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Magnesite


Mineral Properties and Uses


Magnesite

Magnesite from Chewelah, Washington. Specimen is approximately 3-1/2 inches (8.9 centimeters) across.

What Is Magnesite?

Magnesite is a magnesium carbonate mineral with a chemical composition of MgCO3. It is named after the presence of magnesium in its composition. Magnesite usually forms during the alteration of magnesium-rich rocks or carbonate rocks by metamorphism or chemical weathering.

Magnesite is used to produce magnesium oxide (MgO), which serves as a refractory material for the steel industry and as a raw material for the chemical industry. Small amounts of magnesite are also used as a gem and lapidary material.



Magnesite from California

Magnesite from Riverside County, California. Specimen is approximately 4 inches (10 centimeters) across.

How Does Magnesite Form?

Magnesite can form by several processes. Some of the more common are described below.




Physical Properties of Magnesite

Chemical Classification Carbonate
Color White, grayish, yellowish, brownish, colorless
Streak White
Luster Vitreous
Diaphaneity Transparent to translucent
Cleavage Perfect
Mohs Hardness 3.5 to 5.0
Specific Gravity 3.0 to 3.2
Diagnostic Properties Dissolves with warm HCl in the powdered form
Chemical Composition MgCO3
Crystal System Hexagonal
Uses Refractory bricks, cement

Properties of Magnesite

Magnesite can be difficult to identify in hand specimens because it often departs from its anticipated properties. It is often cryptocrystalline, which can obscure its cleavage. Magnesite is often silicified or in an admixture with chert, which makes it deceptively hard. The presence of significant chert will also reduce the apparent effervescence with HCl.

Magnesite vs. Howlite

Magnesite and howlite are two of the most commonly confused gem minerals. Both of these minerals often have a white color with gray to brown markings that look like included matrix. The key to separating these minerals is the fact that magnesite is a carbonate mineral and howlite is a silicate mineral. Although they should immediately be separated by their hardness and acid reaction, magnesite often has a significant chert content which makes it appear to have a hardness of 6 or 7. The hydrochloric acid reaction of magnesite is often more subtle than expected.

Observing the acid reaction of siliceous magnesite works best if it is powdered or tested with warm HCl. You can often observe weak effervescence by scraping the magnesite on a streak plate to produce a small amount of powder, then applying a single drop of cold HCl, then watching for bubble growth with a hand lens.

If you have a specimen of polished magnesite, a refractometer and a white light source can be used to separate it from howlite. Howlite will exhibit a spot or a flat surface refractive index of approximately 1.59. However, magnesite will exhibit a continuous birefringence blink between approximately 1.52 and 1.70. Refractive index is a key separation test for the two minerals.

Magnesite from Washington

Magnesite from Chewelah, Washington. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.

Uses of Magnesite

Magnesite has a chemical composition of MgCO3, and when it is heated it will dissociate into MgO and CO2. MgO has an extremely high melting temperature, and that makes it a good refractory material in many steelmaking, metallurgical, and ceramic processes. MgO is one of the most commonly used materials for making the bricks used to line kilns, industrial ovens, and blast furnaces. MgO is also used to make fertilizers, magnesium chemicals, and magnesium metal.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Magnesite Gemstones

Magnesite is commonly used to make tumbled stones, beads, and cabochons. White magnesite material can be cut and then reliably dyed to almost any color. Magnesite manufacturing costs are very low for several reasons:

Low manufacturing costs and the low durability of magnesite confine its use to costume jewelry and crafts.

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