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The Mineral Sphalerite

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Sphalerite mineral specimen
PHYSICAL PROPERTIES:
Chemistry:(Zn, Fe)S
Composition: Zinc Iron Sulfide
Class: Sulfides
Group: Sphalerite
Crystal System: isometric
Fracture: conchoidal
Hardness: 3.5-4
Specific Gravity: 4
Refractive Index: 2.37 - 2.42
Luster: adamantine to resinous
Streak: yellow to light brown
Color: black to brown also yellow to red and very rarely colorless
Cleavage: perfect in six directions
Transparency: crystals are transparent to translucent
Associated Minerals: Arsenopyrite, Barite, Chalcopyrite, Calcite, Dolomite, Fluorite,Galena, Magnetite, pyrrhotite, Pyrite, Quartz, Siderite, and many other mineral verities.

COMPOSITION:
6 7.0% Zn, 33.0% S with varying amounts of iron and manganese, and other elements.

DISTINGUISHING CHARACTERISTICS: 
Sphalerite can resemble some galena, the streak and the blowpipe tests will determine the difference here. It also resembles some siderite, but can be distinguished by remaining non magnetic after heating and also by its higher specific gravity. The characteristic luster and association with pyrite and galena generally serve to identify sphalerite. Sphalerite's luster really sparkles. It's unusually high index of refraction produces a fire greater than diamond's. When rotated good specimens, with hundreds of small sparkling faces, will produce multiple bright flashes. Sphalerite's structure is analogous to the diamond structure. If every other carbon in the diamond structure is replaced by a sulfur atom and the remaining carbons are replaced with either a zinc or an iron atom then basically the mineral is sphalerite.

ENVIRONMENT:
In suiphide ore veins in all rock classes.

CRYSTAL DESCRIPTION:
Tetrahedral crystals are very common, some times they are so completely developed that they look octahedral. Cube, dodecahedron, and tristetrahedron faces can also be present, the dodecahedron is often rounded so that it is difficult to distinguish the faces. Sphalerite can also be stalactitic, granular, and massive. The twinning of sphalerite is also special. It can form a spinel twin which is a specialty of the mineral spinel. The spinel twin is where a tetrahedral crystal is twisted in the middle so that three points of the tetrahedron are in alignment with the other three points. These crystals are not usually completely developed in sphalerite but the indentations that the twinning causes are usually seen on some crystals in almost every specimen. There is also a twin type called a chicken twin or hen. This twin forms a sort of football shaped twin.

Sphalerite is a polymorph with two minerals, wurtzite and matraite. The three are called polymorphs. Which means they have the same chemistry, (Zn, Fe)S or zinc iron sulfide. But they have different structures and therefore different shapes. Sphalerite is the more common
mineral of the three. There is also an iron rich variety of sphalerite that is called Marmatite. Marmatite is not a commonly seen mineral because it is brilliant black like black sphalerite. So it is very often unrecognized. Marmatite is chemically a ferroan sphalerite. It contains, up to twenty percent iron in addition to zinc sulfide.

TESTS:
Practically infusible on charcoal, but gives coating around chip which is yellow when hot, and white when cold. Touched with cobalt solution, the yellow coating becomes green in the reducing flame. Also adding Na,CO, then dissolve in HCL. The bubbles will be H2S which has a rotten egg smell.

LOCALITIES:
Sphalerite locals are very numerous. We have some nice specimens from Chihuahua, Mexico. Other notable locals are the Tri state area near Joplin, Missouri. Joplin crystals range from black and dull irregular giants to minute red (ruby jack) encrustation's. Very pale light yellow to green crystals are from Franklin, New Jersey. Nice Specimens come from Rosiclare, Illinois; Elmwood, Tennessee; Broken Hill, Australia; Italy; Spain; Burma; Peru; Morocco; Germany and England.

U
SES:
Sphalerite is the principal primary ore of zinc. It alters to hemimorphite, smithsonite, and willemite. The impurities gallium, indium, and cadmium make it also the chief ore of those metals. Sphalerite can make a rather attractive specimen as well. On rare occasion it is cut into a collector only type gemstone.

FACTS & HISTORY:
Black Jack and Ruby Jack are nick names given respectively to the black and red crystal forms of sphalerite.

An aggregate of botryoidal crusts with layers of wurtzite and galena is called "Schalenblende" is sometimes cut and polished as an ornamental stone.

Its cleavage and luminescence make Sphalerite a very interesting mineral. It is the best example of dodecahedral cleavage (six direction cleavage). With care, perfect rhombic dodecahedrons can be cleaved out. Sometimes it fluoresces orange in ultraviolet light. Fluorescent sphalerite also shows the phenomenon of triboluminescence. Which means it emits flashes of orange light when struck lightly with a hard substance like steel or a stone.

The names Sphalerite which is Greek for treacherous rock and Blende which is German for blind or deceiving. Were names used by miners because sphalerite can be so difficult to identify from more valuable minerals such as galena, acanthite and tetrahedrite.