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Kathy Feick

x Al(Al,Si)3O8

(“x” can be sodium and/or calcium and/or potassium)

Feldspar was officially given its name by Johan Gottschalk Wallerius in 1747. It is a contraction of the longer name fieldspar as some early specimens were found in fields. The term spar is a generic term used to refer to any non-metallic mineral with a vitreous (glassy) lustre that breaks on distinct flat surfaces.

The term feldspar is used to identify a very large group of minerals. In fact, it is used to describe any mineral which consists of framework aluminum, oxygen and silicon atoms plus an additive. All minerals in this grouping have the same crystalline structure. The feldspar group has nearly 20 recognized members, but only nine of those are common.  These may be divided into two types of feldspar: plagioclase and alkali.

Plagioclase feldspar:

Each plagioclase grain is made up of stacks of thin crystal with their molecules arranged in opposite directions. This causes the cleavage planes to have fine parallel line across them called striations. This is sometimes plagioclase’s most diagnostic feature. 

Another diagnostic feature is the feldspars colour range. In this case, the colour typically ranges from white to gray. The main feldspars in this grouping include:

  • Oligoclase
  • Andesine
  • Labradorite
  • Bytownite
  • Anorthite
close-up of feldspar specimen

Plagioclase Feldspar and Hornblende. Road cut on highway 515 East of Quadeville, Ontario. University of Waterloo Earth Sciences Museum Collection.

Orthoclase feldspar:

Unlike plagioclase, orthoclase does not have striations across its cleavage plane. Cleavage faces will appear smooth. Orthoclase also typically has a colour range between white and pink, which is different than that of plagioclase. Feldspars which are in this grouping include:

  • Microcline
  • Sanidine
  • Orthoclase
four small pink orthoclase feldspar crystals

Orthoclase Crystals. Goodsprings, Nevada. University of Waterloo Earth Sciences Museum Collection.


The feldspar group is very large and there are many different chemical formulas. Before a feldspar can be used its chemical composition must be determined. Different feldspars will behave differently. Some uses for the feldspar family in general include:

  • Glassware
  • Pottery
  • Scouring solution
  • Fillers (paint, plastics, rubber)
  • Semi-precious gemstones
  • Buildings
  • Monuments
green-blue coloured piece of mineral amazonite

Amazonite. Wood Cox Mine, Bancroft. University of Waterloo Earth Sciences Museum Collection.


Feldspar is an incredibly abundant mineral. It has been found on the moon and in some meteorites. It also makes up roughly 60% of the Earth’s crust. 

Feldspars are found in the every section of the rock cycle. Many feldspars are igneous as they commonly precipitate out in magma as it cools.

They may also be formed as metamorphic minerals in veins of other rocks. This process involves both heat and pressure.

Finally, over time feldspar can weather to produce sediments. Two main weathering agents include wind and water. A feldspar may be worn down to large grains which then turn into conglomerate or breccia, or it may weather to a special kind of clay called kaolin. Kaolin is a very fine-grained particle. When this rock transforms into a sedimentary rock it is called kaolinite. It has many pharmaceutical, cosmetic and paper uses.


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