Fluor-liddicoatite

Fluor-liddicoatite

Liddicoatite from the Ambesabora pegmatite, Madagascar. Photo Rob Lavinsky
General
Category Cyclosilicate
Tourmaline Group
Formula
(repeating unit)		 Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F
Strunz classification 09.CK.05
8/E.19-80
Dana classification 61.3.1.2
Crystal system Trigonal 3 m, space group R 3m
Identification
Formula mass 945.8 g
Color Usually smoky brown, but also pink, red, green, blue, or rarely white.
Crystal habit Stout prismatic, with a curved convex trigonal outline
Cleavage Poor or absent on {0001}[1]
Fracture Uneven to conchoidal
Tenacity Brittle
Mohs scale hardness 7½
Luster Vitreous
Streak White to very light brown
Diaphaneity Transparent to translucent
Specific gravity 3.02
Optical properties Uniaxial (-)
Refractive index No = 1.637, Ne = 1.621
Pleochroism Strong: O dark brown or pink, E light brown or pale pink
Other characteristics Not fluorescent, not radioactive
References [2][3][4][5]

Fluor-liddicoatite[6] is a rare member of the tourmaline group of minerals, elbaite subgroup, and the theoretical calcium endmember of the elbaite-fluor-liddicoatite series; the pure end-member has not yet been found in nature.[2] Fluor-liddicoatite is indistinguishable from elbaite by X-ray diffraction techniques. It forms a series with elbaite and probably also with olenite.[2] Liddiocoatite is currently a non-approved mineral name, but Aurisicchio et al. (1999) and Breaks et al. (2008) found OH-dominant species.[7][8] Formulae are

Fluor-liddicoatite was named in 1977 after Richard T. Liddicoat (1918–2002) gemmologist and president of the Gemological Institute of America,[1] who is well known for introducing the GIA diamond grading system in 1953.

Unit cell

Fluor-liddicoatite belongs to the trigonal crystal system, class 3 m, space group R 3m. It has a rhombohedral lattice, with unit cell parameters

Structure

Fluor-liddicoatite is isostructural with (has the same structure as) all members of the tourmaline group,[2] which are cyclosilicates with the general formula

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

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.[9][10]

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.[2]

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.[11]
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+.[11]
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.[1] 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.

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.[1] 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.[9] Associated minerals are quartz, elbaite, albite and micas.[5]

Localities

A spectacular radiating spray of liddicoatite crystals, from the Minh Tien Mine, Luc Yen, Vietnam. Size: 8.5 x 7.6 x 4.7 cm.

The type locality is Anjanabonoina, Tsilaizina, Antsirabe, Madagascar.[2] Type Material is stored at the National Museum of Natural History, Smithsonian Institution, Washington, D.C., US, catalogue #135815; further type material is stored at the Natural History Museum, London, the Royal Ontario Museum, Canada and the Geological Survey of Canada.[1]

References

  1. 1 2 3 4 5 6 7 American Mineralogist (1977) 62:1121
  2. 1 2 3 4 5 6 7 8 Gaines et al (1997) Dana’s New Mineralogy. Wiley
  3. 1 2 3 http://www.mindat.org/min-2395.html
  4. 1 2 3 http://www.webmineral.com/data/Liddicoatite.shtml
  5. 1 2 3 4 http://rruff.geo.arizona.edu/doclib/hom/liddicoatite.pdf
  6. ↑ Darrell J. Henry, Milan Novák, Frank C. Hawthorne, Andreas Ertl, Barbara L. Dutrow, Pavel Uher, and Federico Pezzotta (2011). "Nomenclature of the tourmaline-supergroup minerals" (PDF). American Mineralogist 96: 895–913. doi:10.2138/am.2011.3636.
  7. ↑ 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: 237–242.
  8. ↑ Breaks, F.W., Tindle, A.G., and 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. 235. Ontario Geological Survey, Miscellaneous Release Data.
  9. 1 2 Deer, Howie and Zussman (1986) Rock-forming minerals, (2nd edition), Volume 1B, Disilicates and Ring Silicates
  10. ↑ American Mineralogist (1948) 33:532
  11. 1 2 extraLapis English No 3: Tourmaline (2002)
  12. ↑ The Mineralogical Record (2006) 37-5:482
  13. 1 2 The Mineralogical Record (2007) 38-3:220

External links

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