Tuperssuatsiaite

Tuperssuatsiaite

Tuperssuatsiaite from the Aris Quarries, Namibia. Field of view 5 mm.
General
Category Phyllosilicate
Formula
(repeating unit)		 NaFe3+3Si8O20(OH)2·H2O[1]
Strunz classification 9.EE.20
Dana classification 74.3.1a.2
Crystal system Monoclinic
Unit cell a = 13.72 Å, b = 18 Å
c = 4.82 Å; β = 104.28°; Z = 2
Identification
Formula mass 818.31
Color Dark to light red-brown
Crystal habit Fan-shaped aggregates, rosettes or fibers
Crystal symmetry Monoclinic - prismatic
H-M symbol (2/m)
Space group: C 2/m
Twinning Common
Cleavage Good on {100}
Fracture Uneven to conchoidal
Tenacity Brittle
Mohs scale hardness Not determined
Luster Vitreous
Streak Brownish yellow
Diaphaneity Transparent
Specific gravity 2.465
Optical properties Biaxial (+)
Refractive index nα = 1.539, nβ = 1.560, nγ = 1.595
Birefringence δ = 0.056
Pleochroism Colorless to yellowish brown or reddish brown
2V angle Measured: 103° to 103°, Calculated: 78°
Other characteristics Neither radioactive[2] nor fluorescent[3]
References [2][4][5][6]

Tuperssuatsiaite is a rare clay mineral found in Greenland, Namibia and Brazil. It is a hydrated phyllosilicate (sheet silicate) of sodium and iron.

Discovery

Tuperssuatsiaite was first found by Karup-Moller and Petersen in Greenland, in 1984, and given the International Mineralogical Association designation IMA1984-002. It was later named after the type locality, Tuperssuatsiat Bay, Ilimaussaq, Greenland.[3] In 1992 Karup-Moller and Petersen, together with von Knorring and Leonardsen, found more specimens from a second find in the Aris Quarry in Namibia, that allowed a better definition of the properties and composition of the mineral.[7] Later still, in 2005, more material was found by a group of researchers from the University of São Paulo at the Bortolan Quarry, Pocos de Caldas, Brazil, and although the crystals were small, accurate determinations were made of their physical and optical properties, which differed slightly from those of the specimens from Greenland and Namibia.[8]

Mineral group

Tuperssuatsiaite is a member of the palygorskite-sepiolite group, palygorskite subgroup.
Subgroup members (formulae according to the IMA[1]):

Iron occurs both in the ferric state Fe3+ and the ferrous state Fe2+. The formula for tuperssuatsiaite contains only ferric iron, and the Greenland material is consistent with this. Analysis of the Namibian material, however, shows that part of the iron is in the ferrous state.[4] Manganese is also present as a substitute for iron,[4] and a zinc-rich material has been reported from Greenland.[3]

Structure

The mineral belongs to the monoclinic crystal class 2/m, meaning that it has a twofold axis of rotational symmetry perpendicular to a mirror plane. At one time it was thought that the material from Namibia might belong to the monoclinic class 2, without the mirror plane,[7] but a more recent study gives it as 2/m, the same as the material from Greenland.[9]

The space group is B2/m, meaning that in the unit cell there is one structural unit at each vertex, and one in the centre of each B face. The palygorskite-sepiolite minerals are clay minerals with a layered structure. In tuperssuatsiaite ribbons of SiO4 tetrahedra, similar to those in the amphibole structure, are aligned parallel to the c crystal axis, and they link to form layers parallel to the plane containing the a and b axes.[9] The spacing between the layers, in the c direction, is about 5 Å, which is typical for minerals with an amphibole-type structure, due to the repeat distance along the chains of tetrahedra.[4] Channels occur that could be occupied by H2O as in palygorskite.[9]

Unit cell

There are two formula units per unit cell (Z = 2), and the cell dimensions vary slightly for specimens from different locations; for all specimens, to the nearest Å, a = 14 Å, b = 18 Å and c = 5 Å, and the angle β = 103° to 105°.

For the three main localities the reported values are:

Appearance

Tuperssuatsiaite occurs as fan-shaped aggregates up to several centimeters across, as rosettes and as fibers elongated parallel to the c axis.[3] It is red-brown in reflected light, and colorless to light yellowish brown in transmitted light, with a brownish yellow streak. Crystals are transparent with a bright vitreous luster, but aggregates may be dull and translucent.[3]

Optical properties

The mineral is biaxial (+), with refractive indices Nx ~ 1.54, Ny ~ 1.56 and Nz ~ 1.58 to 1.60.

For the three main localities the reported values are:

It is mildly pleochroic, with X colorless, Y colorless to pale brown or green and Z generally reddish brown. No fluorescence has been observed.[3]

Physical properties

Cleavage is good on a plane containing the b and c crystal axes, parallel to the layers within the structure, and twinning is common.[4][6]

Fracture is uneven to conchoidal (shell-like) and the mineral is brittle;[6] it is quite light, with specific gravity 2.465,[2][5] which is similar to that of quartz.

Type locality

The type locality is Tuperssuatsiat Bay, Tunugdliarfik Firth (Eriksfjord), Ilimaussaq complex, Narsaq, Kitaa (West Greenland) Province, Greenland,[5] and type material is conserved at the University of Copenhagen, Copenhagen, Denmark, and at the National Museum of Natural History, Washington DC, US, reference number 162402.[6]

Occurrence and associations

References

  1. 1 2 <http://rruff.info/ima>
  2. 1 2 3 Webmineral data
  3. 1 2 3 4 5 6 7 8 9 Dunn et al (1985) summarising Karup-Moller and Petersen( 1984) Neues Jahrbuch Mineral Monatsh: 501. American Mineralogist 70: 1332
  4. 1 2 3 4 5 Gaines et al (1997) Dana’s New Mineralogy Eighth Edition. Wiley
  5. 1 2 3 4 Mindat.org
  6. 1 2 3 4 5 Handbook of Mineralogy
  7. 1 2 3 4 <Jambor (1992) summarising von Knorring, Petersen, Karup-Moller and Leonardsen (1992) Neues Jahrbuch Mineral Monatsh: 145. American Mineralogist 77: 1308>
  8. 1 2 3 4 Atencio, Coutinho and Vlach (2005) The Mineralogical Record 36-3: 275-280
  9. 1 2 3 Cámara, Garvie, Devouard, Groy and Buseck (2002) American Mineralogist 87: 1458
  10. ↑ http://www.koeln.netsurf.de/~w.steffens/aris.htm
  11. ↑ http://www.koeln.netsurf.de/~w.steffens/lovo.htm

External links

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