Fluor-liddicoatite

Fluor-liddicoatite belongs to the trigonal crystal system, class 3 m, space group R 3m. It has a rhombohedral lattice, with unit cell parameters • a = 15.867 Å to 15.875 Å • c = 7.135 Å to 7.126 Å • Z = 3 (there are 3 formula units per unit cell). == Structure ==

Fluor-liddicoatite is isostructural with (has the same structure as) all members of the tourmaline group, == Crystal habit ==

Crystals are stout prismatic, with a curved convex trigonal outline, generally elongated and striated parallel to the c axis. Crystals 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. == Physical properties ==

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 , 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. == Optical properties ==

Fluor-liddicoatite is uniaxial (-), with refractive Indices No = 1.637 and Ne = 1.621 for the type specimen. The refractive indices, however, will vary from specimen to specimen, as they depend on the content of iron and manganese, which are usually present as trace elements. Pleochroism is strong: O dark brown or pink, E light brown or pale pink. == Environment ==

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. == Localities ==

, Vietnam. Size: 8.5 x 7.6 x 4.7 cm. The type locality is Anjanabonoina, Tsilaizina, Antsirabe, Madagascar. • At Ambalabe, Manapa, near Betafo, Madagascar, very lustrous, striated, short prismatic fluor-liddicoatite crystals with trigonal terminations have been found, loose or on pegmatite matrix; they are a very dark greenish brown to black, but have rich red internal highlights, and resting on a few of their surfaces are sharp, lustrous, snow-white dodecahedral crystals of londonite. • In Tsarafara in the Sahatany Valley, Vakinankaratra Region, Madagascar, striated, lustrous, gemmy, color-zoned fluor-liddicoatite crystals have been found. Most of these have red tips and green middle zones, some with as many as five distinct color bands. Some of the crystals rest on greyish quartz. == References ==