Liddicoatite
Encyclopedia
Liddicoatite is a rare member of the tourmaline
group of minerals, elbaite
subgroup, and the theoretical calcium
endmember
of the elbaite-liddicoatite series; the pure end-member has not yet been found in nature. Liddicoatite is indistinguishable from elbaite by X-ray diffraction techniques. It forms a series with elbaite and probably also with olenite.
Formulae are
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.
R 3m. It has a rhombohedral lattice, with unit cell parameters
group, which are cyclosilicates
with the general formula
For 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 .
, 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. Liddicoatite usually has a pedion (a single crystal face) opposite one or two pyramids
.
, 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
.
, 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.
of granitic
pegmatites
. Associated minerals are quartz
, elbaite
, albite
and micas.
is Anjanabonoina, Tsilaizina, Antsirabe
, Madagascar.
Type Material is stored at the National Museum of Natural History, Smithsonian Institution
, Washington, D.C., USA, catalogue #135815; further type material is stored at the Natural History Museum, London, the Royal Ontario Museum
, Canada and the Geological Survey of Canada.
Tourmaline
Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
group of minerals, elbaite
Elbaite
Elbaite, a sodium, lithium, aluminium boro-silicate, is a mineral species belonging to the tourmaline group. Elbaite forms three series, with dravite, with liddicoatite, and with schorl...
subgroup, and the theoretical calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
endmember
Endmember (mineralogy)
An endmember in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity. Minerals often can be described as solid solutions with varying compositions of some chemical elements, rather than as substances with an exact chemical formula...
of the elbaite-liddicoatite series; the pure end-member has not yet been found in nature. Liddicoatite is indistinguishable from elbaite by X-ray diffraction techniques. It forms a series with elbaite and probably also with olenite.
Formulae are
- Liddicoatite Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F
- Elbaite Na(Al1.5Li1.5)Al6(BO3)3Si6O18(OH)4
- Olenite NaAl9B3Si6O27O3(OH)
Liddicoatite was named in 1977 after Richard T. Liddicoat
Richard T. Liddicoat
Richard T. Liddicoat"Father of Gemology"-Timeline:* March 2, 1918 Born in Kearsage, Michigan to Richard T. and Carmen Beryl Liddicoat.* June 28, 1940 Joins the staff of GIA as assistant director of education...
(1918 - 2002) gemmologist and president of the Gemological Institute of America
Gemological Institute of America
The Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...
, who is well known for introducing the GIA diamond grading system
Diamond color
A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice...
in 1953.
Unit Cell
Liddicoatite belongs to the trigonal crystal system, class 3 m, space groupSpace group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
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 formular units per unit cell).
Structure
Liddicoatite is isostructural with (has the same structure as) all members of the tourmalineTourmaline
Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
group, which are cyclosilicates
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
with the general formula
- XY3Z6(BO3)3Si6O18(OH,O)3(OH,F).
For 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
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
by oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
O and hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
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
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...
by O, forming SiO4 groups. These tetrahedra form six-membered rings
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
, 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
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms are also atoms of the aluminum coordination octahedra. The X sites are sandwiched between the units along the c axis .
Crystal Habits
Crystals are stout prismaticPrism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
, with a curved convex trigonal outline, generally elongated and striated parallel to the c axis
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
. Crystals are hemimorphic, meaning that the two ends of the crystal have different forms. Liddicoatite usually has a pedion (a single crystal face) opposite one or two pyramids
Pyramid (geometry)
In geometry, a pyramid is a polyhedron formed by connecting a polygonal base and a point, called the apex. Each base edge and apex form a triangle. It is a conic solid with polygonal base....
.
Physical Properties
The color is usually smoky brown, but also pink, red, green, blue, or rarely white. Color zoning is abundant at the type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
, 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
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
Mn3+ content, and the green color is due to intervalence charge transfer transactions between iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
Fe2+ and titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
Ti4+.
The streak
Streak (mineralogy)
The streak of a mineral is the color of the powder produced when it is dragged across an unweathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus...
is white to very light brown, lighter than the mass color, luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
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
Fracture (mineralogy)
In the field of mineralogy, fracture is a term used to describe the shape and texture of the surface formed when a mineral is fractured. Minerals often have a highly distinctive fracture, making it a principal feature used in their identification....
. It is very hard, with hardness 7½, a little harder than zircon, making it suitable for use as a gemstone. Specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is 3.02, a little lighter than fluorite. It is neither fluorescent
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
nor radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
Optical Properties
Liddicoatite is uniaxial (-)Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
, with
refractive Indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, which are usually present as trace elements.
Pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
is strong: O dark brown or pink, E light brown or pale pink.
Environment
Liddicoatite is detrital in soil at the type locality, presumably derived from the weatheringWeathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
of granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
pegmatites
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
. Associated minerals are quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, elbaite
Elbaite
Elbaite, a sodium, lithium, aluminium boro-silicate, is a mineral species belonging to the tourmaline group. Elbaite forms three series, with dravite, with liddicoatite, and with schorl...
, albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
and micas.
Localities
The type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
is Anjanabonoina, Tsilaizina, Antsirabe
Antsirabe
Antsirabe is the third largest city in Madagascar and has a population of approximately 183,000. It's the capital of the Vakinankaratra region...
, Madagascar.
Type Material is stored at the National Museum of Natural History, Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...
, Washington, D.C., USA, catalogue #135815; further type material is stored at the Natural History Museum, London, the Royal Ontario Museum
Royal Ontario Museum
The Royal Ontario Museum is a museum of world culture and natural history in Toronto, Ontario, Canada. With its main entrance facing Bloor Street in Downtown Toronto, the museum is situated north of Queen's Park and east of Philosopher's Walk in the University of Toronto...
, Canada and the Geological Survey of Canada.
- Deep brilliant red, highly lustrous, prismatic liddicoatite has been found as large sheaves of near parallel, slightly divergent crystals, at the Minh Tien pegmatite, Luc YenLuc YenLục Yên is a rural district of Yen Bai province, in the northeastern region of Vietnam. As of 2003 the district had a population of 100,785. The district covers an area of 811 km². The district capital lies at Yen The. Numerous quarries in the district produce gemstones such as rubies,...
District, in Vietnam. - At Ambalabe, Manapa, near BetafoBetafoBetafo refers to a town and surrounding district in Vakinankaratra Region, Madagascar, and a village in the district of Arivonimamo, in Itasy Region. The village's inhabitants are said to be divided between descendants of andriana and descendants of former slaves . This aspect was the subject...
, Madagascar, very lustrous, striated, short prismatic liddicoatite crystals with trigonal terminations have been found, loose or on pegmatite matrixMatrix (geology)The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
; 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 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.