Tenacity describes the reaction of a mineral to stress such as crushing,
bending, breaking, or tearing. Certain minerals react differently to each type of stress.
Since tenacity is composed of several reactions to various stresses, it is possible for a
mineral to have more than one type of tenacity. The different forms of tenacity are:
Brittle - If a
mineral is hammered and the result is a powder or small crumbs, it is considered brittle.
Brittle minerals leave a fine powder if scratched, which is the way to test a mineral to
see if it is brittle. The majority of all minerals are brittle.
An example is Quartz.
(Minerals that are not brittle
may be referred to as Nonbrittle minerals.)
Sectile - Sectile
minerals can be separated with a knife, much like wax but usually not as soft.
An example is Gypsum.
Malleable - If a mineral can be flattened by pounding with a hammer, it is malleable. All
true metals are malleable.
An example is Silver.
Ductile - A
mineral that can be stretched into a wire is ductile. All true metals are ductile.
An example is Gold.
Flexible but inelastic - Any mineral that can be bent, but remains in the new position after it is
bent is flexible but inelastic. If the term flexible is singularly used, it implies
flexible but inelastic.
An example is Copper.
Flexible and elastic - When flexible and elastic minerals are bent, they spring back to their
original position. All fibrous minerals,
and some acicular minerals belong in
this category.
An example is Chrysotile Serpentine.
How to use tenacity as an identification
mark
Each form of tenacity is tested on its own. A mineral of
questionable status should not be tested for the different forms of tenacity, except for
brittleness; such tests can damage a specimen. Minerals are not tested for malleability or
ductility, since the only way to conduct these tests is by pounding and stretching the
mineral.
How to test using tenacity
When testing for brittleness, one should scratch the
specimen with a harder mineral or material, and see if the mineral leaves a fine powder.
If fine powder is scratched off the mineral, it is brittle. If just a furrow is left
without any fine powder, the mineral is nonbrittle. Brittleness and hardness are usually
tested simultaneously, since the same procedure can be conducted for both of them. When
testing for brittleness, do not scratch a noticeable surface area, as this may decrease a
specimens value. Additionally, the surface being tested by scratching should not be dirty,
coated, or tarnished.
If a mineral is suspected as being sectile, insert a knife
into the mineral and see if it goes through. If the knife penetrates, the mineral is
sectile. Sectility can also be tested with a long fingernail, by slowly pushing it into
the specimen to see if it enters. If the fingernail goes through, the mineral is sectile.
To check for flexibility, slight pressure should be placed
on the mineral to see if it bends. If it moves to the new position after stress is
released, it is inelastic. If it goes back to its original position after stress is
released, it is elastic. Even some flexible minerals will break if there is too much stress is
applied. Therefore, they should be bent carefully and slowly, and only a minimum amount.
Only few minerals are flexible, so thin minerals may be damaged even when slight flexing
pressure is applied.
