5 Cohesive properties of minerals

Hardness

• Hardness is the ability of a mineral to resist scratching; a harder mineral scratches a softer one.
• It is measured numerically on Mohs' scale:

• Common tools: human fingernail (about $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">2.5</span>}$), copper coin ($\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">3.5</span>}$), window glass ($\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">5.5</span>}$) and streak plate ($\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">6.5</span>}$).
• In the field or lab, sets of hardness pens made of alloys, or everyday objects of known hardness, are used.

Cleavage

• Cleavage is the ability of a mineral to split along flat surfaces that represent weaker planes.
• One direction: mica splits into thin sheets (flaky cleavage); graphite has good basal cleavage.
• Several directions: halite and galena have perfect cubic cleavage; calcite has rhombohedral cleavage.
• No cleavage: quartz and chert (flint) break with a conchoidal fracture.

Malleability and ductility

• The ease of beating a mineral into thin sheets or drawing it into wires, as in gold, silver and copper.

Other properties

• Specific gravity: galena $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">7.5</span>}$ and gold $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">19.3</span>}$.
• Magnetic property: magnetite is attracted by a magnet.