A synthetic Cu-rich tourmaline crystal (Lebedev et al. 1988) consists of three different zones. Each zone was characterized by EMPA, SIMS, and single-crystal structure refinement (SREF). The first zone (which crystallized directly on the seed crystal) has the formula ~X(Na0.80.2) Y(Al2.0Cu0.90.1) ZAl6T(Si5.1Al0.9)O18 (BO3)3V(OH)3W[O0.7F0.2(OH)0.1] with lattice parameters a 15.835(1), c 7.093(1) Å (R = 2.4%). The second zone has the formula ~X(Na0.80.2) Y(Al1.8Cu1.10.1) ZAl6T(Si5.1Al0.7B0.2)O18 (BO3)3V(OH)3W[(OH)0.4F0.3O0.3] with a 15.824(1), c 7.087(1) Å (R = 2.3%). The third zone (highest Cu content with ~14 wt.% CuO) has the formula X(Na0.810.19) Y(Cu1.72Al1.210.07) Z(Al5.96Cu0.04) (BO3)3T(Si5.17Al0.48B0.35)O18V(OH)3W[(OH)0.63F0.37] with a 15.849(1), c 7.087(1) Å (R = 2.5%). While [4]Al decreases from zone 1 to zone 3, [4]B increases (by chemistry and SREF), which could be explained by a decreasing temperature during tourmaline crystallization. Such a T-site occupancy is in agreement with the <T–O> distance, which strictly monotonically decreases from 1.625(1) to 1.616(1) Å. We suggest that very small amounts of Cu are present at the Z site of all investigated tourmaline samples, but only in the Cu-richest zone (~14 wt.% CuO) is the refined value for Cu at the Z site (~1% of the total Cu) higher than the 3σ error. The YO6 octahedron of this Cu-richest known tourmaline is mainly occupied by Cu. Two of the six (Cu,Al)–O distances are significantly enlarged: Y–O1 with 2.031(2) Å and Y–O3 with 2.170(2) Å, while the other distances Y–O2 and Y–O6 with ~1.951(2) Å are significantly smaller. The proportion of the average of the two enlarged distances to the average of the other distances in the Cu-richest zone gives a value of ~1.08, which is the highest value known so far for Cu-bearing tourmalines. We conclude that for the Cu-richest zone the Jahn-Teller effect appears to be verified.

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