Magnesium Silicate Hydroxide

OVERVIEW

Magnesium silicate hydroxide (mag-NEE-zee-um SILL-uhkate hye-DROK-side) is also known as hydrated magnesium silicate, hydrous magnesium silicate, magnesium silicate hydrous, talc, talcum, and soapstone. It belongs to a large family of magnesium silicates that occur in nature. Magnesium silicates contain at least one magnesium ion and one or more silicate (SiO₃) ions, and often contain one or more molecules of water of hydration. Other members of the family include magnesium metasilicate (MgSiO₃), magnesium orthosilicate (Mg₂SiO₄), magnesium trisilicate (Mg₂Si₃O₈), and magnesium trisilicate pentahydrate (Mg₂Si₃O₈·5H₂O).

KEY FACTS

OTHER NAMES:

See Overview.

FORMULA:

Mg₃Si₄O₁₀(OH)₂

ELEMENTS:

Magnesium, silicon, oxygen, hydrogen

COMPOUND TYPE:

Hydrated salt(inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

379.27 g/mol

MELTING POINT:

1500°C (2700°F); begins to lose water of hydration above 900°C (1600°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water and most organic solvents

The naturally-occurring form of magnesium silicate hydroxide is called talc, a soft mineral that feels waxy and soapy to the touch. This characteristic has led to another name for the mineral: soapstone. Talc's chemical formula differs somewhat from that of magnesium silicate hydroxide: Mg₃Si₄O₁₀(OH)₂.

Talc is one of the softest minerals known. It has a numerical rank of 1 on the Mohs scale of minerals. The Mohs scale ranks minerals from the softest (1 = talc) to the hardest (10 = diamond). Talc is so soft that it can be scratched with the fingernail. The mineral has a pearly luster and may come in a variety of colors, ranging from white gray, or silver to black, brown, pink, or green. Color variations depend on impurities in the mineral.

Magnesium silicate hydroxide normally occurs as a fine white powder with no odor, insoluble in most solvents, noncombustible, resistant to heat, and with a tendency to absorb moisture from the air.

HOW IT IS MADE

The magnesium silicates are obtained from natural sources, such as talc (magnesium silicate hydroxide), enstatite (magnesium metasilicate), forsterite (magnesium orthosilicate), meerschaum (magnesium trisilicate), or serpentine (Mg₃Si₂O₇). In all cases, extraction of the desired compound involves a number of steps in which the mineral is separated from waste products, crushed, purified, and processed into its desired form (powder or crystal, for example).

Interesting Facts

• The magnesium silicates contain three of the eight most abundant elements in Earth's crust: oxygen (the most abundant element), silicon (the second most abundant element), and magnesium (the eighth most abundant element).

COMMON USES AND POTENTIAL HAZARDS

The form of magnesium silicate hydroxide with which most people are familiar is talc, the main ingredient in talcum powder. Talcum powder is used directly as a skin treatment, especially for babies, primarily as a moisture absorbent and for soothing the skin. It is also used in a number of cosmetic products, including blushes, eye shadows, make-up foundations, and face powders. Talc is also used widely as an additive to give products a smoother texture. Some products in which it is found include paints, rubber products, roofing materials, ceramics and insecticides. Other applications of talc include:

• As a food additive to prevent foods from clumping and sticking together;
• In the polishing of rice;
• In the production of olive oil to improve the product's clarity;
• For countertops in chemical laboratories;
• As a lubricant between sheeted products, such as particleboard, to prevent individual sheets from sticking to each other;
• As a filler in a number of products, including soap, putty, plaster, oilcloth, and rubber products; and
• As a non-caking agent in animal feeds and fertilizers.

Magnesium silicate hydroxide and other forms of magnesium silicates have a number of industrial applications, including:

• In the manufacture of glass and ceramic materials;
• As an insulating material for electrical devices;
• As a refractory material in industrial furnaces;
• In clean-up operations following oil spills; and
• As an odor absorbent.

Exposure to talc dust may cause irritation of the skin, respiratory system, and, especially, the eyes. Harmful effects may include skin rash, eye damage, and coughing and wheezing. These symptoms generally occur only as the result of long-term and consistent exposure to dust. There is no evidence that talc is carcinogenic unless contaminated with other minerals, such as asbestos and/or silica. The amount of talc and other magnesium silicates to which the average person is exposed is probably too low to produce any detectable health effects.

Words to Know

ION

An atom or a group of atoms with either a positive or negative charge, because it has either lost or gained an electron.

MOHS SCALE

A numerical scale used to compare the hardness of a material. Talc has a Mohs value of 1; diamond has a value of 10.

REFRACTORY MATERIAL

A material with a high melting point, resistant to melting, often used to line the interior of industrial furnaces.

WATER OF HYDRATION

Water that has combined with a compound by some physical means.

FOR FURTHER INFORMATION

"Chemical Summary: Talc." CHEC's HealtheHouse. http://www.checnet.org/healthehouse/chemicals/chemicalsdetail.asp?Main_ID=2 (accessed on October 26, 2005).

"Magnesium Silicates." In Pradyot Patnaik. Handbook of Inorganic Chemicals. New York: McGraw-Hil, 2003, 534-535.

"Talc." New Jersey Department of Health and Senior Services. http://www.state.nj.us/health/eoh/rtkweb/1773.pdf (accessed on October 26, 2005).

"Talc Mineral Data." Mineral of the Month Club. http://webmineral.com/data/Talc.shtml (accessed on October 26, 2005).