Liddicoatite - The Calcium Chameleon of Tourmalines

Liddicoatite is an uncommon form of Tourmaline, almost identical in chemical composition to Elbaite, but with a calcium atom replacing a sodium atom in its chemical formula. It was not recognized as a separate Tourmaline species until 1977. Prior to that time, it was thought to be Elbaite.

Liddicoatite Photo : Saphira Minerals e.U

Fluor-liddicoatite was named in 1977 after Richard T. Liddicoat (1918–2002) gemmologist and president of the Gemological Institute of America, who is well known for introducing the GIA diamond grading system in 1953.

The crystals are stout prismatic, with a curved convex trigonal outline, generally elongated and striated parallel to the c axis. They are hemimorphic, meaning that the two ends of the crystal have different forms. Fluor-liddicoatite usually has a pedion (a single crystal face) opposite one or two pyramids.

Properties

• Formula : Ca(Li₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH)
• Colour : Light-brown, pink to red, green, blue, rarely white.
• Lustre : Vitreous
• Hardness : 7.0 - 7.5
• Specific Gravity : 3.02
• Crystal System : Trigonal
• Member of : Tourmaline Group
• Name : Named for Richard T. Liddicoat (1918-2002), gemologist and president of the Gemological Institute of America

The color is usually smoky brown, but also pink, red, green, blue, or rarely white. Color zoning is abundant at the type locality, parallel to pyramid faces. This is due to changes in the solution during crystal growth.

As the concentration of trace elements that serve as coloring agents changes, there will be areas of less or more color in different parts of the crystal. When the crystal is sliced perpendicular to the c axis, triangular zoning may be seen, together with a trigonal star that radiates from the centre of the crystal, with the three rays directed towards the corners of the triangular color patterns.

The pink-red color is due to the manganese Mn3+ content, and the green color is due to intervalence charge transfer transactions between iron Fe2+ and titanium Ti4+.
The streak is white to very light brown, lighter than the mass color, luster is vitreous and crystals are transparent to translucent.

Cleavage is poor perpendicular to the c crystal axis, or it may be totally absent. The mineral is brittle, with an uneven to conchoidal fracture. It is very hard, with hardness 7½, a little harder than zircon, making it suitable for use as a gemstone. Specific gravity is 3.02, a little lighter than fluorite. It is neither fluorescent nor radioactive.

Environment & Associations
Fluor-liddicoatite is detrital in soil at the type locality, presumably derived from the weathering of granitic pegmatites. Associated minerals are quartz, elbaite, albite and micas.

Uses

Liddicoatite forms outstanding and beautiful crystal which are highly regarded by collectors as a rare form of Tourmaline. Large crystals are often cut into thin cross-section slices and polished and sold to collectors in this form, with the multicolored designs and forms and inner crystal colors beautifully displaying.

Especially popular are those slices with a triangular color pattern displaying within the core of the crystal. Liddicoatite is also be used as a Tourmaline gemstone, but its limited occurrences inhibit its use from being more widespread.