Fluor-liddicoatite

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 Mn content, and the green color is due to intervalence charge transfer transactions between iron Fe and titanium Ti. 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 ​7 ⁄2, 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.

Fluor-liddicoatite is isostructural with (has the same structure as) all members of the tourmaline group, which are cyclosilicates with the general formula XY3Z6(BO3)3Si6O18(OH,O)3(OH,F). For fluor-liddicoatite, the X sites are occupied by Ca, the Y sites by Li or Al and the Z sites by Al, giving the formula Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F. The Y sites are octahedrally coordinated by oxygen O and hydroxyl OH ions; three octahedra surround the three-fold axis at the origin, and each octahedron shares an edge with each of its two nearest neighbours. The silicon Si ions are tetrahedrally coordinated by O, forming SiO4 groups. These tetrahedra form six-membered rings, with two of the four Os in each tetrahedron shared between adjacent tetrahedra. So the formula for the ring is Si6O18. In each Si tetrahedron an O at one free apex is shared with one of the Y octahedra. The boron B ions occur in triangular coordination, each triangle sharing a common apex with two Y octahedra. This composite unit is linked to others like it by aluminum Al ions at the Z sites, and its outer oxygen atoms are also atoms of the aluminum coordination octahedra. The X sites are sandwiched between the units along the c axis.