• Abstract and Figures
• Public Full-text

Nbsnadq1//6Lhmdq`knesgdLnmsg9Ektnqhsd Certainly a favorite among collectors for its fascinating forms, colors, and fluorescence, fluorite has graced the cover of The Mineralogical Record no less than eight times! OGXRHB@K OQNODQSHDR Chemistry: CaF2 Calcium Fluoride Class: Halides Group: Fluorite Crystal System: Isometric (Cubic) Crystal Habits: Usually cubic or as penetration twins; less frequently octahedral and rarely dodecahedral; also granular, columnar, earthy, and as cleavage masses. Color: White, colorless, violet, purple, blue, green, yellow, brown, amber, bluish-black, pink, and rose-red. Luster: Vitreous Transparency: Transparent to translucent Streak: White Cleavage: Perfect in four directions, forming an octahedron Fracture: Uneven, brittle Hardness: 4.0 Specific Gravity: 3.0-3.2 Luminescence: Often fluorescent and phosphorescent, sometimes thermoluminescent, and triboluminescent (see “The Many Types of Luminescence”). Refractive Index: 1.433 Distinctive Features and Tests: Best field indicators are cubic crystal form, excellent crystal development, perfect four-directional cleavage, and relative softness. Dana Classification Number: 9.2.1.1 M @L D The name of this month’s mineral, correctly pronounced FLOR-ite, is derived from the Latin fluere, “to flow,” a reference to its ability to lower the melting temperature of metals in smelting processes. Because of its long use as a decorative stone and as a flux, fluorite has many alternative names, including “androdamant,” “bruiachite,” “Derbyshire spar,” “fluor,” “fluores,” “fluoride of calcium,” “fluoride of lime,” “fluorspar,” “fluorspath,” “flusse,” “flusspat,” “liparite,” “murrhina,” and “spath vitreux.” European mineralogists refer to fluorite as “fluorit,” “fluorita,” and “fluorin.” Fluorite also has several variety names based on color and other physical properties. “False emerald” is green, “false amethyst” is purple, “false ruby” is red, “fluorite rose” is pink, “rainbow fluorite” is multi- colored, and “blue john” has alternating bands of yellow, blue, and purple. “Chlorophane” is a thermoluminescent variety. BNL ONRHSHNM Fluorite now joins the honor roll of minerals that we have featured three times, along with pyrite and apatite. In our fourth month of operation, June 1996, we sent eager Club members purple fluorite cubes from the new find at the Elmwood mine, Carthage, Smith County, Tennessee. This mine is no longer producing, and you can be sure that the specimens we sent that month have greatly increased in value–wish we had obtained several flats for ourselves! In March 2002, fluorite became the first mineral to be featured twice, as we sent Club members the highly fluorescent, green fluorite from the Rogerley mine, Wearsdale, Durham County, England, our first and only featured mineral from England, so far. Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 1 Nbsnadq1//6Lhmdq`knesgdLnmsg9Ektnqhsd Fluorite’s chemical formula CaF2 identifies its elemental constituents as calcium (Ca) and fluorine (F). Calcium contributes 51.33 percent of fluorite’s molecular weight; fluorine contributes 48.67 percent. Fluorite is a simple compound mineral, consisting only of a single cation (positively charged ion) and a single anion (negatively charged ion). Within the fluorite molecule, the +2 charge of the calcium cation (Ca2+) balances the collective -2 charge of the double fluorine anion (2F1-) to provide electrical stability. Fluorite is a member of the halides, a group of soft, brittle minerals in which a halogen element is the only anion. The halogen elements fall under Group VII A of the periodic table and include, in order of chemical activity, fluorine, chlorine, bromine, iodine, and astatine. The fluorite molecule and lattice are held together primarily by ionic bonding, the attractive force that joins ions of distinctly metallic elements with those of distinctly nonmetallic elements. Ionic bonding is explained by electronic configuration, that is, the distribution of electrons within atoms that determines which atoms can react with others to form compounds. Electrons occupy space around the atomic nucleus in well- defined energy levels or “shells,” with specific shells accommodating specific numbers of electrons. Elements are stable when their outer shells contain eight electrons (called a “stable octet”). Those with incomplete outer shells can react with other elements by gaining, losing, or sharing electrons to fill their outer shells and thus complete the stable octet. The calcium atom has 20 electrons in four shells—2 in the inner shell, 8 in the second shell, 8 in the third shell, and 2 in the outer shell. It achieves stability by losing its two outer electrons, thus making the third shell with its eight electrons the outer shell. The loss of these two electrons creates a positively charged ion with a +2 charge (Ca2+). Fluorine has only nine electrons in two shells—two in its inner shell and seven in its outer shell. To achieve stability, it borrows a single electron to fill its outer shell, thus creating a negatively charged ion with a -1 charge (F1-). When a calcium atom contacts two fluorine atoms, the fluorine atoms borrow the two outer calcium electrons, transforming both the calcium atom and the two fluorine atoms into oppositely charged ions. Opposite charges attract, forming the ionic bond that joins the calcium and fluorine ions together in the stable fluorite molecule. In the fluorite lattice, eight fluorine ions surround each positively charged calcium ion, while four calcium ions surround each negatively charged fluorine ion. This forms a cubic structure with identical ions at the corners of the cubes and at the center of the six faces. The alternating rows of calcium ions and fluorine ions are arranged in four axial directions and held together mainly by ionic bonding with only a small degree of covalent bonding. Because ionic bonding is weak, fluorite exhibits perfect four-directional cleavage and is relatively soft at Mohs 4.0. It is interesting to compare fluorite with halite [sodium chloride, NaCl], our Mineral of the Month for July 2006. Both minerals crystallize in the isometric system, but different lattice arrangements give halite perfect three-directional cleavage and fluorite perfect four-directional cleavage. Because fluorite has two fluoride anions as opposed to halite’s single chlorine anion, it has stronger overall ionic bonding. Accordingly, fluorite is much less soluble in water and considerably harder than halite (Mohs 2.0). One of fluorite’s most fascinating properties is its broad range of colors, which can be pale and subtle, or so intense as to be nearly opaque. As an allochromatic or “other-colored” mineral, fluorite’s colors are not caused by essential elemental components or the nature of its crystal structure, but by traces of nonessential elements or lattice defects. When pure or nearly pure, fluorite is colorless. But most fluorite contains, among other elements, small amounts of iron and the rare-earth elements yttrium and cerium, which substitute for calcium in the crystal lattice. These impurities, called chromophores (color-causing agents), modify the manner in which the fluorite lattice absorbs and reflects the various wavelengths of white light. Iron imparts green and yellow colors to fluorite, while traces of yttrium and cerium impart pink and rose colors. Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 2 Nbsnadq1//6Lhmdq`knesgdLnmsg9Ektnqhsd Fluorite colors also have other causes, such as lattice defects called “color centers,” which are created either by abnormal crystal growth or exposure to natural geophysical radiation. In fluorite, color centers form when fluorine ions are displaced from their normal lattice positions to create voids that fill with electrons. These “trapped” electrons absorb varying amounts of white-light energy that boost them to higher energy levels. To return to their normal levels, they release this excess energy in the form of visible light of specific wavelengths. The energy emitted by fluorite color centers is usually purple or violet. Yet another color-causing mechanism in fluorite that is still being studied is the presence of “loose” or unbound fluorine ions within the lattice. In some specimens, color is determined by a combination of the effects of chromophoric substitution, color centers, and unbound fluorine ions. Finally, fluorite often exhibits luminescent effects, as explained in the box below. THE MANY TYPES OF LUMINESCENCE Fluorite is a classic example of a luminescent mineral. The general term “luminescence” includes such effects as fluorescence, phosphorescence, thermoluminescence, and triboluminescence. Fluorite, perhaps because its crystal lattice often contains unbound fluorine ions, is one of the few minerals that can—but does not always—exhibit these four types of luminescence. Luminescence must not be confused with the visible light emitted by materials when they undergo combustion or are heated nearly to their melting points (as in the tungsten filaments of incandescent light bulbs). Nor does luminescence rely on the reflection, transmission, or refraction of visible light. Luminescence occurs at ambient temperatures when certain minerals absorb electromagnetic, mechanical, thermal, and radiation

Load more

  10

Copy Link