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 1, space group P1 |
| Identification | |
| Formula mass | 418.33 g[1] |
| Color | White or colourless, pale green (nickel-rich) or brownish or pink (cobalt-rich). 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 450o[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 nickel-rich varieties are pale green[4] and cobalt-rich 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 Mohs 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 crystallisation 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
Talmessite is a rare secondary mineral formed typically in the oxidized zone of some hydrothermal mineral deposits, as an alteration product of realgar, orpiment, or Cu–Ni arsenides.[1] [3] Cobalt-rich varieties are found in the oxidised zone of cobalt arsenide deposits.[2] It occurs associated with gaitite, erythrite, annabergite, picropharmacolite, pharmacolite, austinite, fluorite, baryte, aragonite, calcite and dolomite.[3] At the type locality it is associated with aragonite and dolomite. [4]
Localities
The type locality is the Talmessi Mine, Anarak District, Nain County, Esfahan Province, Iran,[7] and type material is conserved at the National School of Mines, Paris, France and at the Natural History Museum, London, England.[3]
References
- ^ a b c d e f g h http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo
- ^ a b c d e f 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 f g h i j k http://www.handbookofmineralogy.org
- ^ a b c d e f g h i j k l 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 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
- ^ a b c d e f Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley
- ^ a b c d e f http://www.mindat.org/min-3876.html
- ^ a b http://pubsites.uws.edu.au/ima-cnmnc/>
- ^ Cite error: The named reference
AM49was invoked but never defined (see the help page).