Orthoclase

Object/Artifact

-

Crater Rock Museum

Name/Title

Orthoclase

Entry/Object ID

2009.62.136

Description

Chemical Composition: Potasium Aluminum Silicat Crystal System: Monoclinic System Description: Common Name: Orthoclase Chemistry: KAlSi3O8, Potasium Aluminum Silicate. Group: Feldspars Location: Chihuahua, Mexico Description: A tight grouping of cubic, irredescent crystals, tan and brown, with a powder blue refractive color on a reddish-brown rocky matrix. 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: uneven 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. Rock Type: Igneous Specific Gravity: 2.53 - 2.5 Streak: white

Collection

RAGM Mineral Collection

Acquisition

Accession

2009.62

Source or Donor

Museum Collection of Minerals

Acquisition Method

Donation

Dimensions

Width

2-5/8 in

Depth

2-1/4 in

Length

3-3/8 in

Weight

0.59 oz

Location

Location

* Untyped Location

RD-5

Category

Permanent

Moved By

Scott Longan

Date

September 26, 2009

Location

Container

Left

Drawer

3rd Shelf

Shelf

Right

Cabinet

Case #39

Wall

West wall

Room

Delmar Smith Hall

Building

Crater Rock Museum

Category

Permanent