Strontianite

Strontianite is an orthorhombic mineral, belonging to the most symmetrical class in this system, 2/m 2/m 2/m, whose general form is a rhombic dipyramid. The space group is Pmcn. There are four formula units per unit cell (Z = 4) and the unit cell parameters are a = 5.1 Å, b = 8.4 Å, c = 6.0 Å. == Structure ==

Strontianite is isostructural with aragonite. When the CO3 group is combined with large divalent cations with ionic radii greater than 1.0 Å, the radius ratios generally do not permit stable 6-fold coordination. For small cations the structure is rhombohedral, but for large cations it is orthorhombic. This is the aragonite structure type with space group Pmcn. In this structure the CO3 groups lie perpendicular to the c axis, in two structural planes, with the CO3 triangular groups of one plane pointing in opposite directions to those of the other. == Crystal form ==

Strontianite occurs in several different habits. Crystals are short prismatic parallel to the c axis and often acicular. Calcium-rich varieties often show steep pyramidal forms. Crystals may be pseudo hexagonal due to equal development of different forms. Prism faces are striated horizontally. The mineral also occurs as columnar to fibrous, granular or rounded masses. == Optical properties ==

Strontianite is colourless, white, gray, light yellow, green or brown, colourless in transmitted light. It may be longitudinally zoned. It is transparent to translucent, with a vitreous (glassy) lustre, resinous on broken surfaces, and a white streak. It is a biaxial(−) mineral. The direction perpendicular to the plane containing the two optic axes is called the optical direction Y. In strontianite Y is parallel to the b crystal axis. The optical direction Z lies in the plane containing the two optic axes and bisects the acute angle between them. In strontianite Z is parallel to the a crystal axis. The third direction X, perpendicular both to Y and to Z, is parallel to the c crystal axis. The maximum birefringence δ is 0.15 and the measured value of 2V is 7°, calculated 12° to 8°. 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 strontianite the effect is weak, with 2V larger for violet light than for red light r < v. == Luminescence ==

Strontianite is almost always fluorescent. It is also fluorescent and phosphorescent in X-rays and electron beams. All materials will glow red hot if they are heated to a high enough temperature (provided they do not decompose first); some materials become luminescent at much lower temperatures, and this is known as thermoluminescence. Strontianite is sometimes thermoluminescent. == Physical properties ==

Cleavage is nearly perfect parallel to one set of prism faces, {110}, and poor on {021}. Traces of cleavage have been observed on {010}. Twinning is very common, with twin plane {110}. The twins are usually contact twins; in a contact twin the two individuals appear to be reflections of each other in the twin plane. Penetration twins of strontainite are rarer; penetration twins are made up of interpenetrating individuals that are related to each other by rotation about a twin axis. Repeated twins are made up of three or more individuals twinned according to the same law. If all the twin planes are parallel then the twin is polysynthetic, otherwise it is cyclic. In strontianite repeated twinning forms cyclic twins with three or four individuals, or polysynthetic twins. The mineral is brittle, and breaks with a subconchoidal to uneven fracture. It is quite soft, with a Mohs hardness of , between calcite and fluorite. The specific gravity of the pure endmember with no calcium substituting for strontium is 3.78, but most samples contain some calcium, which is lighter than strontium, giving a lower specific gravity, in the range 3.74 to 3.78. Substitutions of the heavier ions barium and/or lead increase the specific gravity, although such substitutions are never very abundant. Strontianite is soluble in dilute hydrochloric acid HCl and it is not radioactive. == Environment and associations ==

Strontianite is an uncommon low-temperature hydrothermal mineral formed in veins in limestone, marl, and chalk, and in geodes and concretions. It occurs rarely in hydrothermal metallic veins but is common in carbonatites. It most likely crystallises at or near 100 °C. Its occurrence in open vugs and veins suggests crystallisation at very low pressures, probably at most equal to the hydrostatic pressure of the ground water. Under appropriate conditions it alters to celestine SrSO4, and it is itself found as an alteration from celestine. These two minerals are often found in association, together with baryte, calcite, harmotome and sulfur. == Occurrences ==

Type locality The type locality is Strontian, North West Highlands (Argyllshire), Scotland, UK. The type material occurred in veins in gneiss. Canada The Francon quarry, Montréal, Québec. Strontianite is very common at the Francon Quarry, in a great variety of habits. It is a late stage mineral, sometimes found as multiple generations. It is found as translucent to opaque, white to pale yellow or beige generally smooth surfaced spheroids, hemispheres and compact spherical and botryoidal aggregates to 10 cm in diameter, and as spheres consisting of numerous radiating acicular crystals, up to 1 cm across. Also as tufts, parallel bundles, and sheaf-like clusters of fibrous to acicular crystals, and as white, finely granular porcelaneous and waxy globular aggregates. Transparent, pale pink, columnar to tabular sixling twins up to 1 cm in diameter have been found, and aggregates of stacked stellate sixling twins consisting of transparent, pale yellow tabular crystals. It also occurs at Yukspor Mountain, Khibiny Massif, Kola Peninsula, Murmanskaja Oblast', Northern Region, in an aegerine-natrolite-microcline vein in foyaite, associated with aegirine, anatase, ancylite-(Ce), barylite, catapleiite, cerite-(Ce), cerite-(La), chabazite-(Ca), edingtonite, fluorapatite, galena, ilmenite, microcline, natrolite, sphalerite and vanadinite. At the same locality it was found in alkaline pegmatite veins associated with clinobarylite, natrolite, aegirine, microcline, catapleiite, fluorapatite, titanite, fluorite, galena, sphalerite, annite, astrophyllite, lorenzenite, labuntsovite-Mn, kuzmenkoite-Mn, cerite-(Ce), edingtonite, ilmenite and calcite. United States In the Gulf coast of Louisiana and Texas, strontianite occurs with celestine in calcite cap rock of salt domes. In the Silurian Lockport Group, Central and Western New York strontianite is observed in cavities in eastern Lockport, where it occurs as small white radiating sprays of acicular crystals. In Schoharie County, New York, it occurs in geodes and veins with celestine and calcite in limestone, and in Mifflin County, Pennsylvania, it occurs with aragonite, again in limestone. Strontianite-118172.jpg|Strontianite from Strontian, Scotland Dresserite-Strontianite-lor03a.jpg|Dresserite and Strontianite from the Francon quarry, Canada Strontianite-Baryte-160390.jpg|Strontianite from the Dreislar Mine, North Rhine-Westphalia, Germany Strontianite-170614.jpg|Strontianite from Texas, US Strontianite-208781.jpg|Strontianite from Illinois, US Strontianite Strontium carbonate Barstow San Bernardino County California 1845.jpg|Strontianite from California == See also ==