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== Physical properties ==
== Physical properties ==
Talmessite is a moderately hard mineral, with [[Moh's hardness]] 5, harder than [[fluorite]] but not as hard as [[quartz]]. The [[specific gravity]] calculated from the [[formula]] and the [[unit cell|cell]] dimensions is 3.49,<ref name=AM49/> <ref name=AM50/> but the measured value is less for ordinary talmessite, at 3.42<ref name=AM45/> <ref name=HOM/> and more for the cobaltoan variety, at 3.57.<ref name=AM45/> <ref name=Dana/> The mineral displays [[polysynthetic twinning]].<ref name=AM42/> It is not [[radioactive]]<ref name=Webmin/> and it loses [[water of crystalllisation]] at 450<sup>o</sup>.<ref name=AM45/>
Molar Mass : 418.33 gm (Webmin)
Polysynthetic twinning (AM42)
Hardness: 5 (All)
Specific Gravity: 3.57 (Dana), 3.421 (HOM), 3.421 (AM45) cobaltoan 3.574 (AM45) 3.2 – 3.5 (AM50)
Other Characteristics: Not radioactive (Webmin). Loss of H2O at 450 deg (AM45)


== Optical properties ==
== Optical properties ==

Revision as of 10:08, 24 February 2015


Talmessite

Talmesite
Talmessite from Gold Hill Mine, Utah, USA
General
CategoryArsenate mineral
Formula
(repeating unit)		Ca2Mg(AsO4)2.2H2O
Strunz classification7/C.17-50 or 8.CG.05
Dana classification40.02.02.05
Crystal systemTriclinic 1, space group P1
Identification
Formula mass418.33 g[1]
ColorWhite or colourless. Nickeloan talmessite is pale green and cobaltoan is brownish or pink. Colourless in transmitted light.
Crystal habitPrismatic crystals[2], radiating fibrous aggregates[3]
TwinningPolysynthetic[2]
Mohs scale hardness5
LusterVitreous
StreakWhite[1]
DiaphaneityTransparent to Translucent
Specific gravity3.421[4] [3], cobaltoan varieties 3.574[4]
Optical propertiesBiaxial (–)
Refractive indexnα = 1.672, nβ = 1.685, nγ = 1.698
Birefringenceδ = 0.026
PleochroismThe cobaltoan variety is pleochroic, colourless to pale rose[4]
Other characteristicsNot radioactive.[1] It loses water of crystallisation at 450 deg[4]
References[2] [4] [5] [6] [7] [1] [3] [8]

Talmessite is a hydrated calcium magnesium arsenate, often with significant amounts of cobalt or nickel.[7] It was named in 1960 for the type locality, the Talmessi mine, Anarak district, Iran. It forms a series with β-Roselite, where cobalt replaces some of the magnesium, and with gaitite, where zinc replaces the magnesium.[7] [3] All these minerals are members of the fairfieldite group. Talmessite is dimorphic with wendwilsonite (which is not a member of this group).[6]

Members of the fairfieldite group[7]

Cassidyite Ca2(Ni2+,Mg)(PO4)2 · 2H2O

Collinsite Ca2(Mg,Fe2+)(PO4)2 · 2H2O

Fairfieldite Ca2(Mn2+,Fe2+)(PO4)2 · 2H2O

Gaitite Ca2Zn(AsO4)2 · 2H2O

Messelite Ca2(Fe2+,Mn2+)(PO4)2 · 2H2O

Nickeltalmessite Ca2Ni(AsO4)2 · 2H2O

Parabrandtite Ca2Mn2+(AsO4)2 · 2H2O

Unnamed (Fe2+-analogue of Parabrandtite) Ca2Fe2+(AsO4)2 · 2H2O

β-Roselite Ca2(Co2+,Mg)(AsO4)2 · 2H2O

Talmessite Ca2Mg(AsO4)2.2H2O

Crystallography

The formula for talmessite is Ca2Mg(AsO4)2.2H2O[8]. It is a triclinic mineral[4], crystal class 1, space group P1. There is one formula unit per unit cell (Z = 1) and the unit cell parameters are variously given as a = 5.87Å, b = 6.94Å, c = 5.53Å α = 97.3°, β = 108.7°, γ = 108.1°[6] [7] [3], or a = 5.89, b = 7.69, c = 5.56, alpha = 112.633°, beta = 70.817°, gamma = 119.417°,[4] [5] [1]. These values give a calculated specific gravity varying from 3.42 to 3.63.
The structure is dominated by chains of tetrahedral AsO4 and octahedral [(cation-O4(H2O)2)] groups that parallel the c crystal axis. The octahedral are compressed, resulting in chain disorder[6]

Appearance

Talmessite occurs as prismatic crystals to 3 mm[2], as radiating fibrous aggregates[3] or as fine crystalline aggregates[6]; it may also be stalactitic or in crusts.[3]. Pure talmessite is white or colourless, and colourless in transmitted light, but nickeloan varieties are pale green[4] and cobaltoan varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The streak is white[1] and crystals are transparent to translucent with a vitreous lustre.

Physical properties

Talmessite is a moderately hard mineral, with Moh's hardness 5, harder than fluorite but not as hard as quartz. The specific gravity calculated from the formula and the cell dimensions is 3.49,[9] [5] but the measured value is less for ordinary talmessite, at 3.42[4] [3] and more for the cobaltoan variety, at 3.57.[4] [6] The mineral displays polysynthetic twinning.[2] It is not radioactive[1] and it loses water of crystalllisation at 450o.[4]

Optical properties

Optic Class: Biaxial (–) (AM45, All) positive (AM42) Refractive Indices: nα = 1.672, nβ = 1.685, nγ = 1.698 (am50, All) nα = 1.680, cobaltoan 1.695, nγ = 1.69, cobaltoan 1.73 (AM45) Maximum birefringence δ = 0.026 (Mindat, Webmin) 2V measured = ~90o (AM50, All), 2V calculated = 88° (Mindat, Webmin) Dispersion of the optic axes: r > v strong (Mindat) Cobaltoan variety is pleochroic colourless to pale rose (AM45)

Occurrence

Occurrence and Associations: A rare secondary species formed typically in the oxidized zone of some hydrothermal mineral deposits, an alteration product of realgar, orpiment, or Cu–Ni arsenides (Webmin, HOM). Zone of alteration of cobalt arsenide deposits (AM42) Zone of oxidation (AM45) Association: Gaitite, erythrite, annabergite, picropharmacolite, pharmacolite, austinite, fluorite, barite, aragonite, calcite, dolomite (HOM). Associated with aragonite and dolomite at the type locality (AM45) Associated with dolomite at the type locality (Dana).

Localities

Type Locality Talmessi Mine (Talmesi Mine; Telmessi Mine; Telmesi Mine), Anarak District, Nain County (Nayin County), Esfahan Province (Isfahan Province; Aspadana Province), Iran (Mindat) Type Material: National School of Mines, Paris, France; the Natural History Museum, London, England (HOM).

Synonyms (Mindat): Arsenate-belovite, Belovite (of Nefedov) Name in Other Languages: German:Arsenatbelowit, Talmessit Russian:Талмессит Spanish:Talmessita


References

  1. ^ a b c d e f g http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo
  2. ^ a b c d e American Mineralogist (1957) 42:583 abstract from Yakhontova, L K and G A Sidorenko (1956) A new mineral – arsenate-belovite. Zapiski V ses Mineralog Obshch 85 No 3, 297-302
  3. ^ a b c d e f g h http://www.handbookofmineralogy.org
  4. ^ a b c d e f g h i j k American Mineralogist (1960) 45:1315 abstract from Bariand, P. & Herpin, P. (1960): Un arséniate de calcium et de magnésium, isomorphe de la β rosélite. Bulletin de la Société française de Minéralogie et de Cristallographie: 83: 118-121>
  5. ^ a b c American Mineralogist (1965) 50:813 abstract from Pierrot, R. (1964): Contribution à la minéralogie des arséniates calciques et calcomagnésiens naturels. Bull. Soc. fr. Minéral. Cristallogr. 87, 169-211
  6. ^ a b c d e f Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley
  7. ^ a b c d e http://www.mindat.org/min-3876.html
  8. ^ a b http://pubsites.uws.edu.au/ima-cnmnc/>
  9. ^ Cite error: The named reference AM49 was invoked but never defined (see the help page).
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