POTASSIUM FELDSPAR
(K-Feldspar) ("K-spar")
Feldspar is a group of common silicate
minerals. Feldspars are silicate minerals having one-fourth of all the
silicons in SiO2 replaced by aluminum (Si4O8
to (Si3Al)O8). When this happens, the (Si3Al)O8
has a -1 electric charge. The charge is satisfied by the addition of one
or more metals. The (Si3Al)O8- structure
has relatively large holes, and the only metals that tend to stay in these
holes are: K (potassium), Na (sodium), Ca (calcium), Cs (cesium), Ba (barium),
Sr (strontium), and Pb (lead). Of these, K & Na & Ca are the most
common metals that enter the matrix. Sometimes, several different metals
enter the structure, resulting in "garbage can minerals".
Chemical analyses of feldspars show that they range in
composition from K-feldspar to Na-feldspar and from Na-feldspar to
Ca-feldspar. Mineralogists have thus established two
"families" of feldspars. There is no chemical gradient between
K-feldspar and Ca-feldspar.
The potassium feldspars (K-feldspars) (aka
alkali feldspars) are those that range in composition from pure K-feldspar to
pure Na-feldspar (actually, feldspars with ~even & random mixes of
potassium and sodium are rare). The feldspars with Na and/or Ca are the plagioclase
feldspars. All feldspars have similar physical properties: a hardness
of about 6, a whitish streak, and two cleavage planes at or very near
90º. Potassium feldspar is whitish to pinkish-orangish to salmon-colored.
"Potassium feldspar" refers to a group of
several different K-rich minerals: orthoclase, microcline, adularia,
sanidine, and anorthoclase. Orthoclase, microcline, and
adularia have the chemical formula KAlSi3O8 - potassium
aluminosilicate. Sanidine and anorthoclase have the formula (K,Na)AlSi3O8.
Potassium feldspar - upper sample is 4.4 cm across; lower sample is 4.6
cm across.
Amazonite (above & below; above: 3.9 cm across; below: 4.7 cm
across) - amazonite is a distinctively green-colored K-feldspar. Specifically, amazonite is green
microcline. It is known from several
localities around the world. Green
orthoclase is also known, but is extremely rare - it is traditionally only
known from the Broken Hill Block in New South Wales, Australia, but it's since
been found elsewhere. The coloration in green microcline (& green
orthoclase) is due to lead impurity.
Amazonite (green) & smoky quartz (black) from the Crystal Peak area,
Colorado, USA (Colorado School of Mines Geology Museum, Golden, Colorado, USA).