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Talmessite

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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 B1, space group PB1 (All) Triclinic (AM45)
Identification
Formula mass418.33 g (Webmin)
ColorWhite, colourless, pale green (nickelian), brownish, pink or brownish pink (cobaltian); colourless in transmitted light (All). Colourless (AM42) Pale green (AM45)
Crystal habitAs crystals, to 3 mm; radiating fibrous aggregates, may be stalactitic or in crusts (HOM). Fine crystalline aggregates (Dana) Prismatic (AM42)
TwinningPolysynthetic twinning (AM42)
Mohs scale hardness5 (All)
LusterVitreous (Mindat, Webmin, HOM)
Streakwhite (Webmin)
DiaphaneityTransparent to Translucent (All)
Specific gravity3.57 (Dana), 3.421 (HOM), 3.421 (AM45) cobaltoan 3.574 (AM45) 3.2 – 3.5 (AM50)
Optical propertiesBiaxial (–) (AM45, All) positive (AM42)
Refractive indexnα = 1.672, nβ = 1.685, nγ = 1.698 (am50, All) nα = 1.680, cobaltoan 1.695, nγ = 1.69, cobaltoan 1.73 (AM45)
Birefringenceδ = 0.026 (Mindat, Webmin)
PleochroismCobaltoan variety is pleochroic colourless to pale rose (AM45)
Other characteristicsNot radioactive (Webmin). Loss of H2O at 450 deg (AM45)
References[1] [2] [3] [4] [5] [6] [7] [8]


Dana: 40 hydrated normal phosphates, arsenates and vanadatesNamed in 1960 for the locality (Dana) Varieties: Cobaltoan Talmessite, Nickeloan Talmessite (Mindat) Fairfieldite group. Forms a series with gaitite (Mindat, HOM), β-Roselite (Mindat) Fairfieldite Group (Dana, Mindat, HOM) Dimorphic with wendwilsonite (Dana) Fairfieldite Group Members: (Mindat) 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

Formula: Ca2Mg(AsO4)2.2H2O (IMA) Crystal Class: Triclinic B1, space group PB1 (All) Triclinic (AM45) Structure: The structure is dominated by chains of tetrahedral AsO4 and octahedral [(cation-O4(H2O)2)] that parallel the c axis. The B octahedral are compressed, resulting in chain disorder (Dana) Unit Cell Parameters: Z = 1 (Dana, Webmin, HOM) a = 5.874 Å, b = 6.943 Å, c = 5.537 Å, α = 97.3°, β = 108.7°, γ = 108.1° (Dana) a = 5.87Å, b = 6.94Å, c = 5.53Å α = 97.3°, β = 108.7°, γ = 108.1° (Mindat) a = 5.89, b = 7.69, c = 5.56, alpha = 112.633°, beta = 70.817°, gamma = 119.417° (AM45, AM50, Webmin) a = 5.874(7), b = 6.943(11), c = 5.537(6), α = 97.3(1)◦ β = 108.7(1)◦ γ = 108.1(2)◦ (HOM) Calculated specific gravity = 3.42 (Dana), 3.63 (Webmin), 3.53 (HOM), 3.491 (AM45) 3.49 (AM50)

Appearance

Crystal habits: As crystals, to 3 mm; radiating fibrous aggregates, may be stalactitic or in crusts (HOM). Fine crystalline aggregates (Dana) Prismatic (AM42) Colour: White, colourless, pale green (nickelian), brownish, pink or brownish pink (cobaltian); colourless in transmitted light (All). Colourless (AM42) Pale green (AM45) Streak: white (Webmin) Lustre: Vitreous (Mindat, Webmin, HOM) Diaphaneity: Transparent to Translucent (All)

Physical properties

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

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. ^ <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>
  2. ^ 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>
  3. ^ <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>
  4. ^ <Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley>
  5. ^ <http://www.mindat.org/min-3876.html>
  6. ^ <http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo>
  7. ^ <http://www.handbookofmineralogy.org>
  8. ^ http://pubsites.uws.edu.au/ima-cnmnc/>