominant Z-site cation and Fe²⁺ is the dominant Y-site cation, the tourmaline is feruvite.

Calcic subgroup 2

If the tourmaline belongs to calcic subgroup 2, it is liddicoatite.

Vacant-group tourmaline – classify vacant subgroups 1 and 2.

If ^TSi is greater than 4.5 and ^YZR²⁺/(2Li+ ^YZR²⁺) is greater than 0.5, the tourmaline belongs to vacant subgroup 1.

If ^TSi is greater than 4.5 and ^YZR²⁺/(2Li+ ^YZR²⁺) is less than or equal to 0.5, the tourmaline belongs to vacant subgroup 2.

Vacant subgroup 1

If Al³⁺ is the dominant Z-site cation, and Fe²⁺ is the dominant Y-site cation, the tourmaline is foitite.

If Al³⁺ is the dominant Z-site cation and Mg²⁺ is the dominant Y-site cation, the tourmaline is magnesio-foitite.

Vacant subgroup 2

If the tourmaline belongs to vacant subgroup 2, it is rossmanite.