Calcite / Fluorite

Object/Artifact

-

Crater Rock Museum

Name/Title

Calcite / Fluorite

Entry/Object ID

2009.62.86

Description

Chemical Composition: CaCO3/ CaF2 Crystal System: Tetragonal System Description: Common Name: Calcite / Fluorite Chemistry: CaCO3/ CaF2 Group: Carbonates / Halides Location: Hardin County, Illinois Description: Creamy yellowish calcite surrounding some small bluish-purplish fluorite crystals Physical Description of Calcite: Color is extremely variable but generally white or colorless or with light shades of yellow, orange, blue, pink, red, brown, green, black and gray. Occasionally iridescent. Luster is vitreous to resinous to dull in massive forms. Transparency: Crystals are transparent to translucent. Crystal System is trigonal; bar 3 2/m Crystal Habits are extremely variable with almost any trigonal form possible. Common among calcite crystals are the scalenohedron, rhombohedron, hexagonal prism, and pinacoid. Combinations of these and over three hundred other forms can make a multitude of crystal shapes, but always trigonal or pseudo-hexagonal. Twinning is often seen and results in crystals with blocky chevrons, right angled prisms, heart shapes or dipyramidal shapes. A notch in the middle of a doubly terminated scalenohedron is a sure sign of a twinned crystal. lamellar twinning also seen resulting in striated cleavage surfaces. Pseudomorphs after many minerals are known, but easily identified as calcite. Also massive, fibrous, concretionary, stalactitic, nodular, oolitic, stellate, dendritic, granular, layered, etc. etc. Cleavage is perfect in three directions, forming rhombohedrons. Fracture is conchoidal. Hardness is 3 (only on the basal pinacoidal faces, calcite has a hardness of less than 2.5 and can be scratched by a fingernail). Specific Gravity is approximately 2.7 (average) Streak is white. Physical Description of Fluorite: Color is extremely variable and many times can be an intense purple, blue, green or yellow; also colorless, reddish orange, pink, white and brown. A single crystal can be multi-colored. Luster is vitreous. Transparency: Crystals are transparent to translucent. Crystal System: Isometric; 4/m bar 3 2/m Crystal Habits include the typical cube and to a lesser extent, the octahedron as well as combinations of these two and other rarer isometric habits. Always with equant crystals; less common are crusts and botryoidal forms. Twinning also produces penetration twins that look like two cubes grown together. Cleavage is perfect in 4 directions forming octahedrons. Fracture is irregular and brittle. Hardness is 4 Specific Gravity is 3.1+ (average) Streak is white. Other Characteristics: Often fluorescent blue or more rarely green, white, red or violet and may be thermoluminescent, phosphorescent and triboluminescent. Associated Minerals are many and include calcite, quartz, willemite, barite, witherite, apatite, chalcopyrite, galena, sphalerite, pyrite and other sulfides. Best Field Indicators are crystal habit, color zoning, hardness (harder than calcite, but softer than quartz or apatite), fluorescence and especially the octahedral cleavage. Fracture: brittle Hardness: 3 Calcite Luster: Vitreous Occurrence: How did Illinois' fluorite deposits form? Hot water containing fluorine and other dissolved chemicals rose from deep in the earth during the Jurassic Period, about 150 to 200 million years ago. The water flowed through northeast-trending faults and fractures in limestones laid down earlier in the Mississippian Period, about 330 million years ago. When the hot brines reached the calcium-rich Mississippian rocks, the temperature and other conditions were just right for crystallizing fluorite along the walls of the faults and in flat-lying layers parallel to the beds of limestone. These host rocks dissolved and were replaced with the fluorite. This makes the combination of calcite with fluorite fairly common occurance in illinois. This calcite formed from oolitic, fossiliferous and massive limestones in sedimentary environments. Specific Gravity: N/A Streak: white Variety: Calcite is one of the most common minerals on the face of the Earth, comprising about 4% by weight of the Earth's crust and is formed in many different geological environments. Calcite can form literally a thousand different shapes by combining the basic forms of the positive rhombohedron, negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid to name a few of the more common forms. There are more than 300 crystal forms identified in calcite and these forms can combine to produce the thousand different crystal variations. Calcite also produces many twin varieties that are favorites among twin collectors. There are also phantoms, included crystals, color varieties, pseudomorphs and unique associations. There simply is no end to the varieties of calcite. Fluorite is a widely occurring mineral which is found in large deposits in many areas and comes in a wide variety of colors and is offen refer to as the worlds most colorful mineral.

Collection

RAGM Mineral Collection

Acquisition

Accession

2009.62

Source or Donor

Museum Collection of Minerals

Acquisition Method

Donation

Dimensions

Width

3 in

Depth

3-1/2 in

Length

5-1/2 in

Weight

0.46 oz

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