Spinel

Authors: Charlotte Salgado-Zerber, Ryan Dean, Shawn Branson

Mineral Name: Magnesium Spinel, also simply known as “Spinel”, is the most common and well known of the Spinel series. Collectors often search for Gahnite (Zinc Spinel) to add to their collection. The rarest two members of the Spinel series are Hercynite (Iron Spinel) and Galaxite (Manganese Spinel). (3)

Synonyms of spinel: Candite, ceylanite, ruby spinel, spinella, spinel-ruby, strongite, talc spinel, and zeilanite. (9)

Chemical Composition: Chemical composition for Magnesium Spinel: MgAl₂O₄; chemical composition for Gahnite: ZnAl₂O₄chemical composition of Hercynite: FeAl₂O₄; chemical composition of Galaxite: MnAl₂O_4.. (9)

Color: Spinel comes in many colors, reds, pinks, oranges, purples, and blues. The most valuable colors of spinel are the reds because that color is the most sought after for making jewelry. (2)

Streak: Spinel has a white streak when scraped on a porcelain tile. (3)

Hardness: Spinel has a hardness of 7.5-8 on Mohs scale. (3)

Cleavage/Fracture: Spinel has conchoidal fracture, which is a break that’s a semicircle shape, like the curved shape of a clam.

Crystal Form: Spinel has an isometric crystal system, meaning that it has four equal sides that come together rather like two pyramids with the bases connecting, forming points on the top and bottom and four sides, or corners. (3).Spinel generally occurs in perfectly shaped octahedral crystals, often with parallel layers of growths or heavy etchings (3). Clusters of crystals or twinning is very common, as this mineral is famous for its spinel twinning where two octahedral crystals twin at their base. A macle, or two flattened crystals forming a triangle, may also be found on the occasion. (3)

Luster: Luster is a subjective term. Spinel may appear to have a vitreous look (glassy), or a submetallic look (somewhat shiny), or it may have a dull, earthy look like unglazed pottery. (3)

Special Features: Spinel comes in many colors that are favored for facetting into gemstones because it is a relatively hard stone. Spinel can form as an octahedron, or two back to back pyramids, though it can also form as a flattened twinned crystal. Under shortwave ultraviolet light, the more red spinel sampled will fluoresce red or neon yellow. (3)

Varieties: Almandine spinel (a violet colored spinel), balas ruby (an outdated name for light pink to red colored gem spinel), alkali-spinel (a spinel containing sodium or potassium oxide originally discovered in Sweden), ceylonite (a black to green colored magnesium spinel colored by traces of iron), chlorospinel (a magnesium spinel with trace amounts of iron and copper giving it a deep green color), chrome spinel (with the synonym picotite, this spinel is chrome rich giving it its name), chromo-hercynite (a chromium-rich variety of hercynite with the synonym of chrysomelane), corundite ( a man-made aluminum-rich spinel), ferropicotite (a variety of spinel containing ferric sulfate or iron(III) sulfate), gahnite (an end-member of the spinel mineral group that is recognized as an individual series of minerals containing large amounts of zinc), gahnospinel (this variety of magnesium spinel is mixed with gahnite giving it the chemical formula(Mg,Zn) Al₂O₄ and could grade into gahnite), galaxite (an end-member of the spinel mineral series very rich in magnesium that is also recognized as its own mineral group), hercynite (another member of the spinel series recognized as its own mineral series, this mineral is rich in iron), iron-magnesia spinel (much like the name implies, this is an iron rich or ferroan variety of spinel), magnochromite ( a variety of spinel containing chromium), picotite ( a magnesium and chromium-rich variety of hercynite that is sometimes used describe brown spinel), pleonast (an iron-rich or ferroan spinel with more complex crystal forms that is opaque and dark blue to green colored), rubicelle (a yellow to orange-red variety of spinel), ruby spinel (a gemstone previously mistaken for corundum ruby that is transparent and red like ruby but not quite as hard with fewer inclusions), titano-spinel (a spinel containing titanium) . (3, 9)

Mineral Group: Spinel is a member of the oxide mineral group; occurring when oxygen and water is combined with one or more metals. Minerals in the oxide group have varying physical properties with hardness that can range from 5-9 on Moh’s scale and colors that can range from black to an array of beautiful and vibrant colors. (3b) Spinel is also a member of a supergroup with their namesake: the Spinel Supergroup. The Spinel supergroup contains minerals that have wide chemical variances and have a spinel type structure, AB₂X₄. Minerals of this supergroup can be used as geothermometers and geobarometers; meaning they are indicators of the range of temperature and pressure under which the mineral formed. (9)

Environment: Spinel forms through contact metamorphism of limestone and marble. Contact metamorphism is a high temperature and low pressure type of metamorphism that changes in mineral composition, structural form, and chemical composition when a limestone or marble comes into contact with high temperatures. (3) (13) Spinel also forms in ultrabasic igneous rock, which is igneous rock with very little silica, or other ultramafic rocks. These igneous rocks containing spinel form at extremely hot temperatures but at lower pressure in the mantle; if higher pressure were to be applied, garnet would form instead of spinel. (14) Spinel is also formed in placer deposits. A placer deposit is a deposit of minerals that have been transported via water and gravity. Water breaks down the rocks containing the minerals, and erodes them to where they are then concentrated in one location. (15)

Associated Rock types: You are likely to find Spinel in the Earth’s upper mantle within peridotite or within other igneous rocks such as dunite, lherzonite, pyrozenite, websterite and kimberlite. Spinel may also be found within metamorphic rocks like granofels, contact-marble, buchite and a skarn. (12)

Occurrence in North America: In the United States, spinel is common in the metamorphosed limestones of the New York-New Jersey highlands belt. (7) Spinel has been found in the following states; California, Colorado, Massachusetts, and North Carolina. (3, 4) Latitude and longitude of Nickel Mountain (Hanna Nickel Mine) in Douglas County., Oregon USA 42° 57' 40'' North, 123° 26' 20'' West, and Read Mine (Read Iron Mine) located in Stevens County Washington USA,48° 6' 23'' North, 118° 7' 21'' West. (9) These are two mines located in the United States that have spinel.

Economic Uses: Spinel can be cut into any of the gemstone cuts and is used for making jewelry. (3) For centuries spinel has been confused with rubies and sapphires due to it being found in the same areas as those minerals. It is spinel that is in many of the crown jewels. When this gem was put in these crowns it was not known that it had a different chemical composition than sapphires or rubies. Spinel is in the Black Prince’s Ruby set in the British Imperial State Crown, as well as in the crown of Catherine the Great, it is also in a necklace that was created for Queen Victoria in 1853. (16)

Industrial Uses: Due to its strength, hardness, and chemical stability at high temperatures, dense spinels are used widely both commercially and industrially in mechanical, thermal and structural applications. (5) Spinel based ceramics (refractories) can be found in cement rotary kilns, in the lining of steel ladles as alumina-rich spinel castables in the iron and steel industry, and checker bricks of glass furnace regenerators. (5) Magnesium aluminate spinel is eco-friendly and poses less health concerns than the alternatives for these applications, although the production cost is much higher. (5)

First Notable Identification: Spinel was given its name in 1779 by Jean Demeste. (9) Spinel is derived from the latin word spinella, meaning “little thorn”, due to it’s sharp octahedral crystals. (9) Prior to the late 1700’s when Jean Demeste identified spinel, it was most often mistaken for corundum like ruby. (10)

How We Identified It: The first thing we did was identify the mineral’s color which appeared to be red. We then tested the minerals hardness using the scratch test and found this mineral was extremely hard with an estimated hardness of 8. Next we tested the minerals streak on a porcelain streak plate and the streak was white. Lastly we took a close look at the mineral and determined that there was fracture present and that the fracture was semicircular in shape (conchoidal fracture). Due to the above factors, we determined the mineral was spinel.

Don’t Confuse It With: Spinel can be confused with the corundum variety ruby, magnetite, franklinite, zircon, chromite, or garnet. Zircon and garnet can can be differentiated from spinel by their crystal forms. Magnetite and franklinite are both softer (5.5-6 moh’s scale) and are attracted to magnets while corundum variety ruby is harder (9 on moh’s scale) and has a different crystal form. Chromite is softer (5.5 moh’s scale) and heavier. (3)

Bibliography:

1)Amethyst Galleries 1995-2014, Accessed 4-14-19

http://www.galleries.com/Spinel

2) Gemological Institute of America 2002-2019 Accessed 4-14-19 (1)

https://www.gia.edu/spinel-quality-factor

3) Minerals.Net The Mineral and Gemstone Kingdom

https://www.minerals.net/mineral/spinel.aspx

https://www.minerals.net/mineral_glossary/oxides.aspx

4) "isometric." Webster's Ninth New Collegiate Dictionary. 1986, p. 642.

Lynch, Dan R. Rock Hound's Logbook & Journal. Adventure Publications, Inc, 2010, p. 82.

5) research.net “Industrial Applications of Refractory Containing Magnesium Aluminate Spinel” April 23 2019

https://www.researchgate.net/publication/279691599_Refractory_applications_of_magnesium_aluminate_spinel