Jump to content

Talmessite: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous edit
Content deleted Content added
VisualWikitext
m BenLinus1214 moved page Draft:Talmessite to Talmessite: Publishing accepted Articles for creation submission (AFCH 0.9)
m Added short description
Tags: Mobile edit Mobile app edit iOS app edit App description add
 
(25 intermediate revisions by 14 users not shown)
Line 1: Line 1:
{{Short description|Hydrated calcium magnesium arsenate}}
{{Userspace draft|source=ArticleWizard|date=February 2015}}

'''Talmessite'''
{{Infobox mineral
{{Infobox mineral
| name = '''Talmesite'''
| name = Talmesite
| category = Arsenate mineral
| category = Arsenate mineral
| boxwidth = 24
| boxwidth =
| boxbgcolor =
| boxbgcolor =
| image = Talmessite-27535.jpg
| image = Talmessite-27535.jpg
| caption = Talmessite from [[Gold Hill Mine]], Utah, USA
| imagesize = 260
| caption = Talmessite from [[Gold Hill Mine]], Utah, US
| formula = {{chem2|Ca2Mg(AsO4)2*2H2O}}
| formula = Ca<sub>2</sub>Mg(AsO<sub>4</sub>)<sub>2</sub>.2H<sub>2</sub>O
| IMAsymbol = Tlm<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight = 418.33 g<ref name=Webmin/>
| molweight = 418.33 g/mol<ref name=Webmin/>
| strunz = 7/C.17-50 or 8.CG.05
| strunz = 8.CG.05 (10 ed) <br/><small>7/C.17-50 (8 ed)</small>
| dana = 40.02.02.05
| dana = 40.02.02.05
| system = Triclinic
⚫
| color = White or colourless, pale green (nickel-rich) or brownish or pink (cobalt-rich). Colourless in transmitted light.
| class = Pinacoidal ({{overline|1}}) <br/><small>(same [[H-M symbol]])</small>
⚫
| habit = Prismatic crystals<ref name=AM42/>, radiating fibrous aggregates<ref name=HOM/>
| system = Triclinic {{overline|1}}, space group P{{overline|1}}
| symmetry = ''P''{{overline|1}}
⚫
| colour = White or colourless, pale green (nickel-rich) or brownish or pink (cobalt-rich). Colourless in transmitted light.
⚫
| habit = Prismatic crystals,<ref name=AM42/> radiating fibrous aggregates<ref name=HOM/>
| twinning = Polysynthetic<ref name=AM42/>
| twinning = Polysynthetic<ref name=AM42/>
| cleavage =
| cleavage =
Line 21: Line 23:
| tenacity =
| tenacity =
| mohs = 5
| mohs = 5
| luster = Vitreous
| lustre = Vitreous
| refractive = n<sub>α</sub> = 1.672, n<sub>β</sub> = 1.685, n<sub>γ</sub> = 1.698
| refractive = n<sub>α</sub> = 1.672, n<sub>β</sub> = 1.685, n<sub>γ</sub> = 1.698
| opticalprop = Biaxial (–)
| opticalprop = Biaxial (–)
Line 27: Line 29:
| pleochroism = The cobaltoan variety is pleochroic, colourless to pale rose<ref name=AM45/>
| pleochroism = The cobaltoan variety is pleochroic, colourless to pale rose<ref name=AM45/>
| streak = White<ref name=Webmin/>
| streak = White<ref name=Webmin/>
| gravity = 3.421<ref name=AM45/> <ref name=HOM/>, cobaltoan varieties 3.574<ref name=AM45/>
| gravity = 3.421,<ref name=HOM/><ref name=AM45/> cobaltoan varieties 3.574<ref name=AM45/>
| melt =
| melt =
| fusibility =
| fusibility =
Line 34: Line 36:
| diaphaneity = Transparent to translucent
| diaphaneity = Transparent to translucent
| other = Not radioactive.<ref name=Webmin/> It loses [[water of crystallisation]] at 450<sup>o</sup><ref name=AM45/>
| other = Not radioactive.<ref name=Webmin/> It loses [[water of crystallisation]] at 450<sup>o</sup><ref name=AM45/>
| references = <ref name=AM42>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</ref> <ref name=AM45>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></ref> <ref name=AM50>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</ref> <ref name=Dana>Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley</ref> <ref name=Mindat>http://www.mindat.org/min-3876.html</ref> <ref name=Webmin>http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo</ref> <ref name=HOM>http://www.handbookofmineralogy.org</ref> <ref name=IMA>http://pubsites.uws.edu.au/ima-cnmnc/></ref> }}
| references = <ref name=AM42>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</ref><ref name=AM45>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></ref><ref name=Webmin>{{cite web|url=http://www.webmineral.com/data/Talmessite.shtml#.VN2Jzi6kFjo|title=Talmessite Mineral Data|author=Dave Barthelmy|work=webmineral.com}}</ref><ref name=HOM>{{cite web|url=http://www.handbookofmineralogy.org|title=Handbook of Mineralogy|work=handbookofmineralogy.org}}</ref><ref name=AM50>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</ref><ref name=Dana>Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley</ref><ref name=Mindat>{{cite web|url=http://www.mindat.org/min-3876.html|title=Talmessite|work=mindat.org}}</ref><ref name=IMA>http://pubsites.uws.edu.au/ima-cnmnc/></ref> }}


Talmessite is a [[Water of crystallization|hydrated]] [[calcium]] [[magnesium]] [[arsenate]], often with significant amounts of [[cobalt]] or [[nickel]].<ref name=Mindat/> 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.<ref name=Mindat/> <ref name=HOM/> All these minerals are members of the [[fairfieldite]] group. Talmessite is dimorphic with [[wendwilsonite]] (which is not a member of this group).<ref name=Dana/>
Talmessite is a [[Water of crystallization|hydrated]] [[calcium]] [[magnesium]] [[arsenate]], often with significant amounts of [[cobalt]] or [[nickel]].<ref name=Mindat/> 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.<ref name=HOM/><ref name=Mindat/> All these minerals are members of the [[fairfieldite]] group. Talmessite is dimorphic with [[wendwilsonite]] (which is not a member of this group).<ref name=Dana/>


== Members of the fairfieldite group<ref name=Mindat/> ==
== Members of the fairfieldite group ==
[[Cassidyite]] Ca<sub>2</sub>(Ni<sup>2+</sup>,Mg)(PO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />
* [[Cassidyite]] {{chem2|Ca2(Ni(2+),Mg)(PO4)2*2H2O}}
* [[Collinsite]] {{chem2|Ca2(Mg,Fe(2+))(PO4)2*2H2O}}

* [[Fairfieldite]] {{chem2|Ca2(Mn(2+),Fe(2+))(PO4)2*2H2O}}
[[Collinsite]] Ca<sub>2</sub>(Mg,Fe<sup>2+</sup>)(PO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />
* [[Gaitite]] {{chem2|Ca2Zn(AsO4)2*2H2O}}

* [[Messelite]] {{chem2|Ca2(Fe(2+),Mn(2+))(PO4)2*2H2O}}
[[Fairfieldite]] Ca<sub>2</sub>(Mn<sup>2+</sup>,Fe<sup>2+</sup>)(PO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />
* [[Nickeltalmessite]] {{chem2|Ca2Ni(AsO4)2*2H2O}}

* [[Parabrandtite]] {{chem2|Ca2Mn(2+)(AsO4)2*2H2O}}
[[Gaitite]] Ca<sub>2</sub>Zn(AsO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />
* unnamed {{chem2|(Fe(2+)}}-analogue of parabrandtite) {{chem2|Ca2Fe(2+)(AsO4)2*2H2O}}

* [[β-Roselite]] {{chem2|Ca2(Co(2+),Mg)(AsO4)2*2H2O}}
[[Messelite]] Ca<sub>2</sub>(Fe<sup>2+</sup>,Mn<sup>2+</sup>)(PO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />
* Talmessite {{chem2|Ca2Mg(AsO4)2*2H2O}}

<ref name=Mindat/>
[[Nickeltalmessite]] Ca<sub>2</sub>Ni(AsO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />

[[Parabrandtite]] Ca<sub>2</sub>Mn<sup>2+</sup>(AsO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />

Unnamed (Fe<sup>2+</sup>-analogue of Parabrandtite) Ca<sub>2</sub>Fe<sup>2+</sup>(AsO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />

[[β-Roselite]] Ca<sub>2</sub>(Co<sup>2+</sup>,Mg)(AsO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O<br />

Talmessite Ca<sub>2</sub>Mg(AsO<sub>4</sub>)<sub>2</sub>.2H<sub>2</sub>O


== Crystallography ==
== Crystallography ==
The formula for talmessite is Ca<sub>2</sub>Mg(AsO<sub>4</sub>)<sub>2</sub>.2H<sub>2</sub>O<ref name=IMA/>. It is a [[triclinic crystal system|triclinic]] mineral<ref name=AM45/>, [[crystal class]] {{overline|1}}, [[space group]] P{{overline|1}}. There is one formula unit per [[unit cell]] (Z = 1) and the unit cell parameters are variously given as a = 5.87 Å, b = 6.94 Å, c = 5.53 Å, α = 97.3°, β = 108.7°, γ = 108.1°<ref name=Dana/> <ref name=Mindat/> <ref name=HOM/>, or a = 5.89 Å, b = 7.69 Å, c = 5.56 Å, α = 112.633°, β = 70.817°, γ = 119.417°,<ref name=AM45/> <ref name=AM50/> <ref name=Webmin/>. These values give a calculated [[specific gravity]] varying from 3.42 to 3.63.<br />
The formula for talmessite is Ca<sub>2</sub>Mg(AsO<sub>4</sub>)<sub>2</sub>.2H<sub>2</sub>O.<ref name=IMA/> It is a [[triclinic crystal system|triclinic]] mineral,<ref name=AM45/> [[crystal class]] {{overline|1}}, [[space group]] P{{overline|1}}. There is one formula unit per [[unit cell]] (Z = 1) and the unit cell parameters are variously given as a = 5.87 Å, b = 6.94 Å, c = 5.53 Å, α = 97.3°, β = 108.7°, γ = 108.1°,<ref name=HOM/><ref name=Dana/><ref name=Mindat/> or a = 5.89 Å, b = 7.69 Å, c = 5.56 Å, α = 112.633°, β = 70.817°, γ = 119.417°,.<ref name=Webmin/><ref name=AM45/><ref name=AM50/> These values give a calculated [[specific gravity]] varying from 3.42 to 3.63.<br />
The structure is dominated by chains of [[tetrahedral]] AsO<sub>4</sub> and [[octahedral]] [(cation-O<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>)] groups that parallel the c [[Crystal structure|crystal axis]]. The octahedral are compressed, resulting in chain disorder<ref name=Dana/>
The structure is dominated by chains of [[tetrahedral]] AsO<sub>4</sub> and [[octahedral]] [(cation-O<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>)] groups that parallel the c [[Crystal structure|crystal axis]]. The octahedral are compressed, resulting in chain disorder<ref name=Dana/>


== Appearance ==
== Appearance ==
Talmessite occurs as [[prism (geometry)|prismatic]] crystals to 3 mm<ref name=AM42/>, as radiating fibrous aggregates<ref name=HOM/> or as fine crystalline aggregates<ref name=Dana/>; it may also be [[Stalactite|stalactitic]] or in crusts.<ref name=HOM/>. Pure talmessite is white or colourless, and colourless in [[transmitted light]], but [[nickel]]-rich varieties are pale green<ref name=AM45/> and [[cobalt]]-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The [[Streak (mineralogy)|streak]] is white<ref name=Webmin/> and crystals are transparent to translucent with a [[vitreous lustre]].
Talmessite occurs as [[prism (geometry)|prismatic]] crystals to 3&nbsp;mm,<ref name=AM42/> as radiating fibrous aggregates<ref name=HOM/> or as fine crystalline aggregates;<ref name=Dana/> it may also be [[Stalactite|stalactitic]] or in crusts.<ref name=HOM/> Pure talmessite is white or colourless, and colourless in [[transmitted light]], but [[nickel]]-rich varieties are pale green<ref name=AM45/> and [[cobalt]]-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The [[Streak (mineralogy)|streak]] is white<ref name=Webmin/> and crystals are transparent to translucent with a [[vitreous lustre]].


== Physical properties ==
== Physical properties ==
Talmessite is a moderately hard mineral, with [[Mohs scale of mineral hardness|Mohs hardness]] 5, harder than [[fluorite]] but not as hard as [[quartz]]. The [[specific gravity]] calculated from the [[Chemical formula|formula]] and the [[unit cell|cell]] dimensions is 3.49,<ref name=AM45/> <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 [[Crystal twinning|polysynthetic twinning]].<ref name=AM42/> It is not [[radioactive]]<ref name=Webmin/> and it loses [[water of crystallisation]] at 450<sup>o</sup>.<ref name=AM45/>
Talmessite is a moderately hard mineral, with [[Mohs scale of mineral hardness|Mohs hardness]] 5, harder than [[fluorite]] but not as hard as [[quartz]]. The [[specific gravity]] calculated from the [[Chemical formula|formula]] and the [[unit cell|cell]] dimensions is 3.49,<ref name=AM45/><ref name=AM50/> but the measured value is less for ordinary talmessite, at 3.42<ref name=HOM/><ref name=AM45/> and more for the cobaltoan variety, at 3.57.<ref name=AM45/><ref name=Dana/> The mineral displays [[Crystal twinning|polysynthetic twinning]].<ref name=AM42/> It is not [[radioactive]]<ref name=Webmin/> and it loses [[water of crystallisation]] at 450<sup>o</sup>.<ref name=AM45/>


== Optical properties ==
== Optical properties ==
Line 73: Line 67:
The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is [[inversely proportional]] to speed, so the refractive indices for the X, Y and Z directions increase from X to Z.<ref name=MOM>Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition. Wiley</ref>
The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is [[inversely proportional]] to speed, so the refractive indices for the X, Y and Z directions increase from X to Z.<ref name=MOM>Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition. Wiley</ref>
The refractive indices are n<sub>α</sub> = 1.672, n<sub>β</sub> = 1.685 and n<sub>γ</sub> = 1.698<ref name=AM50/> with slightly higher values for the cobaltoan variety of n<sub>α</sub> = 1.695 and n<sub>γ</sub> = 1.73 (n<sub>β</sub> not specified).<ref name=AM45/>
The refractive indices are n<sub>α</sub> = 1.672, n<sub>β</sub> = 1.685 and n<sub>γ</sub> = 1.698<ref name=AM50/> with slightly higher values for the cobaltoan variety of n<sub>α</sub> = 1.695 and n<sub>γ</sub> = 1.73 (n<sub>β</sub> not specified).<ref name=AM45/>
The maximum birefringence δ is the difference between the highest and lowest refractive index; for talmessite δ = 0.026.<ref name=Mindat/> <ref name=Webmin/> <br />
The maximum birefringence δ is the difference between the highest and lowest refractive index; for talmessite δ = 0.026.<ref name=Webmin/><ref name=Mindat/> <br />
The angle between the two optic axes is called the optic angle, 2V, and it is always [[Acute angle|acute]], and [[Bisection|bisected]] either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative.<ref name=MOM/> Talmessite may be biaxial (–)<ref name=AM45/> or biaxial (+).<ref name=AM42/> The measured value of 2V is about 90<sup>o</sup>.<ref name=AM50/> Also 2V can be calculated from the values of the refractive indices, giving a value of 88°<ref name=Mindat/> <ref name=Webmin/><br />
The angle between the two optic axes is called the optic angle, 2V, and it is always [[Acute angle|acute]], and [[Bisection|bisected]] either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative.<ref name=MOM/> Talmessite may be biaxial (–)<ref name=AM45/> or biaxial (+).<ref name=AM42/> The measured value of 2V is about 90<sup>o</sup>.<ref name=AM50/> Also 2V can be calculated from the values of the refractive indices, giving a value of 88°<ref name=Webmin/><ref name=Mindat/><br />
2V depends on the refractive indices, but refractive index varies with [[wavelength]], and hence with [[colour]]. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with [[chromatic dispersion]]). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For talmessite dispersion is strong, with r > v.<ref name=Mindat/> The cobaltoan variety is [[pleochroic]] colourless to pale rose.<ref name=AM45/>
2V depends on the refractive indices, but refractive index varies with [[wavelength]], and hence with [[colour]]. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with [[chromatic dispersion]]). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For talmessite dispersion is strong, with r > v.<ref name=Mindat/> The cobaltoan variety is [[pleochroic]] colourless to pale rose.<ref name=AM45/>


== Occurrence ==
== Occurrence ==
Talmessite is a rare [[Supergene (geology)#Oxidized zone|secondary mineral]] formed typically in the oxidized zone of some [[Ore genesis#Hydrothermal processes|hydrothermal]] mineral deposits, as an alteration product of [[realgar]], [[orpiment]], or Cu–Ni [[Arsenide|arsenides]].<ref name=Webmin/> <ref name=HOM/> [[Cobalt]]-rich varieties are found in the oxidised zone of cobalt arsenide deposits.<ref name=AM42/> It occurs associated with [[gaitite]], [[erythrite]], [[annabergite]], [[picropharmacolite]], [[pharmacolite]], [[austinite]], [[fluorite]], [[baryte]], [[aragonite]], [[calcite]] and [[dolomite]].<ref name=HOM/> At the [[Type locality (geology)|type locality]] it is associated with [[aragonite]] and [[dolomite]]. <ref name=AM45/>
Talmessite is a rare [[Supergene (geology)#Oxidized zone|secondary mineral]] formed typically in the oxidized zone of some [[Ore genesis#Hydrothermal processes|hydrothermal]] mineral deposits, as an alteration product of [[realgar]], [[orpiment]], or Cu–Ni [[arsenide]]s.<ref name=Webmin/><ref name=HOM/> [[Cobalt]]-rich varieties are found in the oxidised zone of cobalt arsenide deposits.<ref name=AM42/> It occurs associated with [[gaitite]], [[erythrite]], [[annabergite]], [[picropharmacolite]], [[pharmacolite]], [[austinite]], [[fluorite]], [[baryte]], [[aragonite]], [[calcite]] and [[Dolomite (mineral)|dolomite]].<ref name=HOM/> At the [[Type locality (geology)|type locality]] it is associated with [[aragonite]] and [[Dolomite (mineral)|dolomite]].<ref name=AM45/>


== Localities ==
== Localities ==
Line 88: Line 82:


<!--- Categories --->
<!--- Categories --->

[[Category:Articles created via the Article Wizard]]
[[Category:Arsenate minerals]]
[[Category:Arsenate minerals]]
[[Category:Triclinic minerals]]

[[Category:Minerals in space group 2]]
{{AFC submission|||ts=20150225104458|u=Strickja|ns=2}}

Latest revision as of 15:02, 17 December 2024

Talmesite
Talmessite from Gold Hill Mine, Utah, US
General
CategoryArsenate mineral
Formula
(repeating unit)		Ca2Mg(AsO4)2·2H2O
IMA symbolTlm[1]
Strunz classification8.CG.05 (10 ed)
7/C.17-50 (8 ed)
Dana classification40.02.02.05
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Identification
Formula mass418.33 g/mol[2]
ColourWhite or colourless, pale green (nickel-rich) or brownish or pink (cobalt-rich). Colourless in transmitted light.
Crystal habitPrismatic crystals,[3] radiating fibrous aggregates[4]
TwinningPolysynthetic[3]
Mohs scale hardness5
LustreVitreous
StreakWhite[2]
DiaphaneityTransparent to translucent
Specific gravity3.421,[4][5] cobaltoan varieties 3.574[5]
Optical propertiesBiaxial (–)
Refractive indexnα = 1.672, nβ = 1.685, nγ = 1.698
Birefringenceδ = 0.026
PleochroismThe cobaltoan variety is pleochroic, colourless to pale rose[5]
Other characteristicsNot radioactive.[2] It loses water of crystallisation at 450o[5]
References[3][5][2][4][6][7][8][9]

Talmessite is a hydrated calcium magnesium arsenate, often with significant amounts of cobalt or nickel.[8] 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.[4][8] All these minerals are members of the fairfieldite group. Talmessite is dimorphic with wendwilsonite (which is not a member of this group).[7]

Members of the fairfieldite group

[edit]

[8]

Crystallography

[edit]

The formula for talmessite is Ca2Mg(AsO4)2.2H2O.[9] It is a triclinic mineral,[5] 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 = 5.87 Å, b = 6.94 Å, c = 5.53 Å, α = 97.3°, β = 108.7°, γ = 108.1°,[4][7][8] or a = 5.89 Å, b = 7.69 Å, c = 5.56 Å, α = 112.633°, β = 70.817°, γ = 119.417°,.[2][5][6] 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[7]

Appearance

[edit]

Talmessite occurs as prismatic crystals to 3 mm,[3] as radiating fibrous aggregates[4] or as fine crystalline aggregates;[7] it may also be stalactitic or in crusts.[4] Pure talmessite is white or colourless, and colourless in transmitted light, but nickel-rich varieties are pale green[5] and cobalt-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The streak is white[2] and crystals are transparent to translucent with a vitreous lustre.

Physical properties

[edit]

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,[5][6] but the measured value is less for ordinary talmessite, at 3.42[4][5] and more for the cobaltoan variety, at 3.57.[5][7] The mineral displays polysynthetic twinning.[3] It is not radioactive[2] and it loses water of crystallisation at 450o.[5]

Optical properties

[edit]

Triclinic crystals (and orthorhombic and monoclinic crystals) have two directions in which light travels with zero birefringence; these directions are called the optic axes, and the crystal is said to be biaxial. Talmessite is triclinic, so it is biaxial.
The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is inversely proportional to speed, so the refractive indices for the X, Y and Z directions increase from X to Z.[10] The refractive indices are nα = 1.672, nβ = 1.685 and nγ = 1.698[6] with slightly higher values for the cobaltoan variety of nα = 1.695 and nγ = 1.73 (nβ not specified).[5] The maximum birefringence δ is the difference between the highest and lowest refractive index; for talmessite δ = 0.026.[2][8]
The angle between the two optic axes is called the optic angle, 2V, and it is always acute, and bisected either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative.[10] Talmessite may be biaxial (–)[5] or biaxial (+).[3] The measured value of 2V is about 90o.[6] Also 2V can be calculated from the values of the refractive indices, giving a value of 88°[2][8]
2V depends on the refractive indices, but refractive index varies with wavelength, and hence with colour. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with chromatic dispersion). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For talmessite dispersion is strong, with r > v.[8] The cobaltoan variety is pleochroic colourless to pale rose.[5]

Occurrence

[edit]

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.[2][4] Cobalt-rich varieties are found in the oxidised zone of cobalt arsenide deposits.[3] It occurs associated with gaitite, erythrite, annabergite, picropharmacolite, pharmacolite, austinite, fluorite, baryte, aragonite, calcite and dolomite.[4] At the type locality it is associated with aragonite and dolomite.[5]

Localities

[edit]

The type locality is the Talmessi Mine, Anarak District, Nain County, Esfahan Province, Iran,[8] and type material is conserved at the École nationale supérieure des mines de Paris, France and at the Natural History Museum, London, England.[4]

References

[edit]
  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c d e f g h i j Dave Barthelmy. "Talmessite Mineral Data". webmineral.com.
  3. ^ a b c d e f g 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
  4. ^ a b c d e f g h i j k "Handbook of Mineralogy". handbookofmineralogy.org.
  5. ^ a b c d e f g h i j k l m n o p 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>
  6. ^ a b c d e 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
  7. ^ a b c d e f Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley
  8. ^ a b c d e f g h i "Talmessite". mindat.org.
  9. ^ a b http://pubsites.uws.edu.au/ima-cnmnc/>
  10. ^ a b Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition. Wiley
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Talmessite&oldid=1263592838"