Name: Tourmali was a generic name used in Ceylon for colored gems, mostly zircons.
Synonym: Pierre de Ceylan;
Tourmaline Garnet;
Tourmaline group.
Clades:
Borosilicates; 6-member single ring;
Cyclosilicates;
Silicates;
Lapidae.
Chemical: (Na,Ca)(Al,Li,Mg)3?(Al,Fe,Mn)6(Si6O18)(BO3)3(OH)4.
Hardness: 7.
Specific gravity: 2.9 to 3.1.
Crystal system: Trigonal.
MineralogyThe structure for the group is one in which SiO4 tetrahedra are linked into six-membered rings having a hexagonal pattern and are stacked up with intervening distorted triangular BO3 groups, linked by D site cations. The SiO4 tetrahedra are linked vertically by G site cations and X site anions, while the columns are linked horizontally by both D site cations and X site anions. Additionally, tetrahedral boron may be present in both natural and synthetic tourmalines.
ChemicalGeneral formula: AD3G6 (T6O18)(BO3)3X3Z
A = Ca, Na, K, Pb or is vacant
D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+, Ti
G = Al, Cr3+, Fe3+, V3+
T = Si, sometimes Al or B3
In Adachiite the T sites include one essential Al
X = O and/or OH
Z = F, O and/or OH