Micro­cline is a wide­spread rock-form­ing min­er­al belong­ing to the sil­i­cate class, specif­i­cal­ly the feldspar group. It is a frame­work potas­si­um alu­mi­nosil­i­cate. Its col­or varies from col­or­less and white to cream, pale yel­low, pink, red, bright green, or bluish-green.

Micro­cline forms short pris­mat­ic or tab­u­lar crys­tals, often large in size: sin­gle crys­tals can weigh sev­er­al tons and reach meters in length. Crys­tals are often mul­ti­ple bina­ries, with two sets of thin lines at right angles to each oth­er. This gives a “check­ered” effect unique to micro­cline among feldspars. The micro­cline can also be mas­sive. The min­er­al is found in feldspar-rich rocks such as gran­ite, syen­ite, and gra­n­odi­or­ite.

Micro­cline is includ­ed in the list of min­er­als of nation­al impor­tance, approved by Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 of Decem­ber 12, 1994, as raw mate­ri­als for fac­ing mate­ri­als (dec­o­ra­tive stones) and glass and for­for-faience raw mate­ri­als..

Formation of the mineral

Micro­cline (KAlSi₃O₈) crys­tal­lizes in tri­clin­ic syn­go­nia and is a low-tem­per­a­ture poly­mor­phic mod­i­fi­ca­tion of potas­si­um feldspar. Its for­ma­tion occurs at tem­per­a­tures below 500 °C. This min­er­al is formed main­ly in gran­ite and peg­matite mag­mas, where it is one of the first crys­tal­liz­ing min­er­als. With slow cool­ing of the mag­ma, the con­di­tions are ide­al for the for­ma­tion of large, well-formed micro­cline crys­tals, espe­cial­ly in peg­matites.

In meta­mor­phic rocks, micro­cline forms through the recrys­tal­liza­tion of potas­si­um-bear­ing min­er­als under high pres­sure and tem­per­a­ture, for exam­ple in gneiss­es and shales. Micro­cline for­ma­tion can also occur by alter­ing oth­er potas­si­um feldspars, such as ortho­clase, result­ing from grad­ual cool­ing.

A micro­cline may be chem­i­cal­ly the same as a mon­o­clin­ic ortho­clase, but since it belongs to a tri­clin­ic crys­tal sys­tem, the angle of the prism is slight­ly less than a right angle; hence the name “micro­cline” from the Greek “small slope”. How­ev­er, there are sev­er­al key dif­fer­ences between ortho­clase and micro­cline.

First, ortho­clase is a high-tem­per­a­ture mon­o­clin­ic poly­morph of potas­si­um feldspar with a par­tial­ly ordered struc­ture. In con­trast, micro­cline is a medi­um- to low-tem­per­a­ture tri­clin­ic poly­morph of potas­si­um feldspar with a high­ly ordered struc­ture.

It is a ful­ly ordered tri­clin­ic mod­i­fi­ca­tion of potas­si­um feldspar, dimor­phous with ortho­clase. Micro­cline is iden­ti­cal to ortho­clase in many phys­i­cal prop­er­ties and can be dis­tin­guished by X‑ray or opti­cal analy­sis. When viewed under a polar­iza­tion micro­scope, the micro­cline exhibits a small mul­ti­ple bina­ry for­ma­tion that forms a lat­tice struc­ture that can­not be con­fused.

Varieties of microcline

The only well-known vari­ety of micro­cline is ama­zonite, which ranges in col­or from light to dark green. The fol­low­ing vari­eties are also dis­tin­guished:

• microcline–albite (anortho­clase);
• beryl­li­um micro­cline (same as beryl­li­um feldspar);
• micro­cline green (same as ama­zonite);
• max­i­mum micro­cline (micro­cline with the high­est degree of tri­clin­ic dis­tor­tion of the crys­tal lat­tice);
• sodi­um micro­cline (anortho­clase);
• micro­cline-perthite (mutu­al ger­mi­na­tion of micro­cline by albite, which is formed as a result of the dis­in­te­gra­tion of a sol­id solu­tion);
• micro­cline rubid­i­um (a type of micro­cline that con­tains up to 3% Rb2O3).

Microcline spread

Wide­spread in plu­ton­ic fel­site rocks, such as gran­ites, gran­ite peg­matites, syen­ites; in meta­mor­phic rocks of green shale and amphi­bo­lite facies; in hydrother­mal veins. Detri­tal com­po­nent in sed­i­men­ta­ry rocks and as authi­genic growths.

Sig­nif­i­cant occur­rences of micro­cline have been record­ed in North Amer­i­ca (par­tic­u­lar­ly in the USA — Col­orado and Vir­ginia), as well as in Ger­many, Switzer­land, Nor­way, and Mada­gas­car. In Ukraine, it is com­mon in Volyn and the Azov region.

The largest doc­u­ment­ed sin­gle crys­tals of micro­cline were found in the Devil’s Hole Beryl Mine, Col­orado, USA, mea­sur­ing approx­i­mate­ly 50 × 36 × 14 m. These may rep­re­sent some of the largest crys­tals of any mate­r­i­al ever dis­cov­ered.

Use of mineral

Micro­cline is not a wide­ly used min­er­al today, but this was not always the case. In Ancient Egypt and Mesopotamia, it was wide­ly used both for jew­el­ry and dec­o­ra­tive pur­pos­es. Ama­zonite beads were even found in the tomb of King Tut.

Today, micro­cline is main­ly used in the pro­duc­tion of porce­lain. It is first fine­ly ground and then mixed with kaolin (a type of clay) and quartz. When heat­ed, feldspar melts and acts as a cement that binds mate­ri­als togeth­er.

Molten feldspar is also a major com­po­nent of ceram­ic glazes. It is some­times used as a source of alu­mi­na in glass pro­duc­tion.