Orthoclase

Object/Artifact

-

Crater Rock Museum

Orthoclase

Orthoclase

Name/Title

Orthoclase

Entry/Object ID

2009.62.130

Description

Chemical Composition: Potassium aluminum silica Crystal System: Monoclinic System Description: Common Name: Orthoclase Chemistry: KAlSi3O8, Potasium Aluminum Silicate. Group: Feldspars Location: Colorado Description: Course grained, cylindrical crystal affixed to a rocky, granular matrix, both tan to light brown in color with small amounts of refractive crystals. 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 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 Rock Type: Sedimentary Specific Gravity: 2.5 Streak: white

Collection

RAGM Mineral Collection

Acquisition

Accession

2009.62

Source or Donor

Museum Collection of Minerals

Acquisition Method

Donation

Dimensions

Width

2 in

Depth

1-1/2 in

Length

2-1/4 in

Weight

0.15 oz

Location

Location

Shelf

CS-E-1

Room

Curation Storage

Building

Crater Rock Museum

Category

Storage

Moved By

Curator

Date

February 23, 2024