Selenite

Object/Artifact

-

Crater Rock Museum

Name/Title

Selenite

Entry/Object ID

78.63.348

Description

Chemical Composition: CaSO4-2(H2O), Crystal System: Monoclinic System Description: Common Name: Gypsum Group Name: Sulfates Location: Sicily, Italy Chemistry: CaSO4-2(H2O), Descriptionh: Delmar described this specimen as one large gypsum crystal with sulfur. Crystals on it. A large clear, like glass, piece with yellow sulfur crystals. Has a kined of fish shape with yellow over nose and tail. PHYSICAL CHARACTERISTICS: Color is usually white, colorless or gray, but can also be shades of red, brown and yellow. Luster is vitreous to pearly especially on cleavage surfaces. Transparency crystals are transparent to translucent. Crystal System is monoclinic; 2/m Crystal Habits include the tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces dominate with jutting prism faces on the edges of the tabular crystals. Long thin crystals show bends and some specimens bend into spirals called "Ram's Horn Selenite" Two types of twinning are common and one produces a "spear head twin" or "swallowtail twin" while the other type produces a "fishtail twin". Also massive, crusty, granular, earthy and fiberous. Cleavage is good in one direction and distinct in two others.. Fracture is uneven but rarely seen. Hardness is 2 and can be scratched by a fingernail. Specific Gravity is approximately 2.3+ (light) Streak is white. Associated Minerals are halite, calcite, sulfur, pyrite, borax and many others. Other Characteristics: thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Also some samples are fluorescent. Gypsum has a very low thermal conductivity (hence it's use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz. Notable Occurances include Naica, Mexico; Sicily; Utah and Colorado, USA; and many other locallities throughout the world. Best Field Indicators are crystal habit, flexible crystals, cleavage and hardness. Fracture: uneven Hardness: 2 Gypsum Luster: Vitreous Occurrence: "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 Rock Type: Sedimentary Specific Gravity: 2.3 Streak: white Variety: Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur silky and fibrous, in which case it is commonly called satin spar. Finally it may also be granular or quite compact.

Collection

Delmar Smith Mineral Collection

Acquisition

Accession

78.63

Source or Donor

Delmar Smith Crystal Collection

Acquisition Method

Donation

Dimensions

Height

3 in

Width

10 in

Length

1 in

Dimension Notes

Dimension taken at widest points

Location

Location

Display Case

FS-9

Room

Frieda Smith Hall

Building

Crater Rock Museum

Category

Permanent

Moved By

Ian Cunningham

Date

December 27, 2023