Apatite is the name given to a group of common phosphate minerals found across the globe, often occurring in beautifully colored crystalline forms. This group includes minerals like fluorapatite, chlorapatite, and hydroxylapatite, which is a major component of human bones and teeth. Understanding apatite’s hardness is key to determining its suitability for various uses, from industrial applications to jewelry.
Defining Mineral Hardness
Mineral hardness is defined as a material’s resistance to being scratched by another substance. This property is distinct from toughness, which relates to a mineral’s resistance to breaking or chipping. Geologists have used a simple, comparative system to measure this characteristic.
The most widely recognized method is the Mohs Scale of Mineral Hardness, developed in 1812 by German mineralogist Friedrich Mohs. This scale ranks ten specific minerals from the softest to the hardest on a qualitative, ordinal scale from 1 to 10. The test involves attempting to scratch a mineral of unknown hardness with a known mineral from the scale.
Talc is rated 1, and Diamond, the hardest naturally occurring substance, is rated 10. The scale works because a harder mineral will always scratch a softer one. The Mohs scale is useful for fieldwork as it provides a quick, relative measure of scratch resistance.
Apatite’s Specific Mohs Rating
Apatite serves as the reference mineral for the midpoint of the scale, possessing a Mohs hardness of 5. This placement means that apatite can be scratched by any mineral with a Mohs rating greater than 5, and it can scratch any mineral with a rating lower than 5.
The placement of apatite at 5 puts it directly above Fluorite (hardness 4) and below Orthoclase Feldspar (hardness 6). Apatite will easily scratch Fluorite, while Orthoclase Feldspar can scratch apatite, confirming its moderate resistance to abrasion.
Real-World Practicality of Apatite’s Hardness
The Mohs 5 rating places apatite in a category of moderate hardness, which has practical implications for its use and care. Apatite can scratch materials like a copper penny (hardness 3.5), but it is susceptible to scratching from many common household items. For example, a steel knife blade or ordinary window glass has a hardness that ranges from 5.5 to 6.5, meaning both can easily scratch apatite.
When used as a gemstone, this moderate hardness requires careful handling and storage. Apatite jewelry is far more susceptible to surface abrasion than gems like Quartz (rated 7) or Topaz (rated 8). To avoid scratches, apatite must be stored away from harder materials and cleaned with soft cloths and mild solutions.
Beyond its use as a gemstone, apatite’s structure is biologically significant because of its similarity to hydroxyapatite. This calcium phosphate compound forms the mineral component of human teeth and bones, providing them with their structural rigidity. The hardness of tooth enamel, the body’s hardest tissue, is around Mohs 5, reflecting the resilience of this apatite-like structure. This structural relationship highlights the mineral’s fundamental role in natural systems, even though its scratch resistance is not high enough for industrial applications.