View source for Alstonite

{{short description|Hydrothermal mineral}}
{{infobox mineral
| name        = Alstonite
| image       = Alstonite-Witherite-oldeuro-107d.jpg
| imagesize   = 260px
| alt         = 
| caption     = 
| category    = [[Carbonate mineral]]
| formula     = BaCa(CO<sub>3</sub>)<sub>2</sub>
|IMAsymbol=Asn<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   = 297.42 g/mol<ref name="Webmin" />
| strunz      = 5.AB.35
| dana        = 14.02.05.01
| system      = [[Triclinic]] <br/>Unknown space group
| unit cell   = a = 17.38, b = 14.40 <br/>c = 6.123&nbsp;[Å]; α = 90.35° <br/>β = 90.12°, γ = 120.08°; Z&nbsp;=&nbsp;24<ref name="atlas" />
| color       =
| colour      = Colourless to snow-white; also pale gray, pale cream, pink to pale rose-red
| habit       = Steep pseudohexagonal dipyramids, pseudo-orthorhombic
| twinning    = Common on pseudo-orthorhombic {1{{overline|1}}0} and {310}<ref name="HOM" />
| cleavage    = Imperfect on pseudo-orthorhombic {110}<ref name="HOM" />
| fracture    = Uneven
| tenacity    =
| mohs        = 4 to 4.5
| lustre      = Vitreous
| streak      = White
| diaphaneity = Transparent to translucent
| gravity     = 3.70
| density     =
| polish      =
| opticalprop = Biaxial (−)
| refractive  = n<sub>α</sub> = 1.526 n<sub>β</sub> = 1.671 n<sub>γ</sub> = 1.672
| birefringence = δ = 0.146
| pleochroism = None
| 2V          = Measured 6°, calculated 8°
| dispersion  = Weak, r > v
| extinction  =
| length fast/slow =
| fluorescence = Weak yellow under LW and SW
| absorption  =
| melt        =
| fusibility  =
| diagnostic  =
| solubility  = Soluble in dilute HCl
| impurities  =
| alteration  = Colour may fade on exposure to light
| other       = Not radioactive
| references  = <ref name="Webmin">[http://www.webmineral.com/data/Alstonite.shtml Webmineral data]</ref><ref name="atlas">[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Alstonite Mineralienatlas]</ref><ref name="Dana">Gaines et al. (1997) Dana’s New Mineralogy, Eighth Edition. Wiley</ref><ref name="Mindat">[http://www.mindat.org/min-146.html Mindat.org]</ref>
}}
'''Alstonite''', also known as '''bromlite''',<ref name="Mindat" /> is a low temperature [[hydrothermal synthesis|hydrothermal]] mineral<ref name="Mindat" /> that is a rare [[double salt|double]] [[carbonate]] of [[calcium]] and [[barium]] with the formula {{chem|BaCa(CO|3|)|2}}, sometimes with some [[strontium]].<ref name="Dana" /> [[Barytocalcite]] and [[paralstonite]] have the same formula but different structures, so these three minerals are said to be [[polymorphism (materials science)|trimorphous]]. Alstonite is [[triclinic crystal system|triclinic]] but barytocalcite is [[monoclinic crystal system|monoclinic]] and paralstonite is [[trigonal crystal system|trigonal]]. The species was named Bromlite by [[Thomas Thomson (chemist)|Thomas Thomson]] in 1837 after the Bromley-Hill mine,<ref>Thomas Thomson, This mine was owned by Thomas Shaw and business partner Jacob Walton between 1816 and 1874, The owners then Subcontracted to groups of independent miner, basing their pay on the amount of drifting accomplished, the amount of ore produced, or a combination of the two, . This system worked well, and the Brownley Hill mine became one of the more productive mines on Alston Moor during the Middle 1800s. by the early 1870s the seems of lead were being exhausted and in 1874 the lease was then taken over by the Brownley Hill Mining which concentrated on Zinc mining, which failed to make a profit.  "On the Right Rhombic Baryto-Calcite, with reference to Prof. ''Johnston’s'' Paper in the Phil. Mag. for May 1837", ''The London and Edinburgh Philosophical Magazine and Journal of Science'', vol. XI, july-december 1837, [https://books.google.com/books?id=KC9DAQAAMAAJ&pg=PA48 p. 48]</ref> and alstonite by [[August Breithaupt]] of the [[Freiberg University of Mining and Technology|Freiberg Mining Academy]] in 1841, after [[Alston, Cumbria]], the base of operations of the mineral dealer from whom the first samples were obtained by Thomson in 1834. Both of these names have been in common use.<ref name="EB1911" />

==Structure==
Alstonite is [[triclinic crystal system|triclinic]], but appears pseudo-[[orthorhombic crystal system|orthorhombic]] because of [[crystal twinning|twinning]].<ref name="HOM" /> The [[space group]] is P1 or P{{overline|1}}. Alstonite appears to have a superstructure based on [[paralstonite]] without long range order of the metal [[cations]] or the CO<sub>3</sub> groups. The structure of paralstonite is similar to that of other double [[carbonates]].<ref name="Dana" />

==Unit cell==
The number of formula units, Z, in the triclinic [[unit cell]] is given as 10<ref name="Webmin" /> or 12,<ref name="Dana" /> and the unit cell parameters are a = 17.38 Å, b = 14.40 Å, c = 6.123 Å, α = 90.35°, β = 90.12°, γ = 120.08°.<ref name="Webmin" /><ref name="Dana" /><ref name="Mindat" /> The Handbook of Mineralogy, however, describes the mineral in terms of a pseudo-orthorhombic unit cell, with space group C1 or C{{overline|1}}, Z = 24, and unit cell parameters a = 30.14 Å, b = 17.40 Å, c = 6.12 Å, α = β = γ = 90°.<ref name="HOM" />

==Appearance==
Simple crystals of alstonite are not known. The crystals are invariably complex [[crystal twinning|twins]] formed by repeated twinning, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of [[witherite]] but more acute.<ref name="EB1911">{{EB1911 |inline=1 |wstitle=Bromlite |volume=4 |page=634 }}</ref> The faces are horizontally striated perpendicular to the pseudohexagonal c [[crystal axis]]<ref name="HOM">[http://www.handbookofmineralogy.org Handbook of Mineralogy]</ref> and they are divided vertically by a medial, slightly reentrant twinning line parallel to the pseudohexagonal c axis.<ref name="Mindat" />

Crystals are colourless to snow white, yellow-gray, pale gray, pale cream, pink, or pale rose-red, but the colour may fade on exposure to light. They are transparent to translucent with a white [[streak (mineralogy)|streak]] and vitreous [[lustre (mineralogy)|lustre]].<ref name="Mindat" /> The examination in [[polarized light]] of a transverse [[thin section|section]] shows that each compound [[crystal]] is built up of six differently oriented individuals arranged in twelve segments.<ref name="HOM" />

==Optical properties==
Alstonite is a [[birefringence#Positive or negative|biaxial (−)]] mineral with [[refractive indices]] n<sub>α</sub> =1.526, n<sub>β</sub> = 1.671, n<sub>γ</sub> = 1.672. The maximum [[birefringence]] (the difference in refractive index between light travelling through the crystal with different [[polarization (waves)|polarizations]]) is δ = 0.146.<ref name="Webmin" /><ref name="Mindat" />

The optic angle 2V is the angle between the two [[optic axis of a crystal|optic axes]] in a [[birefringence|biaxial]] crystal. The measured values of 2V for this mineral is 6°. It is also possible to calculate a theoretical value of 2V from the measured values of the [[refractive indices]]. The calculated value for alstonite is 8°.<ref name="Webmin" /><ref name=HOM/><ref name="Dana" /><ref name="Mindat" /> If the colour of the incident light is changed, then the refractive indices are modified, and the value of 2V changes. This is known as dispersion of the [[optic axis of a crystal|optic axes]]. For alstonite the effect is weak, with 2V larger for red light than for violet light (r > v).<ref name="Webmin" /><ref name=HOM/><ref name="Dana" /><ref name="Mindat" />

The optical directions X, Y, and Z are the directions of travel of light with refractive indices n<sub>α</sub>, n<sub>β</sub>, and n<sub>γ</sub> respectively. In general they are not the same as the directions a, b, and c of the crystallographic axes. For alstonite X, Y, and Z are parallel to the c, a, and b crystal axes respectively.<ref name="HOM" /><ref name="Dana" />

Alstonite fluoresces weak yellow under shortwave and longwave [[ultraviolet light]].<ref name="Webmin" /><ref name="HOM" /><ref name="Dana" /><ref name="Mindat" />

==Physical properties==
[[Crystal twinning|Twinning]] in alstonite is ubiquitous, forming pseudohexagonal groups.<ref name="HOM" /><ref name="Mindat" /> The mineral has one imperfect cleavage and it breaks with an uneven fracture. It is not very hard, with a [[Mohs hardness]] of just 4 to {{frac|4|1|2}}, a little harder than fluorite, and its specific gravity is 3.70. It is soluble in dilute [[hydrochloric acid|HCl]]<ref name="Mindat" /> and it is not radioactive.<ref name="Webmin" /> The [[polymorphism (materials science)|trimorphs]] alstonite, paralstonite, and barytocalcite all have similar physical properties.

==Type locality==
[[File:Alstonite-mrz258a.jpg|thumb|left|Alstonite crystal group from the [[type locality (geology)|type locality]] in Cumbria (size: 5.1 × 4.1 × 2.7 cm)]]

There are two [[type locality (geology)|type localities]], both in the north of England. One is the Bromley Hill Mine (Bloomsberry Horse Level), Nenthead, [[Alston Moor]] District, North Pennines, Cumbria, and the other is the Fallowfield Mine, Acomb, [[Hexham]], Tyne Valley, Northumberland.<ref name="Mindat" /> The [[type specimen (mineralogy)|type material]] is held at the [[Freiberg University of Mining and Technology|Freiberg Mining Academy]], Germany, 15818.<ref name="HOM" /><ref name="Mindat" />

At the type locality at Brownley Hill, alstonite occurs in low-temperature [[lead]]–[[zinc]] hydrothermal deposits associated with witherite, calcite, and baryte.<ref name="Dana" /><ref name="Mindat" /> The crystals are white to colourless or faintly pink acute pseudohexagonal pyramids or dipyramids up to 6&nbsp;mm long. In some specimens the alstonite is intergrown with very thin hexagonal platy crystals of nailhead [[calcite]]. Alstonite commonly encrusts compact crystalline white to pale pink [[baryte]].<ref name="MR">Mineralogical Record supplement to 41-1 (2010).</ref> Similar crystals have been found at Fallowfield.<ref name="MR" /> It occurs typically in low-temperature [[hydrothermal]] [[lead]]–[[zinc]] ore deposits, as is the case at the type localities, and it has also been reported as a rare phase in [[carbonatite]]s. It occurs associated with [[calcite]], [[baryte]], [[ankerite]], [[siderite]], [[benstonite]], [[galena]], [[sphalerite]], [[pyrite]], and [[quartz]].<ref name="HOM" />
{{clear left}}

==Other localities==
*USA: Tiny sharp pseudohexagonal dipyramids of alstonite with [[benstonite]] and more commonly with [[witherite]] are found at the Minerva Mine, [[Cave-In-Rock, Illinois]].<ref name="R&M">Rocks & Minerals 85-3:211 (2010)</ref>
*England: Alstonite has been found in veins at [[New Brancepeth]], near Durham.<ref name="Dana" />
*England: At Nentsberry Haggs Mine, which, like the Brownley Hill Mine, is on [[Alston Moor]], Cumbria, [[baryte]] has been found as [[pseudomorphs]] after alstonite up to 2.5&nbsp;cm long, associated with [[sphalerite]], [[galena]], and [[witherite]].<ref name="MR" />

==References==
{{Reflist}}

==Bibliography==
*<small>Palache, P.; Berman H.; Frondel, C. (1960). "''Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)"'' John Wiley and Sons, Inc., New York, pp. 218-219.</small>

{{Commons}}

[[Category:Barium minerals]]
[[Category:Calcium minerals]]
[[Category:Carbonate minerals]]
[[Category:Triclinic minerals]]