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 MRapfu. However, element R can be present on multiple sites, so the total apfu of R is designated as totRapfu. 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 (Siapfu) 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 Siapfu (totSiapfu) is greater than 6, the T-site is only occupied by Si and TSiapfu is set to equal 6. Otherwise, TSiapfu equals Siapfu. TAlapfu equals 6 – Siapfu.
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 totAlapfu – TAlapfu is greater than 6, ZAlapfu equals 6. Otherwise, ZAlapfu equals totAlapfu – TAlapfu.
If ZAlapfu + totCrapfu + totVapfu < 6, ZCrapfu equals totCrapfu. ZVapfu equals totVapfu. If ZAlapfu + totCrapfu + totVapfu > 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 ZCrapfu and/or ZVapfu.
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 totMgapfu is greater than 3, ZMgapfu equals 6– ZCrapfu – ZVapfu– ZAlapfu. Otherwise, ZMgapfu 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 totMgapfu is greater than 3, YMgapfu equals totMgapfu – ZMgapfu. Otherwise, YMgapfu equals totMgapfu.
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 totAlapfu – TAlapfu – ZAlapfu is greater than 0, YAlapfu equals totAlapfu – TAlapfu – ZAlapfu. Otherwise, YAlapfu equals 0.
YCrapfu equals totCrapfu – ZCrapfu.
YVapfu equals totVapfu – ZVapfu.
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 – XNaapfu – XCaapfu – XKapfu. 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., VOHapfu equals 3.
W-site: The W-site is fully occupied by OH; i.e., WOHapfu 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.