Köttigite

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Köttigite
Specific gravity
3.33
Optical propertiesBiaxial (+)
Refractive indexnα = 1.622
nβ = 1.638
nγ = 1.671
Birefringenceδ = 0.049
PleochroismVisible. X, Y = colourless, Z = pale red.
SolubilitySoluble in acids
Other characteristicsNot fluorescent
References[2][3][4][5][6][7]

Köttigite is a rare hydrated

dimorph of metaköttigite, which means that the two minerals have the same formula, but a different structure: köttigite is monoclinic and metaköttigite is triclinic.[8]
There are several minerals with similar formulae but with other
vivianite group
.

The Vivianite Group is a group of

cations. The group members are annabergite, arupite, babanekite, baricite, erythrite, hornesite, kottingite, manganhornesite, pakhomovskyite, parasymplesite and vivianite.[6]

Structure

Köttigite belongs to the monoclinic crystal system, so it has two crystal axes, a and c, inclined to each other at angle β, and a third axis, the b axis, at right angles to both a and c. It belongs to point group 2/m, which means that it has a two-fold axis of rotational symmetry parallel to b, and a mirror plane perpendicular to it. The space group is C2/m, which means that the unit cell is centred on the C face.[2][3]

Although zinc is the only transition metal that appears in the formula, köttigite usually contains significant quantities of

hydrogen bonding alone, which is quite weak, hence the perfect cleavage in this direction.[3]

There are two formula units per unit cell (Z = 2) and the cell parameters are a = 10.24 Å, b = 13.405 Å, c = 4.757 Å and β = 105.21°.[3]

Appearance

Pure end-member köttigite is colourless, but frequently samples are coloured pink, red, red-orange or brown by elements substituting for the zinc. It is light rose-pink in transmitted light, translucent with a reddish-white to white streak and a resinous or waxy lustre, silky on fractures.[6] Crystals are small, prismatic parallel to the c axis and flattened perpendicular to the b axis. It also occurs as massive crusts with a crystalline surface and fibrous structure.[4][6]

Optical properties

The mineral is

refractive indices nα = 1.622, nβ = 1.638 and nγ = 1.671.[2]
The maximum birefringence δ is the difference between the largest and the smallest refractive index, and is equal to 0.049.

Biaxial crystals have two optic axes, and the angle between them is known as the optic angle, 2V.[9] For köttigite 2V has a measured value of 74°, and a calculated value of 72°.[6]

Biaxial crystals have three mutually perpendicular principal optical direction, named X, Y and Z. Light travels at different speeds in different directions through the crystal. X is the direction of travel at the highest speed, Z at the lowest, and Y intermediate. The orientation is given by expressing the relationship of X, Y and Z to the crystallographic axes a, b and c. In monoclinic crystals one of the principal optical directions X, Y and Z coincides with the b axis. Since X, Y and Z are mutually perpendicular, it suffices to define just two of them, then the third is determined.[9] For köttigite X=b and Z^c=37°.[2]

fluorescent.[6]

Physical properties

Köttigite is soft, with

specific gravity 3.33.[2] Because of its sheetlike structure it has perfect cleavage perpendicular to the b axis;[2] it is flexible, and has a fibrous fracture giving it a silky lustre on cleavage surfaces.[6] It is soluble in acids.[4][6]

Occurrence and associations

It is formed by the alteration of

ore deposit, associated with roselite Ca2(Co2+,Mg)(AsO4)2·8H2O.[6]

At the

Ojuela Mine, Mapimí Municipality, Mexico, it occurs in sprays of bladed crystals to 6 mm, which is large for the species,[4] associated with symplesite Fe2+3(AsO4)2·8H2O, parasymplesite Fe2+3(AsO4)2·8H2O, adamite Zn2(AsO4)(OH), legrandite Zn2(AsO4)(OH)·H2O, metaköttigite Zn3(AsO4)2·8H2O and gypsum Ca(SO4)·2H2O.[5]

At the Hilton Mine, Cumbria, England, köttigite has been found in a specimen of galena PbS and gersdorffite NiAsS (but no sphalerite), on a surface coated with annabergite Ni3(AsO4)2·8H2O and an earthy crust. The individual crystals are colourless, transparent, and very small, the largest being about 1 mm.[8]

At Bou Azzer, Taznakht, Morocco, köttigite has been identified in a sample of vein quartz SiO2 rich in chalcopyrite CuFeS2 and sphalerite ZnS. The sample has turquoise-blue secondary minerals including devilline CaCu4(SO4)2(OH)6·3H2O, and also lath-shaped, blue-grey to pinkish grey crystals of köttigite with a habit resembling erythrite Co3(AsO4)2·8H2O, measuring less than 2 mm. The crystals are relatively rich in iron and cobalt, with traces of copper and nickel.[10]

References

  1. S2CID 235729616
    .
  2. ^ a b c d e f Wolfe, C.W. (1940). "Classification of minerals of the type A3(XO4)2·nH2O (Concluded)". American Mineralogist. 25 (12): 804. Retrieved 7 August 2022.
  3. ^ a b c d e Hill, R.J. (1979). "The crystal structure of koettigite". American Mineralogist. 64 (3–4): 376–382. Retrieved 7 August 2022.
  4. ^ a b c d e f Gaines et al (1997) Dana's New Mineralogy Eighth Edition. Wiley
  5. ^ a b Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2005). "Köttigite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved 7 August 2022.
  6. ^ a b c d e f g h i j k l Köttigite, Mindat.org, retrieved 8 August 2022
  7. ^ a b Barthelmy, David (2014). "Kottigite Mineral Data". Webmineral.com. Retrieved 27 June 2022.
  8. ^ a b c Bridges, T.F.; Green, D.I. (2006). "The first British occurrence of kottigite, from Hilton Mine, Scordale, Cumbria" (PDF). Journal of the Russell Society. 9: 3. Retrieved 7 August 2022.
  9. ^
    ISBN 047157452X
    .
  10. ^ Meisser, Nicolas; Favreau, Georges; Brugger, Joël; Haddouch, Lahcen Ait; Maacha, Lhou; Dietrich, Jacques Emile (2007). "Famous mineral localities: Bou Azzer, Morocco". The Mineralogical Record. 38 (5): 381.
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