Name/Title
OrthoclaseEntry/Object ID
2009.62.141Description
Chemical Composition: Potasium Aluminum Silica
Crystal System: Monoclinic System
Description: Common Name: Orthoclase
Chemistry: KAlSi3O8, Potasium Aluminum Silicate.
Group: Feldspars
Location: Minas Gerias, Brazil
Description: White, opaque center section with a thick outer shell of beige and tan material with some areas of gray crystalline mineral imbedded in the outer material.
PHYSICAL CHARACTERISTICS:
Color is off-white, yellow, or shades of red, orange to brown.
Luster is vitreous to dull if weathered.
Transparency crystals are usually opaque, may be translucent or rarely transparent.
Crystal System is monoclinic; 2/m
Crystal Habits include blocky or tabular crystals. Crystals have a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. Twinning is common. (see above). A psuedo-orthorhombic or psuedo-trigonal variety, found in alpine veins is called adularia, and forms more flattened tabular crystals.
Cleavage is good in 2 directions forming nearly right angled prisms.
Fracture is conchoidal or uneven
Hardness is 6
Specific Gravity is approximately 2.53 - 2.56 (average)
Streak is white.
Associated Minerals are quartz, plagioclase feldspars, micas, garnets, tourmalines and topaz.
Other Characteristics: some crystals may show opalescence and are called moonstone.
Best Field Indicators color, lack of striations, cleavage, twinning if present and occurrence.
Fracture: Conchoidal
Hardness: 6 Orthoclase
Luster: Vitreous
Occurrence: Orthoclase forms at intermediate temperatures between the stability fields of sanidine and microcline. At 400 degrees C or less, microcline is the stable structure for KAlSi3O8. Between approximately 500 degrees C and 900 degrees C, orthoclase is the stable structure. And above approximately 900 degrees C, sanidine is the stable structure. The difference between the structures is only in the randomness of the aluminum and silicon atoms. In microcline the ions are ordered, and this produces the lower symmetry of triclinic (yes, more order produces lower symmetry, see discussion in symmetry). With higher temperatures the positions of the aluminums and silicons become more disordered and produce the monoclinic symmetry of orthoclase and finally, sanidine.
Rock Type: Igneous
Specific Gravity: 2.53 - 2.5
Streak: white
Variety: Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.
Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8). While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.
The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia (from Adular) is found in low temperature hydrothermal deposits. The largest documented single crystal of othoclase was found in Ural mountains, Russia. It measured ~10x10x0.4 m3 and weighed ~100 tons.[4]Collection
RAGM Mineral CollectionAcquisition
Accession
2009.62Source or Donor
Museum Collection of MineralsAcquisition Method
DonationLocation
Location
Display Case
FS-3Room
Frieda Smith HallBuilding
Crater Rock MuseumDate
January 5, 2024Location
* Untyped Location
Needs Updated Location - 2022Category
PermanentMoved By
Scott LonganDate
September 26, 2009Location
Container
RightDrawer
3rd ShelfShelf
RightCabinet
Case #39Wall
West wallRoom
Delmar Smith HallBuilding
Crater Rock MuseumCategory
Permanent