Selenite

Object/Artifact

-

Crater Rock Museum

Selenite (Gypsum)

Selenite (Gypsum)

Name/Title

Selenite

Entry/Object ID

78.63.273 A&B

Description

Chemical Composition: CaSO4-2H2O Crystal System: Monoclinic System Description: Common Name: Selenite (Gypsum) Group Name: Sulfates Chemistry: CaSO4-2H2O Location: Oklahoma Description: Two crystal clusters of "Hair glass Selenite" (Gypsum) from Oklahoma. Colors: One is tan and one is grey and white. Both have thin bladed crystals with textured surfaces. Pieces vary in size. Physical Characteristics: Composition Hydrous calcium sulfate Color Colorless, white, gray, brown, beige, orange, pink, yellow, light red, green Streak White Hardness 2 Crystal System Monoclinic 3D Crystal Atlas (Click for animated model) Monoclinic Tabular Contact Swallowtail Twin Complex Tabular Penetration Twin Crystal Forms and Aggregates Commonly occurs as tabular crystals, sometime perfect with no imperfections. Also occurs prismatic, acicular, bladed, and as dense bundles of fragile acicular crystals. Other forms are as fibrous veins, scaly, grainy, lenticular, rosette, massive, and as parallel, cactus-like growths. Crystals and fibrous masses may be curved, sometimes severely, forming formations that are sometimes called "Rams Horns". Crystals frequently twin, forming perfect fishtail twins or swallowtail twins. Crystals can be enormous in size. In fact, the largest crystals ever found on earth were of Gypsum. Transparency Transparent to opaque Specific Gravity 2.3 - 2.4 Luster Vitreous to pearly Cleavage 1,1 - micaceous ; 2,2 Fracture Uneven Tenacity Sectile and slightly flexible Other ID Marks Often fluorescent light yellow in shortwave ultraviolet light, and occasionally also phosphorescent. In Group Sulfates; Hydrous Sulfates Striking Features Crystal habits, low hardness, flexibility, and sectility Environment In enormous deposits and beds in sedimentary rock, specifically limestone. Also in clay sedimentary deposits, and in dry caves. Occasionally also in igneous traprock and in the oxidation zone of sulfide deposits. Rock Type Igneous, Sedimentary Popularity (1-4) 1 Prevalence (1-3) 1 Demand (1-3) 1 Gypsum ON - See more at: http://www.minerals.net/mineral/gypsum.aspx#sthash.NhsKgv5t.dpuf Fracture: Uneven Hardness: 2 Gypsum Luster: Vitreous Occurrence: Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water. Gypsum has several variety names that are widely used in the mineral trade. •"Selenite" is the colorless and transparent variety that shows a pearl like luster and has been described as having a moon like glow. The word selenite comes from the greek for Moon and means moon rock. •Another variety is a compact fiberous aggregate called "satin spar" . This variety has a very satin like look that gives a play of light up and down the fiberous crystals. •A fine grained massive material is called "alabaster" and is an ornamental stone used in fine carvings for centuries, even eons. Crystals of gypsum can be extremely colorless and transparent, making a strong contrast to the most common usage in drywall. The crystals can also be quite large. Gypsum is a natural insulator, feeling warm to the touch when compared to a more ordinary rock or quartz crystal. Sheets of clear crystals can be easily peeled from a a larger specimen. Gypsum crystals can be extremely large - among the largest on the entire planet. A cave in Naica, Mexico contains crystals that dwarf the people inside. Apparently, ideal conditions for the slow growth of gypsum were maintained for thousands of years, allowing a few crystals to grow to enormous sizes. Click on the photos for larger images, and see this abstract for an article in the April 2007 Geology magazine that describes how the growth of these gypsum megacrystals occurred. Plaster of Paris is made by heating gypsum to about 300 degrees Fahrenheit, driving 75% of the water out of the mineral. This reaction absorbs energy, enabling a sheet of drywall to resist fire for a while. Heating further to about 350 degrees F drives out the remaining water and results in conversion to the mineral anhydrite. - See more at: http://www.galleries.com/Gypsum#sthash.llQy32ax.dpuf Rock Type: Sedimentary Specific Gravity: 2.3-2.7 Streak: white

Collection

Delmar Smith Crystal Collection

Acquisition

Accession

78.63.273

Dimensions

Width

3-1/2 in

Depth

1/2 in

Length

2-1/4 in

Dimension Notes

Dimension taken at widest points

Location

Location

Shelf

CS-E-1

Room

Curation Storage

Building

Crater Rock Museum

Category

Storage

Moved By

Curator

Date

February 23, 2024