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.

If you want to identify magnesite, the steps below will probably be helpful. Some assume that you have a specimen that can be used for destructive testing.

Check for an acid reaction: Produce some powder by scraping the specimen across a streak plate. Then place a drop of dilute (5%) hydrochloric acid on the specimen and look for an effervescent reaction. You might need a hand lens to see tiny bubbles growing very slowly from the powder.

Test the specific gravity: Magnesite usually has a specific gravity of between 3.00 and 3.20. It might be as low as 2.8 if it has a significant quartz or chert content. This low specific gravity will be paired with a higher than normal hardness if your specimen contains significant chert or is silicified.

Test with a refractometer: If your specimen has a polished surface and you have a refractometer (and are good at using it), you will be able to do one of the most reliable tests for magnesite. Magnesite has an index of refraction that ranges from about 1.509 to 1.700, and a birefringence of 0.191. But the most important property is that it exhibits birefringence blink across the 1.509 to 1.700 range.

Uses of Magnesite

Magnesite has a chemical composition of MgCO_۳, and when it is heated it will dissociate into MgO and CO_۲. 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 refined into magnesium metal.