Welcome

The non-profit, Friends of Mineralogy (FM), a national organization founded in 1970, includes nearly a dozen chapters from coast to coast, whose members share a common love of minerals. FM’a objective is to promote, support, protect, and expand the collecting of mineral specimens, while furthering the recognition of the scientific, economic, and aesthetic value of minerals and mineral collecting. Membership includes collectors, museum curators, mineralogists, and earth science educators.  The organization is affiliated with Mindat.org, the Mineralogical Society of America (MSA), and the Mineralogical Association of Canada (MAC).

Among its many activities, FM regional chapters sponsor symposiums, and collecting trips to quarries, mine dumps, and mines across the country.  FM has made its voice heard whenever proposals are made to close mineral museums or collecting sites.  National FM co-sponsors symposiums held annually at the Tucson Gem & Mineral Show, and contributes support and speakers to other mineral symposiums.  FM presents annual awards for best article published in each of the following publications: The Mineralogical RecordRocks & MineralsMineral News, and Mineral Monographs.  It also gives awards to the best institutional and individual educational exhibit cases displayed at the Denver and Tucson gem and mineral shows.


Please send tax-deductible donations to support Friends of Mineralogy, a non-profit 501(c)3 organization, in care of Bruce W. Bridenbecker, Treasurer, 7528 Lucerne Vista Ave, Yucca Valley, CA 92284, made out to Friends of Mineralogy.

INSTAGRAM

  • by friendsofmineralogy 3 days ago
    Black Friday - that traditional day of mass shopping following Thanksgiving Day in the U.S.A. - involves a web of processes and technologies that depend on geology. But let’s look at just one big piece that makes the whole thing work: the credit card. Ever wonder what is actually inside these ubiquitous little marvels of technology? The body of the card is a plastic laminate, with name and number details mechanically imprinted. Lamination increases durability and allows for a wide variety of designs and coatings. The first credit cards, without the whiz-bang technology they all have now, used carbon paper copies in manual mechanical imprinting machines at the point of sale. These are still around for use when more analog methods are needed. The next big innovation in credit card technology was the magnetic strip used when you "swipe" your card. The strip is a magnetic tape laminated to the card and embedded with iron oxides derived from minerals like hematite that can be magnetically coded with the card information. While many cards still retain the magnetic strip for backwards compatibility, they are being phased out in favor of EMV chip technology. Chip cards have sophisticated microprocessors that are much more capable and secure. The chips start as boules of almost pure silicon derived from quartz sands, cut thinly and carefully to chip size. Because silicon on its own in nonconductive at room temperature, small quantities of specific "donor" atoms are added, such as boron and phosphorus, obtained from minerals like borax and phosphate rock. These donor atoms supply electrons that can then move through the silicon crystal lattice and make a semiconductive material. Transistors are built onto the doped layers of the chip. Conductive contact pads made of precious metals like gold and silver are arrayed on top. Many of the newest cards also have tiny antennae to enable contactless transactions. All of this is constructed on microscopic scales in sophisticated clean rooms.  #science   #blackfriday   #minerals   #geologyrocks   #resources   #technology  FM graphic: Erin Delventhal
  • by friendsofmineralogy 5 days ago
    Collector specimens of millerite are full of personality, often fuzzy-looking rays or clusters of dark or brassy metallic fibers like this one from Halls Gap, Lincoln Co., Kentucky, USA. Millerite is nickel sulfide, and can be an important ore of nickel when found in high enough concentrations. Halls Gap is a well-known locality for millerites in quartz-lined geodes and pockets, but is reportedly closed or cleaned out these days. Erin Delventhal collection and photo. ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!  #minerals   #crystals   #sulfides   #millerite   #science   #mineralcollecting   #geologyrocks 
  • by friendsofmineralogy 1 week ago
    Birefringence is defined as the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. In plain terms, this means that a ray of light will be split by polarization into two paths. This often causes what is called "double refraction" when two images can be seen where once was one. This property can be useful in identifying minerals, because not all minerals are birefringent, and the different minerals have different refractive indices depending on their crystal structure. This piece of polished optical calcite shows a clear example of this property. Although "birefringence" is written three times on the paper underneath, the word shows six times through the crystal. As the crystal is rotated, the words also appear to rotate as the angle of polarization changes relative to the eye. Birefringence is a very useful property. You're most likely reading this text right now through the polarized light of a flat panel liquid crystal display. It is even used in medical diagnostics. For example, gout patients' painful joints are caused by accumulated crystals in their joint fluids. The birefringent properties of these crystals helps in diagnosis, because monosodicum urate crystals are negatively birefringent, calcium pyrophosphate crystals shows weak positive birefringence, and the colors of these crystals will change depending on the filters used.  #minerals   #crystals   #science   #birefringence   #calcite   #mineralcollecting   #geologyrocks 
  • by friendsofmineralogy 2 weeks ago
    Looking for a great introduction to studying mineralogy? One that we recommend is "Mineralogy for Amateurs" by John Sinkankas. Sinkankas covers subjects ranging from the history of mineralogy, an overview of atoms and how they arrange within (and thus affect) minerals, and classification schemes of minerals to crystallography and crystal growth, the physical and optical properties of minerals, formation and association of minerals, and identification procedures and tests. The latter portion of the book is a section of descriptive mineralogy, listing properties and information about a wide range of mineral species you might encounter. If all of that sounds like it might be too much and you're a little intimidated - it's okay! Sinkankas' writing style takes even the toughest concepts and explains them in a way that is approachable to everyone - this is the go-to starter guide for anyone looking to learn something more about the fascinating world of minerals. Pro tips: this same book was also published with a different cover simply as "Mineralogy." The social media team went to great lengths to compare our various different-looking copies to verify they contained the same material! Because it is long out of print, online sellers may charge quite a bit for any of these editions, however it can frequently be found for much more reasonable prices at neat old used bookstores and mineral shows, so keep an eye out! : Jessica Robertson  #minerals   #mineralogy   #books   #antiquebooks   #geologyrocks   #mineralcollecting 
  • by friendsofmineralogy 3 weeks ago
    Pyrrhotite is sometimes called magnetic pyrite, because it is weakly magnetic and has a pyrite-like brassy color. It's also an iron sulfide like pyrite, but the chemical formula is intriguingly different. Instead of simple FeS2 like pyrite, the formula is Fe(1-x)S : it's non-stoichiometric! If your chemistry is a little rusty, that means its elemental composition cannot be represented by a ratio of small natural numbers because a small percentage of atoms are missing, or too many atoms are packed into a the lattice work. There are multiple polytypes of pyrrhotite, including pyrrhotite-4C (Fe7S8), pyrrhotite-5C (Fe9S10), 6C (Fe11S12), 7C (Fe9S10) and 11C (Fe10S11) . The number and the "C" definine the size of the superlattice relative to the main symmetry axis of the crystal. Each polytype can have monoclinic (M) or hexagonal (H) symmetry. Several polytypes can exist within the same specimen. Pyrrhotite is often found alongside marcasite, pyrite, and magnetite. Although it does not have specific applications, it is mined in association with pentlandite, another sulfide mineral that can contain cobalt and nickel. Photo: Grouping of pyrrhotite crystals • Dalnegorsk, Primorsky Krai, Russia • 4.7cm • Erin Delventhal collection and photo ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!  #minerals   #crystals   #pyrrhotite   #sulfides   #mineralcollecting   #geologyrocks   #chemistry 
  • by friendsofmineralogy 3 weeks ago
    What is this pretty rock? A common question, but one that can lead to a lot of confusion over many of the sorts of rocks found in gift shops and in streambeds and roadsides, which are truly variations on the same thing: quartz! When a quartz crystal is that classic pointed hexagonal prism shape, it is obviously "macrocrystalline," a crystal large enough to be determined by the eye or a simple hand lens. (Varieties of large quartz crystals is a topic for another day.) "Druzy quartz" such as that found in the center of a geode is macrocrystalline, too; the little quartz points are easily determined with a close squint. A bit smaller is quartzite, sometimes called "sugar agate" regionally, in which all those 'sugary' grains are interlocking quartz grains. Every quartz with a grain size smaller than can be seen by eye or hand lens is "microcrystalline" or "cryptocrystalline." All microcrystalline quartz is chalcedony, but a piece might only be called "chalcedony" if it's a translucent sort of color, because there are hundreds if not thousands more subtype names of chalcedony, some well defined, others overlapping, still more having been given vague or misleading trade names. We've included some of the more common defined varieties in our graphic. "Agate" is chalcedony with translucent banding, and "onyx" is a dark and light banded agate. "Fire agate" has iridescent rainbow colors. Varieties can also be named based on inclusions, like chrysoprase, which is a milky green from small quantities of nickel, and pietersite, which has a sheen of blue and brown from embedded fibers of amphibole minerals. When the chalcedony is opaque, it is jasper, chert, and/or flint, depending on who is talking. The same piece of opaque chalcedony might rightfully be called any one of these three names, depending on region and geologic and geographic context. "Jasper" tends to be a gemological name for more lustrous and attractive material, while geologists tend to use the word "chert", and historians or archaeologists tend to use the word "flint." But it's all quartz, and it's beautiful.  #science   #quartz   #minerals   #geologyrocks   #crystals   #agate  FM graphic: Erin Delventhal
  • by friendsofmineralogy 4 weeks ago
    Greenockite (cadmium sulfide) doesn't often form large crystals; a few millimeters wide is about as big as they get. It can also occur as a sort of powdery yellow to red encrustation. While most cadmium is recovered as a byproduct of lead, zinc, and copper mining, greenockite is the the main ore mineral of cadmium. Cadmium, among many other applications, is used to make the yellow to red pigments such as "cadmium yellow," "cadmium ochre," and "cadmium red." Cadmium pigments were discovered around 1820 and first used commercially by artists’ by the mid 1840’s. It is cadmium that gives the warmth to Vincent Van Gogh's Sunflowers and Edvard Munch’s The Scream. Cadmium is one of those toxic elements that is regulated due to health and environmental concerns. You may remember when 12 million "Shrek" drinking glasses by McDonald's were recalled in 2010 due to relatively high cadmium concentrations in the paints. Although the concentration was not something that would cause immediate harm to anyone, it is chronic exposure that is the issue and that children's bodies take up cadmium more readily than adults. Because the glasses were targeted to young children who are much more likely to lick paint, new standards were adopted for allowable cadmium concentrations in children's jewelry and toys. This specimen of an isolated Greenockite crystal in prehnite is from Houdaille Quarry, Springfield, Union County, New Jersey, USA Photo: Rob Lavinsky & irocks.com ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!  #sulfides   #greenockite   #minerals   #crystals   #geologyrocks 
  • by friendsofmineralogy 4 weeks ago
    Happy Halloween from Friends of Mineralogy! Sphalerite slab under a dramatic cloudy sky. Boo! : Jessica Robertson Friends of Mineralogy graphic: Erin Delventhal
  • by friendsofmineralogy 4 weeks ago
    Marie Curie is the only person to have won two Nobel Prizes in different sciences: the Nobel Prize in Physics in 1903 and the Nobel Prize in Chemistry in 1911. These accomplishments in a period when women were almost non-existent in the sciences have given her legacy almost mythic status. A pioneer in the study of radioactivity, she, along with her husband Pierre and her assistant Andre Debierne, discovered and isolated polonium and radium through painstaking processes that took several years and multiple tons of pitchblende ore. These new substances were a wonder: they glowed! Marie said, "One of our joys was to go into our workroom at night....The glowing tubes looked like faint, fairy lights." The Curies published their detailed processes, seeing no reason to patent them and having no initial expectation that radium would be a business interest. An industry quickly sprang up around these glowing substances, using radium-containing substances as novelties and in a wide variety of medical and pseudo-medical applications. During their research the Curies were experiencing adverse health effects from handling so much radioactive material, although they did not yet understand the link and likely thought their symptoms were the result of working so hard and the drafty conditions of their laboratory shed. When Pierre died unexpectedly of an accident in 1906, Marie carried on in their research. Over time, Marie's interests shifted to the medical applications of radioactive materials, including radiology and the potential of radium as a cancer therapy. Her legacy is and was large and far-reaching, and among other awards and accolates, she had three radioactive minerals named after her and/or her husband (curite, sklodowskite, and cuprosklodowskite) as well as an element (curium). There are so many great resources that go into more depth on Marie Curie's fascinating life and science. For a start, consider American Institute of Physics' online exhibit "Marie Curie and the Science of Radioactivity" by Naomi Pasachoff.  #MarieCurie   #radioactivity   #minerals   #mineralogy   #history   #science 
  • by friendsofmineralogy 1 month ago
    Chalcopyrite (copper iron sulfide) is the most common copper ore mineral. It is often confused with gold and pyrite, since all three are metallic and yellowish or brassy. But they are easily distinguished with a closer look-- pyrite usually crystallizes in cubes or octahedrons while chalcopyrite forms in the tetragonal system. They are all different hardnesses, too. While pyrite can't be scratched with a knife, chalcopyrite can, and gold is the softest of all and can be scrated with copper. This specimen is from Alice Mine, Clear Creek County, Colorado, USA, one of the classic Colorado localities for Chalcopyrite, often producing brilliantly metallic twinned specimens like this one. Photo credit: Rob Lavinsky & irocks.com ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!
  • by friendsofmineralogy 1 month ago
    Geology of a...Pumpkin? Yes, of course pumpkins on their own are biological, not geological. However, a lot of earth science goes into the pumpkin you pick out of a seasonal supermarket display or those at a very instagrammable commercial pumpkin patch. Pumpkins are frequently grown in abundant sun and water on the rich soil of farmland along river floodplains, but because a whole lot of nice plump gourds need a lot of additional nutrients, supplemental fertilization is usually needed. Different types of fertilizers are used, including nitrogen fertilizers, which help to produce healthy vines and leaves in the early growing season, and phosphate fertilizers, which help produce lots of pumpkin blossoms. Potassium rich fertilizers may also be used to help make heavy fruit. Nitrogen fertilizers are often made from manure, fish wastes, and other nitrogen rich material. Phosphate fertilizers are derived from phosphate rock that contains apatite group phosphate minerals. "Time-release" fertilizers can be a sort of technological combination of the two, in which nanoparticles of hydroxyapatite are coated with urea. This combination can target the delivery of both nutrients to the soil when they are needed in the growing cycle. Most of the world’s current production of phosphate rock is mined from sedimentary deposits formed by deposition of phosphate-rich materials in marine environments--deposits in Florida currently supply up to about 25 percent of total world phosphate rock production. And of course, petroleum is also needed for the fuels that take supplies and workers to the fields and that bring pumpkin crop to market. That's a topic for another day, but a note: more than a billion pounds of pumpkin get tossed into U.S. landfills every year. Pumpkins thrown into landfill or allowed to rot outside contribute to greenhouse gases like other rotting food waste. So please consider donating your used pumpkins to community pumpkin collection programs that will turn them into compost, reuse them as animal feed, or even turn them into biogas that can be used to generate electricity.  #science   #pumpkins   #minerals   #geologyrocks   #resources  FM graphic: Erin Delventhal
  • by friendsofmineralogy 1 month ago
    Sphalerite is zinc iron sulfide and an ore of zinc. It can vary in appearance, and was named from Greek word sphaleros, meaning "deceiving", due to the difficulty of identification. When the iron content is high, sphalerite is an opaque black variety called marmatite. It is also sometimes called ruby blende, zinc blende, or black-jack depending on color and appearance. Cleiophane is a gemmy, yellow to green variety. Zinc is one of those terribly useful metals, used in batteries, galvanization, nutrient supplements, zinc oxide sunscreens, and much more. Around 95% of all primary zinc is extracted from sphalerite ore, but because sphalerite can also include variable trace elements, it is also a source of other metals like gallium, indium, and germanium. Sphalerite is one of the most common sulfide minerals, and it is found worldwide and in a variety of deposit types. Each of the specimens in our image includes sphalerite, and together represent four continents. Backlighting of translucent specimens can reveal red, orange, green, and yellow colors-- sometimes multiple colors in the same crystal! Photo credit: Jessica Robertson ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!
  • by friendsofmineralogy 1 month ago
    National  #Fossil  Day! Why do fossils exist? Because they are the remains of former living creatures, turned to stone. What is stone made of? Minerals! So, Friends of Mineralogy wishes you a happy National Fossil Day. The million dollar question is this: can fossils be considered pseudomorphs? Though this might come as a shock in context of the typical modern concept of a “pseudomorph,” (a “false form” resulting strictly from one mineral replacing another mineral) the first usage of the word is found in René Just Haüy’s Traité de Mineralogie (1801). Haüy describes mineral bodies that owe their outward appearance to circumstances beyond their own crystallization or formation (“pseudo” = false, “morph” = form). The text goes into great length about fossils (in particular fossil shells and petrified wood), with only a brief mention at the end of minerals replacing other minerals. Thus, historically, it could be argued that the term  #pseudomorph  originated to describe fossils with mineral replacements as simply an afterthought. Photo: Calcite ps. Snail • Atienza, Guadalajara, Castile-La Mancha, Spain • 5.1cm • Erin Delventhal collection and photo
  • by friendsofmineralogy 2 months ago
    Galena (lead sulfide) is the main ore of lead. It's been used since antiquity (as far back as 9,000 years ago) since lead can be smelted from galena in an everyday wood fire. Galena also can contain impurities of silver-- if heating is prolonged and the smelted lead is oxidized into powdery ash, a small drop of pure silver may remain. In this way, ancient lead mines were also silver mines. Galena is one of the most abundant sulfide minerals, found worldwide--the specimens in our image represent four continents. Galena crystallizes in the cubic system and often is found in massive deposits, or crystals in cubes or octahedrons. The galena crystals in the center specimen shown here (from Dal'negorsk, Primorskiy Kray, Russia), are twinned into sharp triangular or hexagonal crystals that are almost as shiny as mirrors. Photo credit: Jessica Robertson ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!
  • by friendsofmineralogy 2 months ago
    Martine de Bertereau was the first recorded female mineralogist. With her husband Jean de Chastelet, she traveled extensively through Europe studying mines and mineral deposits. In 1626, at the request of King Henry IV, the Bertereaus established a base in Brittany to survey France for new mines to revive the French mining industry. At that time, the sciences of mineralogy and chemistry were still intertwined with alchemy, and mining prospectors sometimes included astrology-based techniques and dowsing. While in Brittany, provincial clergymen suspected them of sorcery and they searched their châteaux looking for incriminating material. Although nothing was found they were temporarily forced to leave France. Martine wrote two pamplets on her practice and theory of mining. The second, Restitution de Pluton, is an appeal to Cardinal Richelieu to be paid for their completed work, and also details some of her mine prospecting practices, like observing the local vegetation and water, watching for rising vapors, and dowsing and divining instruments. It is possible that de Bertereau wanted to keep her real prospecting techniques secret by confusing the reader with inclusion of divining and astrology, or used these techniques as a sort of theatrical proof of her skill. It is thought that the demand for money did not please the King. Charges of witchcraft were moved against them, and Martine and her family were imprisoned. While in prison Martine did not give up her passion. At one point, Martine scared a visitor with a 'magical' transformation, which she explained was a demonstration of the ‘végétal’ of silver and mercury, which grew and regenerated in a flask. This demonstration was probably a "Diana's Tree" precipitation of metallic silver from a solution of silver nitrate with mercury, like the one shown in the background of our image. The couple died in prison around 1642. France's mining industry was again in disarray. Other mining experts fled to other areas of Europe, and in 1667, it was remarked that there was nowhere in France a skilled mining expert to be found, only pretenders.  #history   #alchemy   #mining   #minerals   #silver   #witchcraft   #science 
  • by friendsofmineralogy 2 months ago
    Bornite is a copper iron sulfide and an important copper ore. It's one of those minerals that usually forms as massive material and has been known by many names including "purple copper ore," "variegated copper ore," "buntkupfererz" and "phillipsite." In 1845 it was officially named "bornite" by Wilhelm Karl von Haidinger in honor of Ignaz von Born, an Austrian mineralogist. Tarnished bornite can have a purplish sort of iridescence, however, the strong bright blue and purple "peacock ore" labeled as bornite in some rock and gift shops is very often chalcopyrite treated with acid rather than bornite. Bornite as a mineral, and especially as crystals, is more unusual than chalcopyrite. This excellently crystallized "tree" of bornite is from Carn Brea Mine, Pool, Carn Brea, Cornwall, England, UK. Carn Brea Mine was one of many 19th century mines in Cornwall that helped supply the raw materials for the industrial revolution. In 1847 alone, Carn Brea produced 10,372 tons of copper. Over the following years, Carn Brea amalgamated with neighboring mines as production decreased. The Carn Brea section closed in 1913-14.  #bornite   #sulfides   #peacockore   #copper   #minerals   #geologyrocks   #mineralcollection   #rockcollection  Photo credit: Rob Lavinsky & irocks.com ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!
  • by friendsofmineralogy 2 months ago
    Detail from an allegorical painting titled "Mineralogy," in which a goddess sits among a crowd of mythological figures, with babies or cherubs at her feet holding minerals and rock hammers. Coloured stipple engraving by J. Chapman for the Encyclopaedia Londinensis, 1817. Start 'em young!  #mineralogy   #crystals   #art   #history   #mineralcollecting 
  • by friendsofmineralogy 2 months ago
    Chalcocite: copper sulfide. Stunning, shining crystals of chalcocite like these from Mammoth Mine, Queensland, Australia are unusual for the species. Much more often, the mineral occurs as massive ore deposits. Chalcocite is an important copper ore known for centuries, and has a variety of historical names to match, including chalcosine, redruthite, vitreous copper and copper-glance. Photo credit: Rob Lavinsky & irocks.com ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!  #sulfides   #chalcocite   #copper   #geologyrocks   #minerals   #mineralcollecting   #crystals 
  • by friendsofmineralogy 2 months ago
    September's birthstone is sapphire! One of the most famous sapphires of all is the Star of India, a 563-carat star sapphire. Mined in Sri Lanka and now located at the American Museum of Natural History in New York City, the Star of India is a historic gem that is also a dazzling example of the phenomenon of 'asterism'. Sapphire is a variety of the mineral corundum (aluminium oxide, α-Al2O3). Corundum crystallizes in a hexagonal system, with elongated, six-sided crystals. Star sapphires are included with microscopic inclusions of rutile (titanium dioxide) that are arranged perpendicularly to the rays of six-fold symmetry. A polished cross-section of this crystal can therefore reflect a pattern of a six-sided star, which appears to move and shimmer with slight changes in the angle of reflected light. The Star of India was among several gems commissioned from mineralogist George Kunz by J.P. Morgan in the early years of the 1900s. Kunz wrote in 1913 that the Star of India "has a more or less indefinite historic record of some three centuries," however, details of this historic record are now unknown. The Star of India's fame was increased even higher when it was one of several noted gems stolen from the American Museum in October 1964. It was recovered in early 1965 and returned to the museum, where it remains on display today. Photo credit: Kelly Nash  #sapphire   #corundum   #gemstones   #crystals   #science   #asterism 
  • by friendsofmineralogy 2 months ago
    Acanthite! Ever wonder what the dark tarnish is on your silver jewelry? That tarnish is the result of the reaction of silver with sulfur-containing molecules in the air and is silver sulfide: chemically the same as acanthite, the stable form of silver sulfide below 177 °C. The "acanthite" name comes from the Greek word meaning "thorn", which is reminiscent of its crystal shape. The sulfides, which tend to crystallize with a metallic luster and to be more brittle than the native metals, are of great economic importance, because many metal ores are in this group. Some field guides lists argentite, not acanthite, as the primary silver sulfide. These names are sometimes used almost interchangeably, however, because argentite is the stable form of silver sulfide above 177 °C, the only stable form in normal air temperature is acanthite. So, if you see a speciment for sale as "argentite", know it is really an Acanthite pseudomorph after Argentite. Photo credit: Rob Lavinsky & irocks.com ICYMI: Each  #mineralmonday  we present a mineral species with some history and science, following along the order presented in Frederick Pough's definitive Field Guide to Rocks and Minerals; currently working through the sulfides!  #sulfides   #argentite   #acanthite   #geologyrocks   #minerals   #mineralcollecting   #crystals