Fluor-liddicoatite[7] is a rare member of the tourmaline group of minerals, elbaite subgroup, and the theoretical calciumendmember of the elbaite-fluor-liddicoatite series; the pure end-member has not yet been found in nature.[3] Fluor-liddicoatite is indistinguishable from elbaite by X-ray diffraction techniques. It forms a series with elbaite and probably also with olenite.[3] Liddiocoatite is currently a non-approved mineral name, but Aurisicchio et al. (1999) and Breaks et al. (2008) found OH-dominant species.[8][9] Formulae are
Fluor-liddicoatite is isostructural with (has the same structure as) all members of the tourmaline group,[3] 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.[10][11]
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.[3]
Physical properties
A polished slice of liddicoatite from Madagascar. Photo Rob Lavinsky
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.[12] 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+.[12]
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.[2] The mineral is brittle, with an uneven to conchoidalfracture. It is very hard, with hardness7+1⁄2, 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.
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.[2]Pleochroism is strong: O dark brown or pink, E light brown or pale pink.
Deep brilliant red, highly lustrous, prismatic fluor-liddicoatite has been found as large sheaves of near parallel, slightly divergent crystals, at the Minh Tien pegmatite, Luc Yen District, in Vietnam.[13]
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.[14]
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.[14]
^Aurisicchio, C., Demartin, F., Ottolini, L. & Pezzotta, F. (1999). "Homogeneous liddicoatite from Madagascar. A possible reference material? First EMPA, SIMS, and SREF data". European Journal of Mineralogy. 11 (2): 237–242. Bibcode:1999EJMin..11..237A. doi:10.1127/ejm/11/2/0237.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Breaks, F.W.; Tindle, A.G. & Selway, J.B. (2008). Electron microprobe and bulk rock and mineral compositions from rare-element pegmatites and peraluminous, S-type granitic rocks from the Fort Hope pegmatite field, north-central Superior Province of Ontario. Vol. 235. Ontario Geological Survey, Miscellaneous Release Data.
^ abDeer, Howie and Zussman (1986) Rock-forming minerals, (2nd edition), Volume 1B, Disilicates and Ring Silicates
