This semi-transparent piece of fluorite has a deep purple color with a glassy surface, as shown in the lower right corner of the first image. The photo highlights the stepped growth of the cubic crystals.
Approximate Photo Location (Side 1)
Field of view: ~7/16” x 5/8“ (11.0mm x 16.5mm)
Images in focus stack: 109
The image below shows the impact of the lighting on the piece. On the left is the view through the camera’s viewfinder, and on the right a single frame as captured by camera. The flash was positioned so that most of the light reflected off the face, emphasizing the surface details/depth at the cost of reproducing the color.
Fluorite is composed of calcium and fluorine (CaF2)(1, 2). It is a relatively soft material that ranges from transparent to translucent and is found in purple, green, yellow, blue, pink, colorless, and other varieties (1, 2, 3, 4)(i, ii). In some cases it also glows blue-violet, pale yellow, or other colors under ultraviolet light (2, 4, 5, 6)(iii). This is typically caused by impurities of rare earth elements (6). Electrons in these elements are excited to a higher energy level when exposed to UV light, and emit photons/glow as they fall back to their ground state (7). The word “fluorescence” was coined after this phenomenon in the mid-19th century by George Gabriel Stokes (2, 5). Some fluorite is also phosphorescent, thermoluminescent, or triboluminescent (4, 6, 8)(iv). Industrially, fluorspar (concentrates of fluorite) is predominantly used in the manufacture of hydrofluoric acid, which is then used in a wide range of chemicals like refrigerants, non-stick coatings (e.g., Teflon), anesthetics, etc (8, 9)(v). Fluorspar is also commonly used in steel production, where it serves as a flux that lowers the melting point of the iron ore, helps remove impurities, and improves the fluidity of the slag (5, 8, 10). (Slag is a by-product that is removed when the iron is separated from the ore.) The use of fluorspar as a flux was known and recorded in the 16th century by Georgius Agricola (see the illustration below)(9, 11, 12). He called the mineral “fluores,” as derived from the Latin word “fluere,” which means to flow (2, 4, 9, 11, 12). By the mid- to late 18th century, scientists had crudely produced hydrofluoric acid (10, 11, 12)(vi). Isolating fluorine, however, proved to be a difficult and dangerous task and a number of scientists (a.k.a. the “fluorine martyrs”) were injured and even killed by their experiments (10, 11, 12, 13)(vii). The feat was finally accomplished by Henri Moissan in 1886 (11, 12, 14). Fluorine is a pale greenish-yellow gas at room temperature (11, 12). It is the most reactive element on the periodic table and very rarely exists in nature in free form (i.e., not as a compound)(10, 11, 14, 15). When in a jet/thin stream, the gas can apparently ignite organic materials (like water) on contact without a spark (11, 15). It is also toxic (11, 15). While fluorine is highly reactive, fluorine compounds like carbon-fluorine are highly stable (11, 15).
Illustration of an iron foundry, where German miners used fluxes as part of the smelting process (11). From G. Agricola’s De Re Metallica, first published in 1556. Image courtesy of The Linda Hall Library of Science, Engineering & Technology (p. 315)(CC BY-NC-SA 4.0 (Edits: rotation/cropping, levels/saturation, sizing)). For an English translation of the text and illustration, see p. 386 of the Project Gutenberg eBook here. A footnote referring to fluorspar is here.
i. At ~150 on the Knoop scale, fluorite has roughly the same hardness as copper (16). It is also the representative mineral for the level 4 hardness on the Mohs scale (16).
ii. Pure fluorite is colorless and rarely occurs in nature (1, 2, 3, 8).
iii. It is estimated that 15% of the ~5,000 known minerals fluoresce under UV light (7).
iv. In phosphorescence, the emission of light is on a longer timescale (milliseconds to seconds) than fluorescence (nanoseconds), so you can likely see the specimen continue to glow after the energy is turned off or removed (8, 17). Thermoluminescence is the emission of light from heating (8, 18). Triboluminescence is the emission of light due to mechanical energy breaking the chemical bonds within the substance (8, 18). You might see a light/glow when crushing, scratching, or hitting a triboluminescent specimen (18).
v. In the past, one of the main uses of hydrofluoric acid was the production of chlorofluorocarbons (CFCs)(8, 9). Due to their destructive impact on the ozone layer, non-essential CFCs were phased out in the late 1980s/early 1990s through the Montreal Protocol global agreement, and global production of fluorspar consequently dropped by nearly a third (8, 9, 10, 14). Global production of fluorspar returned to pre-ban levels by 2009 (8).
vi. In the 1670s, Heinrich Schwanhard reportedly treated fluorspar with sulfuric (or another) acid and produced a substance that could etch glass (11, 12). A century later, Carl Wilhelm Scheele investigated this process and concluded that sulfuric acid liberated another acid from fluorspar, which he named fluoric acid (i.e., hydrofluoric acid)(10, 11, 12). Scheele’s premature death (at the age of 43) may well have been caused by his habit of tasting his preparations (which also included other toxic gases)(11). Other scientists whose work contributed to the eventual isolation of fluorine included André-Marie Ampère, Sir Humphry Davy, Thomas and George Knox, Edmond Frémy, and George Gore (11, 12).
vii. Paulin Louyet and Jerome Nickles died from their experiments with fluorine, likely due to inhaling hydrogen fluoride (11, 12, 13). Other scientists injured during this period included Sir Humphry Davy, Thomas and George Knox, Louis-Joseph Gay Lussac, and Louis-Jacques Thenard (10, 12). Davy injured his eyes and fingernails, while the Knox brothers suffered hydrofluoric acid poisoning (from which Thomas Knox apparently nearly died)(10, 12). A student of Moissan’s, Alfred Stock, wrote that Moissan had told him that his life had been shortened by ten years because of his work on fluorine (10). Moissan died of acute appendicitis in 1907, less than three months after being awarded the Nobel Prize for isolating the element (11, 12).
1. D.L. Reinertsen, D. L., & Masters, J. M. (n.d.). Fluorite: Illinois’ State mineral. Retrieved from the Illinois State Geological Survey.
2. The University of Minnesota. (n.d.). Fluorite. Retrieved from the U of M Department of Geology.
3. Jones, B. (n.d.). Introduction to fluorite. Retrieved from the Tidewater Gem & Mineral Society.
4. Anthony, J.W., Bideaux, R. A., Bladh, K. W., & Nichols, M.C. (Eds.). (2001). Fluorite. In The Handbook of Mineralogy. Retrieved from the Mineralogical Society of America.
5. King, H. M. (n.d.). Fluorite (also known as fluorspar). Retrieved from Geology.com.
6. Barmarin, G. (n.d.). Fluorite. Retrieved from the Database of Luminescent Minerals.
7. Verbeek, E. R. (n.d.). Why minerals fluoresce. Retrieved from the Sterling Hill Mining Museum.
8. Bide, T., Gunn, G., Brown, T., & Rayner, D. (2011). Fluorspar. Retrieved from the British Geological Survey.
9. US Geological Survey (USGS). (2010). Mineral resource of the month: Fluorspar. Retrieved from the USGS.
10. Wisniak, J. (2002). The history of fluorine: From discovery to commodity. Indian Journal of Chemical Technology, 9(4), pp.153-173. Retrieved from ResearchGate.
11. Tressaud, A. (2019). Fluorine: A paradoxical element [Chapter 1]. Amsterdam: Elsevier. Retrieved from Google Books.
12. Toon, R. (2011). The discovery of fluorine. Retrieved from the Royal Society of Chemistry.
13. Princeton University Environmental Health & Safety, & American Chemical Society (ACS) Division of Chemical Health and Safety. (2019). Periodic table of the elements of safety [safety martyrs]. Retrieved from the ACS Division of Chemical Health and Safety.
14. Riedel, S., & Pröhm, P. (2019, June 9). Fluorine: The most reactive and indispensable element in our daily lives. Retrieved from De Gruyter Conversations.
15. Fluorine. (n.d.). Retrieved from ChemEurope.
16. Ted Pella, Inc. (n.d.). Hardness tables. Retrieved from Ted Pella.
17. Olympus. (n.d.). Confocal microscopy: Glossary of terms in confocal microscopy [Fluorescence, luminescence, phosphorescence]. Retrieved from Olympus.
18. Conservation and Art Materials Encyclopedia Online (CAMEO). (2018). Fluorescent minerals. Retrieved from CAMEO–Museum of Fine Arts Boston.
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