ADAMITE

Zn₂(AsO₄)(OH)

Adamite [Zn₂ (AsO₄) (OH)] belongs to Arsenate group of minerals and it was named after Gilbert Joseph Adam, French mineralogist. It naturally occurs in the oxidation zone of zinc-copper-arson bearing ore deposits.  Structure of adamite is orthorhombic and Zn has to be substituted by Cu near 80 atom% Zn/(Cu+Zn) before there is a changeover in symmetry to olivenite (olivenite-monoclinic (Cu₂(AsO₄)(OH)) ¹. In pure adamite, one Zn atom is octahedrally coordinated by four oxygen atoms and two hydroxyl groups in a cis arrangement, and the other Zn is coordinated by a trigonal bipyramid of four oxygen atoms and one apical hydroxyl group. The most apparent difference between adamite structure and its dimorphous pair, paradamite, is the coordination of the zinc atoms ². 

IR spectroscopic studies on adamite are usually reported in combination with olivenite,  showing the distinguishing features between the two crystal structures. According to Braithwaite¹, pure adamite displays a single very sharp absorption at 3540 cm^-1 for OH-stretching bond non-hydrogen bonded while Farmer ³ records additional bands at 3555 and 3510 cm^-1. Bands observed around 885, 826, 805 and 740 cm^-1 are assigned to the ( AsO₄)^3- v₁ and  v₃ stretching modes ³. The v₄ vibrational mode is found around 475 and 410 cm^{-1 3}, while splitting of the Zn-O vibrational modes is observed at 515 and 535 cm^{-1 4}.

In the Raman spectra of adamite, strong characteristic bands are found at 894, 851, and 327 cm^-1, which are accompanied by smaller intensity bands at 825, 810, 540, 427, 418, 316, 258 and 236 cm^{-1 5}.

1.    Braithwaite, R.S.W. (1983) Infrared spectroscopic analysis of the olivenite-adamite series, and of phosphate substitution in olivenite. Mineral. Mag. 47(342), 51-7.

2.    Hawthorne, F.C. (1976) A refinement of the crystal structure of adamite. American Mineralogist, 61(9-10), 979-986.  

3.    V.C. Farmer, 1974 Infrared spectra of minerals in Mineralogical Society Monograph 4 VC Farmer (Ed) pp 399. Mineralogical Society, London.

4.    Sumin De Portilla, V.I. (1974) Infrared spectroscopic investigation of the structure of some natural arsenates and the nature of hydrogen-bonds in their structures. Can. Mineral., 12(4), 262-8.

5.    Griffith, W.P. (1970) Raman studies on rock-forming minerals. II. Minerals containing MO3, MO4 and MO6 groups. J. Chem. Soc. A(2), 286-91.

SELECTED REFERENCES ON SPECTROSCOPY OF ADAMITE:  

3.    Sumin De Portilla, V.I. (1974) Infrared spectroscopic investigation of the structure of some natural arsenates and the nature of hydrogen-bonds in their structures. Can. Mineral., 12(4), 262-8. 

4.    J.A. Gadsden 1975. Infrared spectra of minerals and related inorganic compounds. Butterworths, London. Pp126.  

Original spectra shown for this mineral can be obtained on request from J.T. Kloprogge (E-mail t.kloprogge@qut.edu.au), or R.L. Frost (E-mail r.frost@qut.edu.au).

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