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Heulandite is the name of a series of
tecto-silicateThe silicate minerals make up the largest and most important class of rock-forming minerals, comprising approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
s of the
zeoliteZeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by the
International Mineralogical AssociationThe International Mineralogical Association is an international group of 38 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 4000 plus known mineral species...
changed it to a series name, with the mineral species being named heulandite-Ca, heulandite-Na, heulandite-K, and heulandite-Sr. Heulandite-Ca, the most common of these, is a hydrous hydrous calcium and
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
, (Ca,Na)
2-3Al
3(Al,Si)
2Si
13O
36·12H
2O. Small amounts of
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
and
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
are usually present replacing part of the calcium.
StrontiumStrontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
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.
Crystals are monoclinic. 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
inclusionInclusion may refer to:Social inclusion of persons:*Inclusion *Inclusion *Inclusion Other:*Inclusion , a type of metal casting defect*Inclusion...
s of
celadoniteCeladonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide with formula: K[Si4O10]2....
. It varies from transparent to translucent. Isomorphous with heulandite is the
strontiumStrontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
and
bariumBarium is a chemical element. It has the symbol Ba, atomic number 56, and is the fifth element in Group 2. Barium is a soft silvery metallic alkaline earth metal. It is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts...
zeolite
brewsteriteBrewsterite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, brewsterite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named...
.
The
MohsThe Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science...
' hardness is 3-4, and the specific gravity 2.2. Heulandite is similar to
stilbiteStilbite is a tectosilicate mineral of the zeolite group consisting of hydrated calcium aluminium silicate, NaCa2Al5Si13O36·14H2O....
. The two minerals may, however, be readily distinguished by the fact that in heulandite the acute positive bisectrix of the optic axes emerges perpendicular to the cleavage.
Heulandite was first separated from stilbite by
August BreithauptJohann Friedrich August Breithaupt was a German mineralogist and professor at Freiberg Mining Academy in Freiberg, Saxony. He received his doctorate at the Universities of Jena and Marburg. He studied under Abraham Gottlob Werner at the Freiberg Mining Academy where he became professor of...
in 1818, and named by him "euzeolite" (meaning beautiful zeolite); independently, in 1822, H. J. Brooke arrived at the same result, giving the name heulandite, after the mineral collector, Henry Heuland (1778-1856).
Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of
basaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most...
ic
volcanic rockVolcanic rock is an igneous rock of volcanic origin.- Texture :Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye...
s, and occasionally in
gneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
and hydrothermal veins.
The best specimens are from the basalts of Berufjord, near Djupivogr, in
IcelandThe Republic of Iceland is a European island country located in the North Atlantic Ocean. It has a population of about 320,000 and a total area of 103,000 km². Its capital and largest city is Reykjavík, whose surrounding area is home to approximately two thirds of the national population...
and the
Faroe IslandsThe Faroe Islands, sometimes Faeroe Islands, Faroe, or Faeroes are an island group situated between the Norwegian Sea and the North Atlantic Ocean, approximately halfway between Scotland and Iceland...
, and the Deccan traps of the Sahyadri Mountains near Bombay. Crystals of a brick-red colour are from
Campsie FellsThe Campsie Fells are a range of hills in central Scotland, stretching east to west, from south Stirling to Dumgoyne in East Dunbartonshire. The highest point in the range is Earl's Seat which is 578 m high. The range overlooks the villages of Strathblane and Blanefield to the South, Killearn...
in
StirlingshireStirlingshire or the County of Stirling is a registration county of Scotland, based around Stirling, the former county town. It borders Perthshire to the north, Clackmannanshire and West Lothian to the east, Lanarkshire to the south, and Dunbartonshire to the south-west.Until 1975 it was a county...
and the Fassathal in
TirolTirol is a comune in the province of Bolzano-Bozen in the Italian region Trentino-Alto Adige/Südtirol, located about 70 km north of the city of Trento and about 25 km northwest of the city of Bolzano .As of 31 December 2004, it had a population of 2,361...
. A variety known as beaumontite occurs as small yellow crystals on syenitic
schistThe schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced...
near Baltimore in
MarylandMaryland is a state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia and the District of Columbia to the south and west, Pennsylvania to the north, and Delaware to the east. It is comparable in size to the European country of Belgium. According to the U.S...
.