Heulandite is the name of a series of tecto-silicate

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s of the

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by the

International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. Th ...

changed it to a series name, with the mineral species being named: * Heulandite-Ca * Heulandite-Na * Heulandite-K * Heulandite-Sr * Heulandite-Ba (described in 2002). Heulandite-Ca, the most common of these, is a hydrous calcium and

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...

, . Small amounts of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

are usually present replacing part of the calcium.

Strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is e ...

replaces calcium in the heulandite-Sr variety. The appropriate species name depends on the dominant element. The species are visually indistinguishable, and the series name heulandite is still used whenever testing has not been performed.

Crystallography and properties

Crystals are

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

. They may have a characteristic coffin-shaped habit, but may also form simple rhombic prisms. Frequently, a crust of fine crystals will form with only the ends of the rhombs visible, making the crystals look like wedges. They have a perfect cleavage parallel to the plane of symmetry, on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or white, but may be orange, brown, yellow, brick-red, or green due to inclusions of

celadonite Celadonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide with formula . It crystallizes in the monoclinic system and usually forms massive aggregates of prismatic crystallites ...

. It varies from transparent to translucent. Isomorphous with heulandite is the

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is e ...

and

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

zeolite brewsterite. The Mohs hardness is 3-4, and the specific gravity 2.2. Heulandite is similar to stilbite. The two minerals may, however, be readily distinguished by the fact that in heulandite the acute positive

bisectrix In geometry, bisection is the division of something into two equal or congruent parts, usually by a line, which is then called a ''bisector''. The most often considered types of bisectors are the ''segment bisector'' (a line that passes through ...

of the optic axes emerges perpendicular to the cleavage. Heulandite-290459

Discovery and occurrence

Heulandite was first separated from stilbite by

August Breithaupt Johann Friedrich August Breithaupt (May 16, 1791 – September 22, 1873) was a German mineralogist and professor at Freiberg Mining Academy in Freiberg, Saxony. Biography He was born in Probstzella. He received his doctorate at the Universities ...

in 1818, and named by him "euzeolite" (meaning beautiful zeolite); independently, in 1822,

H. J. Brooke H is the eighth letter of the Latin alphabet. H may also refer to: Musical symbols * H number, Harry Halbreich reference mechanism for music by Honegger and Martinů * H, B (musical note) * H, B major People * H. (noble) (died after 127 ...

arrived at the same result, giving the name heulandite, after the mineral collector, John Henry Heuland (1778–1856). Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

ic volcanic rocks, and occasionally in

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

and hydrothermal veins. It forms at temperatures below about , and so its presence in

sedimentary rocks Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...

indicates that these have experienced shallow diagenesis. Good specimens have been found in the basalts of

Berufjörður Berufjörður () is a fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Gree ...

, near Djúpivogur, in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...

, the

Faroe Islands The Faroe Islands ( ), or simply the Faroes ( fo, Føroyar ; da, Færøerne ), are a North Atlantic island group and an autonomous territory of the Kingdom of Denmark. They are located north-northwest of Scotland, and about halfway bet ...

and the

Deccan Traps The Deccan Traps is a large igneous province of west-central India (17–24°N, 73–74°E). It is one of the largest volcanic features on Earth, taking the form of a large shield volcano. It consists of numerous layers of solidified flo ...

of the Sahyadri Mountains of

Maharashtra Maharashtra (; , abbr. MH or Maha) is a state in the western peninsular region of India occupying a substantial portion of the Deccan Plateau. Maharashtra is the second-most populous state in India and the second-most populous country subdi ...

near

Mumbai Mumbai (, ; also known as Bombay — the official name until 1995) is the capital city of the Indian state of Maharashtra and the ''de facto'' financial centre of India. According to the United Nations, as of 2018, Mumbai is the secon ...

. Crystals of a brick-red colour are from Campsie Fells in Stirlingshire and the Fassa valley in

Trentino Trentino ( lld, Trentin), officially the Autonomous Province of Trento, is an autonomous province of Italy, in the country's far north. The Trentino and South Tyrol constitute the region of Trentino-Alto Adige/Südtirol, an autonomous region ...

. A variety known as beaumontite occurs as small yellow crystals on syenitic

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

near

Baltimore Baltimore ( , locally: or ) is the most populous city in the U.S. state of Maryland, fourth most populous city in the Mid-Atlantic, and the 30th most populous city in the United States with a population of 585,708 in 2020. Baltimore was ...

Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean t ...

Notes

References

Mineral GalleriesIMA Zeolite Classification
;Attribution *

External links

{{Commons, Heulandite
Structure type HEU
Tectosilicates Sodium minerals Calcium minerals Aluminium minerals Zeolites Monoclinic minerals Minerals in space group 12 Luminescent minerals