<p>Potassium feldspar (upper specimen is 4.4 cm across; lower specimen is 4.6 cm across)
</p><p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  "Silica" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.
</p><p>The simplest &amp; most abundant silicate mineral in the Earth's crust is quartz (SiO2).  All other silicates have silica + impurities.  Many silicates have a significant percentage of aluminum (the aluminosilicates).
</p><p>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 &amp; Na &amp; Ca are the most common metals that enter the matrix.  Sometimes, several different metals enter the structure, resulting in "garbage can minerals".
</p><p>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.
</p><p>The potassium feldspars (K-feldspars) (also known as alkali feldspars) are those that range in composition from pure K-feldspar to pure Na-feldspar (actually, feldspars with ~even &amp; 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 usually whitish to cream-colored to pinkish-orangish-salmon colored.
</p><p>"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.
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<hr><p>Photo gallery of orthoclase:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=3026">https://www.mindat.org/gallery.php?min=3026</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3026&lt;/a&gt;
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<hr><p>Photo gallery of microcline:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=2704">https://www.mindat.org/gallery.php?min=2704</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=2704&lt;/a&gt;
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<hr><p>Photo gallery of adularia:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=28">https://www.mindat.org/gallery.php?min=28</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=28&lt;/a&gt;
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<hr><p>Photo gallery of sanidine:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=3521">https://www.mindat.org/gallery.php?min=3521</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3521&lt;/a&gt;
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<hr><p>Photo gallery of anothoclase:
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&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=248">https://www.mindat.org/gallery.php?min=248</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=248&lt;/a&gt;
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/32499528651/">Potassium feldspar</a>