Chapter 2.3

How can we identify Minerals?  Minerals come in all different shapes, colors, textures, and properties.  For example, minerals like halite and calcite have large crystals that form in predictable patterns, magnetite is magnetic, and sulfur smells like rotten eggs. All of these properties can be used by geologists to identify minerals.

The Property of Color  Although sometimes color can help identify certain minerals like hematite, which is red, often times color can be misleading because many minerals can be found in a wide variety of colors, like fluorite. HematiteDifferent Shades of Fluorite

The Property of Luster  Luster is similar to color because it has to do with a mineral’s outer appearance.  Luster is a property that describes how light is reflected off a surface.  For example, luster can be dull, earthy, vitreous (glassy), pearly, metallic, silky, or adamantine (brilliant luster).  Diamond for example has an adamantine or brilliant luster.

The Property of Luster Diamond has an Adamantine Luster Quartz has a Vitreous or Glassy Luster Galena has a Metallic Luster Hematite has a Dull or Earthy Luster

The Property of Streak  Some minerals may have different shades or colors.  Often times, geologists can use a piece of unglazed porcelain (called a streak plate) to determine the color of the mineral in the form of a powder.  This is a very useful property because although a mineral’s color may vary, a mineral’s streak color stays the same.

The Property of Streak The Mineral Hematite sometimes appears black and sometimes appears red, but the color of its streak will always show as reddish/brown. The Mineral Pyrite (Also known as Fool’s Gold), usually is yellowish-gold, but the color of its streak is always Gray.

Cleavage, Fracture, and Crystal Arrangements and Patterns  Often times, a mineral will break or cleave along an even plane or surface.  This is a property known as cleavage.  Minerals that do not break along even surfaces when broken are said to have fracture and not cleavage.  All minerals have an internal crystal arrangement, but some exhibit large and unique crystal patterns

Mineral Cleavage and Crystal Forms

Conchoidal Fracture Obsidian with Conchoidal FractureQuartz with Conchoidal Fracture

The Property of Hardness  The hardness of a mineral is also a useful property used to identify different minerals.  One way to determine a mineral’s hardness is to compare its hardness to other substances of known hardness.  The Mohs hardness scale is a scale of 1-10, with 1 being the softest and 10 being the hardest.

The Mohs Hardness Scale

Density or Specific Gravity  Another property that can be used to identify a mineral is a mineral’s density or specific gravity.  Both methods give the same number, but density is easier to calculate and specific gravity is usually more precise because it is harder to find the volume of a rock than it is to find the mass.

What’s the difference?  To find the density, you divide an object’s mass by its volume.  To find the specific gravity you must weigh a mineral in air and in water.  Then you divide the mass of the mineral in air by the difference between the mass in air and under water.  Density = Mass/Volume  Specific Gravity = Mass in Air/(Mass in Air – Mass in Water)

Special Properties  Many minerals have special properties.  Calcite reacts with acid and has double refraction.  Magnetite is magnetic.  Halite tastes salty.  Sulfur smells like rotten eggs.  Graphite and Talc feel soapy or slippery.  Graphite writes on paper.

Special Properties Calcite has a Special Property known as Double Refraction