Appendix 4: Guideline for Simplified Distribution of Site Occupancy for LA-ICP-MS Data

This classification method is modified from Henry et al. (2011). The general chemical formula for tourmaline supergroup minerals is XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W. The atoms per formula unit (apfu) of hypothetical element R on a hypothetical site M is designated as ^MR_apfu. However, element R can be present on multiple sites, so the total apfu of R is designated as ^totR_apfu. We determine the site occupancy using the following method. The number of atoms of each element is determined using the calculation method above.

B-site: The B-site is fully occupied by B. Therefore, the equivalent weight percent for three B atoms in the tourmaline formula is calculated.

T-site: The T-site is occupied by six atoms of Si and Al (T-site B is not considered in this method). If the number of Si atoms per formula unit (Si_apfu) is less than six, Al is added to fill the site. Cations with a +4 charge, such as Si⁴⁺, take priority in this site over those with a +3 charge, such as Al³⁺. 

If total Si_apfu (^totSi_apfu) is greater than 6, the T-site is only occupied by Si and ^TSi_apfu is set to equal 6. Otherwise, ^TSi_apfu equals Si_apfu. ^TAl_apfu equals 6 – Si_apfu.

Z-site: The Z-site is occupied by six atoms of Al, Cr, V, and Mg. Cations with a +3 charge (e.g., Al³⁺) take priority over those with a +2 charge (e.g., Mg²⁺). Further, smaller +3 cations (Al³⁺) take priority over larger Cr³⁺ and V³⁺ cations. In shorthand form:

^ZR³⁺ >> ^ZR²⁺

For ^ZR³⁺: Al³⁺ >> Cr³⁺ > V³⁺

The only common ^ZR²⁺ cation is Mg²⁺. 

If ^totAl_apfu – ^TAl_apfu is greater than 6, ^ZAl_apfu equals 6. Otherwise, ^ZAl_apfu equals ^totAl_apfu – ^TAl_apfu.

If ^ZAl_apfu + ^totCr_apfu + ^totV_apfu < 6, ^ZCr_apfu equals ^totCr_apfu. ^ZV_apfu equals ^totV_apfu. If ^ZAl_apfu + ^totCr_apfu + ^totV_apfu > 6 and there is any deficiency in the Z-site occupancy from a total of 6 after assigning Al to the Z-site, the remainder is filled with Cr first, and then V if present. This gives ^ZCr_apfu and/or ^ZV_apfu.

If, after assigning Al, Cr and V to the Z-site, there remains a deficiency in the Z-site occupancy from a total of 6, and if ^totMg_apfu is greater than 3, ^ZMg_apfu equals 6– ^ZCr_apfu – ^ZV_apfu– ^ZAl_apfu. Otherwise, ^ZMg_apfu equals 0. Note that Z-site disordering in oxy species is not considered in this method.

Y-site: The Y-site is fully occupied by Al, Ti, Mn, Fe, Mg, Cu, Cr, V, Zn, and Li. The priority of ions with different valence states entering the Y-site is:

^YR²⁺ > ^YR³⁺ > ^YR¹⁺ > ^YR⁴⁺ 

For ^YR²⁺: Mg²⁺ ~ Fe²⁺ > Mn²⁺ >>> Zn²⁺, Cu²⁺, etc.

For ^YR³⁺: Al³⁺ >> Cr³⁺ >> V³⁺

The only common ^YR¹⁺ cation is Li¹⁺.

The only common ^YR⁴⁺ cation is Ti⁴⁺.

If ^totMg_apfu is greater than 3, ^YMg_apfu equals ^totMg_apfu – ^ZMg_apfu. Otherwise, ^YMg_apfu equals ^totMg_apfu.

^YFe²⁺_apfu equals ^totFe²⁺_apfu.

After assigning Mg and Fe to the Y-site, any ^totMn²⁺, ^totZn²⁺, and ^totCu²⁺ is added to the Y-site.

After assigning Mg, Fe, Mn, Zn, and Cu to the Y-site, if ^totAl_apfu – ^TAl_apfu – ^ZAl_apfu is greater than 0, ^YAl_apfu equals ^totAl_apfu – ^TAl_apfu – ^ZAl_apfu. Otherwise, ^YAl_apfu equals 0. 

^YCr_apfu equals ^totCr_apfu – ^ZCr_apfu.

^YV_apfu equals ^totV_apfu – ^ZV_apfu.

After assigning all R²⁺ and R³⁺ ions above to the Y-site, the Li that was calculated in the data processing method described above is added. (Only low-Mg tourmaline contains substantial Li.) Finally, Ti is added to the Y-site.

X-site: The X-site is occupied by Ca, Na, and K or has vacancies (and any minor Pb, Ba, Rb, and Cs present). The priority of ions with different valence states entering the X-site is (R¹⁺ > R²⁺ > □ (vacancy)). Any deficiency in the X-site occupancy from a total of 1 is equal to X-site vacancies (^X□_apfu); i.e., ^X□_apfu equals 1 – ^XNa_apfu – ^XCa_apfu – ^XK_apfu. X-site charge controls the amount of F in the structure—if it is an X-site vacant tourmaline, it will have low F (Henry and Dutrow, 2012), which eliminates fluor-species in the vacant subgroup.

V-site: The V-site is fully occupied by OH; i.e., ^VOH_apfu equals 3.

W-site: The W-site is fully occupied by OH; i.e., ^WOH_apfu equals 1. The species based on this anionic site occupancy cannot be definitively determined (i.e., oxy, hydroxy, or fluor species). The oxy component can be calculated based on charge balance. However, this is a topic for future work.