Alstonite
Template:Short description Template:Infobox mineral Alstonite, also known as bromlite,<ref name="Mindat" /> is a low temperature hydrothermal mineral<ref name="Mindat" /> that is a rare double carbonate of calcium and barium with the formula Template:Chem, 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 trimorphous. Alstonite is triclinic but barytocalcite is monoclinic and paralstonite is trigonal. The species was named Bromlite by 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, p. 48</ref> and alstonite by August Breithaupt of the 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, but appears pseudo-orthorhombic because of twinning.<ref name="HOM" /> The space group is P1 or PTemplate:Overline. Alstonite appears to have a superstructure based on paralstonite without long range order of the metal cations or the CO3 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 CTemplate:Overline, 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 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">{{#if: |
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}}{{#ifeq: ||}}</ref> The faces are horizontally striated perpendicular to the pseudohexagonal c crystal axis<ref name="HOM">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 and vitreous lustre.<ref name="Mindat" /> The examination in polarized light of a transverse 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 biaxial (−) mineral with refractive indices nα =1.526, nβ = 1.671, nγ = 1.672. The maximum birefringence (the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.146.<ref name="Webmin" /><ref name="Mindat" />
The optic angle 2V is the angle between the two optic axes in a 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 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 nα, nβ, and nγ 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
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 Template:Frac, a little harder than fluorite, and its specific gravity is 3.70. It is soluble in dilute HCl<ref name="Mindat" /> and it is not radioactive.<ref name="Webmin" /> The trimorphs alstonite, paralstonite, and barytocalcite all have similar physical properties.
Type locality
There are two 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 material is held at the 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 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 carbonatites. It occurs associated with calcite, baryte, ankerite, siderite, benstonite, galena, sphalerite, pyrite, and quartz.<ref name="HOM" /> Template: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 cm long, associated with sphalerite, galena, and witherite.<ref name="MR" />
References
Bibliography
- 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.