Talmessite
Talmessite
| Talmesite | |
|---|---|
 Talmessite from Gold Hill Mine, Utah, USA | |
| General | |
| Category | Arsenate mineral |
| Formula (repeating unit) | Ca2Mg(AsO4)2.2H2O |
| Strunz classification | 7/C.17-50 or 8.CG.05 |
| Dana classification | 40.02.02.05 |
| Crystal system | Triclinic B1, space group PB1 |
| Identification | |
| Formula mass | 418.33 g[1] |
| Color | White or colourless. Nickeloan talmessite is pale green and cobaltoan is brownish or pink. Colourless in transmitted light. ( |
| Crystal habit | Prismatic crystals[2], radiating fibrous aggregates[3] |
| Twinning | Polysynthetic[2] |
| Mohs scale hardness | 5 |
| Luster | Vitreous |
| Streak | white[1] |
| Diaphaneity | Transparent to Translucent |
| Specific gravity | 3.421[4] [3], cobaltoan varieties 3.574[4] |
| Optical properties | Biaxial (–) |
| Refractive index | nα = 1.672, nβ = 1.685, nγ = 1.698 |
| Birefringence | δ = 0.026 |
| Pleochroism | The cobaltoan variety is pleochroic, colourless to pale rose[4] |
| Other characteristics | Not 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
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
- ^ a b c d e http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo
- ^ a b c 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
- ^ a b c d e http://www.handbookofmineralogy.org
- ^ a b c d e f g 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>
- ^ a b 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
- ^ a b c d Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley
- ^ a b c d e http://www.mindat.org/min-3876.html
- ^ a b http://pubsites.uws.edu.au/ima-cnmnc/>