Definition of a Mineral Each Mineral Species Has Unique and Identifiable Physical Properties. Form and Habit Terms (Crystal Grow

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Definition of a Mineral Each Mineral Species Has Unique and Identifiable Physical Properties. Form and Habit Terms (Crystal Grow

Definition of a Mineral • A mineral is a naturally occurring, • homogeneous solid with a Physical Properties of Minerals • definite, but not fixed, composition, and Color, shape, density, • an ordered atomic arrangement that is hardness, etc • formed by inorganic processes. • A mineral is a natural, crystalline phase. Form and Habit Terms Each mineral species has (Crystal Growth Forms) unique and identifiable • Prismatic (well developed prism physical properties. faces) (Pyramidal) • Form and habit (Shape) • Columnar (Irregular prism faces) • Luster • Acicular (needle-like) • Color and Streak • Botryoidal (rounded growth surface) • Cleavage and Fracture • Tabular (Platey) • Hardness • Stellate (Radiating) • Density • Fibrous (fibers, asbestiform) • Tenacity • Dendritic (tree-like) Acicular Fibrous 1 Dendritic Dendritic Radiating Radiating Bladed Botryoidal 2 Prismatic Luster and Transparency • Luster • Transparency – Metallic – Opaque – Resinous – Translucent (waxy) – Transparent – Pearly – Greasy – Adamantine – Vitreous Metallic Adamantine Color and Streak • The reflectance color of minerals is strongly affected by transition metals (V, Cr, Mn, Fe, Co, Ni, and Cu). (Also rare earths) Vitreous Pearly • Color in hand specimen may not be diagnostic. • Color in streak generally indicates presence of iron or other transition metals. Chatoyance, Asterism, Asterism: Star Sapphire and Luminescence • Chatoyance and asterism are optical effects due to diffraction of light from small inclusions. • Luminescence is emission of light. – Visible or UV (black light) – Tribo-luminescence: glow when rubbed – Cathodo-luminescence is emission of light from electron bombardment 3 Cleavage and Fracture Hardness What scratches what? • Crystals tend to break on planes of • 1. Talc 6. Orthoclase weakness. • 2. Gypsum 7. Quartz – Cleavage: perfect – Parting: irregular • 3. Calcite 8. Topaz – Hackly: very irregular • 4. Fluorite 9. Corundum – Conchoidal Fracture: no cleavage, • 5. Apatite 10. Diamond breaks like glass. Density Tenacity: How does it deform? • Density units are g/cm3 (water is 1.0) • Densities range from 0.92 for ice to • Brittle: Fractures (quartz) ~1.8 for some zeolites to 22 for Os. • Most silicates are 2.5 to 3.5. • Ductile: Malleable (gold) • Most sulfides are 4.5 to 6.0 • Iron metal is ~8 • Sectile: Cut with a knife (mica) • Lead is ~13 • Gold and platinum are 19-22. Other Properties Unique Properties • Optical (Index of refraction) • Optical (Birefringence) • Ferro-magnetism • Optical (Spectroscopy) • Taste (Don’t do it. It might be • Chemistry (X-ray and electron witherite.) fluorescence) • Radioactivity (U and Th minerals) • X-ray diffraction (+ electron and • UV Fluorescence neutron) • Piezoelectricity (acentric crystals) • Other spectroscopies • Pyroelectricity (acentric crystals) – IR Raman (visible) – Mössbauer() Auger (electron) 4.

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Download PDF About Minerals Sorted by Mineral Name
MINERALS SORTED BY NAME Here is an alphabetical list of minerals discussed on this site. More information on and photographs of these minerals in Kentucky is available in the book “Rocks and Minerals of Kentucky” (Anderson, 1994). APATITE Crystal system: hexagonal. Fracture: conchoidal. Color: red, brown, white. Hardness: 5.0. Luster: opaque or semitransparent. Specific gravity: 3.1. Apatite, also called cellophane, occurs in peridotites in eastern and western Kentucky. A microcrystalline variety of collophane found in northern Woodford County is dark reddish brown, porous, and occurs in phosphatic beds, lenses, and nodules in the Tanglewood Member of the Lexington Limestone. Some fossils in the Tanglewood Member are coated with phosphate. Beds are generally very thin, but occasionally several feet thick. The Woodford County phosphate beds were mined during the early 1900s near Wallace, Ky. BARITE Crystal system: orthorhombic. Cleavage: often in groups of platy or tabular crystals. Color: usually white, but may be light shades of blue, brown, yellow, or red. Hardness: 3.0 to 3.5. Streak: white. Luster: vitreous to pearly. Specific gravity: 4.5. Tenacity: brittle. Uses: in heavy muds in oil-well drilling, to increase brilliance in the glass-making industry, as filler for paper, cosmetics, textiles, linoleum, rubber goods, paints. Barite generally occurs in a white massive variety (often appearing earthy when weathered), although some clear to bluish, bladed barite crystals have been observed in several vein deposits in central Kentucky, and commonly occurs as a solid solution series with celestite where barium and strontium can substitute for each other. Various nodular zones have been observed in Silurian–Devonian rocks in east-central Kentucky.
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Phenomenal Gemstones Possess Striking Optical Effects, Making Them Truly a Sight for Sore Eyes
THE PHENOMENAL PROPERTIES OF GEMS Phenomenal gemstones possess striking optical effects, making them truly a sight for sore eyes. Here is GIA’s guide to understanding what makes each phenomenon so uniquely brilliant. ASTERISM CROSSING BANDS OF REFLECTED LIGHT CREATE A SIX-RAYED STAR-LIKE APPEARANCE. ASTERISM OCCURS IN THE DOME OF A CABOCHON, AND CAN BE SEEN IN GEMS LIKE RUBIES AND SAPPHIRES. ADULARESCENCE THE SAME SCATTERING OF LIGHT THAT MAKES THE SKY BLUE CREATES A MILKY, BLUISH-WHITE GLOW, LIKE MOONLIGHT SHINING THROUGH A VEIL OF CLOUDS. MOONSTONE IS THE ONLY GEM THAT DISPLAYS IT. AVENTURESCENCE FOUND IN NATURAL GEMS LIKE SUNSTONE FELDSPAR AND AVENTURINE QUARTZ, IT DISPLAYS A GLITTERY EFFECT CAUSED BY LIGHT REFLECTING FROM SMALL, FLAT INCLUSIONS. CHATOYANCY OTHERWISE KNOWN AS THE “CAT’S EYE” EFFECT, BANDS OF LIGHT ARE CAUSED BY THE REFLECTION OF LIGHT FROM MANY PARALLEL, NEEDLE-LIKE INCLUSIONS INSIDE A CABOCHON. NOTABLE GEMS THAT DISPLAY CHATOYANCY INCLUDE CAT’S EYE TOURMALINE AND CAT’S EYE CHRYSOBERYL. IRIDESCENCE ALSO SEEN IN SOAP BUBBLES AND OIL SLICKS, IT’S A RAINBOW EFFECT THAT IS CREATED WHEN LIGHT IS BROKEN UP INTO DIFFERENT COLORS. LOOK FOR IT IN FIRE AGATE AND OPAL AMMONITE (KNOWN BY THE TRADE AS AMMOLITE). LABR ADORESCENCE A BROAD FLASH OF COLOR THAT APPEARS IN LABRADORITE FELDSPAR, IT’S CAUSED BY LIGHT INTERACTING WITH THIN LAYERS IN THE STONE, AND DISAPPEARS WHEN THE GEM IS MOVED. INSIDER’S TIP: THE MOST COMMON PHENOMENAL COLOR IN LABRADORITE IS BLUE. PLAY OF COLOR THE FLASHING RAINBOW-LIKE COLORS IN OPAL THAT FLASH AT YOU AS YOU TURN THE STONE OR MOVE AROUND IT.
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Barite (Barium)
Barite (Barium) Chapter D of Critical Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply Professional Paper 1802–D U.S. Department of the Interior U.S. Geological Survey Periodic Table of Elements 1A 8A 1 2 hydrogen helium 1.008 2A 3A 4A 5A 6A 7A 4.003 3 4 5 6 7 8 9 10 lithium beryllium boron carbon nitrogen oxygen fluorine neon 6.94 9.012 10.81 12.01 14.01 16.00 19.00 20.18 11 12 13 14 15 16 17 18 sodium magnesium aluminum silicon phosphorus sulfur chlorine argon 22.99 24.31 3B 4B 5B 6B 7B 8B 11B 12B 26.98 28.09 30.97 32.06 35.45 39.95 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.10 40.08 44.96 47.88 50.94 52.00 54.94 55.85 58.93 58.69 63.55 65.39 69.72 72.64 74.92 78.96 79.90 83.79 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.47 87.62 88.91 91.22 92.91 95.96 (98) 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 cesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.9 186.2 190.2 192.2 195.1 197.0 200.5 204.4 207.2 209.0 (209) (210)(222) 87 88 104 105 106 107 108 109 110 111 112 113 114 115 116
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Selective Flotation of Witherite from Calcite Using Potassium Chromate As a Depressant
Physicochem. Probl. Miner. Process., 55(2), 2019, 565-574 Physicochemical Problems of Mineral Processing ISSN 1643-1049 http://www.journalssystem.com/ppmp © Wroclaw University of Science and Technology Received May 31, 2018; reviewed; accepted September 2, 2018 Selective flotation of witherite from calcite using potassium chromate as a depressant Yangshuai Qiu 1, Lingyan Zhang 1,2, Xuan Jiao 1, Junfang Guan 1,2, Ye Li 1,2, Yupeng Qian 1,2 1 School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China 2 Hubei Key Laboratory of Mineral Resources Processing & Environment, Wuhan 430070, China Corresponding author: [email protected] (Lingyan Zhang) Abstract: Witherite has been widely used as an industrial and environmental source of barium, with calcite being the primary associated carbonate mineral. However, few studies have been conducted to effectively concentrate witherite from barium ores. In this work, with the treatment of potassium chromate (K2CrO4) and sodium oleate (NaOL), witherite was selectively separated from calcite through selective flotation at different pH conditions. In addition, contact angle, Zeta potential, adsorption and X-ray photoelectron spectroscopy measurements were performed to characterize the separation mechanisms. The results demonstrated that NaOL had a strong collecting ability for both witherite and calcite; nevertheless, witherite could be effectively selected from calcite with the highest recovery at pH 9 in the presence of K2CrO4. From the XPS measurements, NaOL and K2CrO4 were found to be primarily attached to the surfaces of witherite and calcite through chemisorption. The presence of K2CrO4 on the surface of calcite adversely influenced the NaOL adsorption, which could make the flotation separation efficient and successful.
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Conservation of Coated and Specialty Papers
RELACT HISTORY, TECHNOLOGY, AND TREATMENT OF SPECIALTY PAPERS FOUND IN ARCHIVES, LIBRARIES AND MUSEUMS: TRACING AND PIGMENT-COATED PAPERS By Dianne van der Reyden (Revised from the following publications: Pigment-coated papers I & II: history and technology / van der Reyden, Dianne; Mosier, Erika; Baker, Mary , In: Triennial meeting (10th), Washington, DC, 22-27 August 1993: preprints / Paris: ICOM , 1993, and Effects of aging and solvent treatments on some properties of contemporary tracing papers / van der Reyden, Dianne; Hofmann, Christa; Baker, Mary, In: Journal of the American Institute for Conservation, 1993) ABSTRACT Museums, libraries, and archives contain large collections of pigment-coated and tracing papers. These papers are produced by specially formulated compositions and manufacturing procedures that make them particularly vulnerable to damage as well as reactive to solvents used in conservation treatments. In order to evaluate the effects of solvents on such papers, several research projects were designed to consider the variables of paper composition, properties, and aging, as well as type of solvent and technique of solvent application. This paper summarizes findings for materials characterization, degradative effects of aging, and some effects of solvents used for stain reduction, and humidification and flattening, of pigment-coated and modern tracing papers. Pigment-coated papers have been used, virtually since the beginning of papermaking history, for their special properties of gloss and brightness. These properties, however, may render coated papers more susceptible to certain types of damage (surface marring, embedded grime, and stains) and more reactive to certain conservation treatments. Several research projects have been undertaken to characterize paper coating compositions (by SEM/EDS and FTIR) and appearance properties (by SEM imaging of surface structure and quantitative measurements of color and gloss) in order to evaluate changes that might occur following application of solvents used in conservation treatments.
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Star Stones (Asterism)
Learning Series: Basic Rockhound Knowledge Star Stones (Asterism) When parallel, needle-like inclusions, or tube-like channels, are oriented along two or more of the crystal faces of a mineral, and when that stone is cut as a domed cabochon, a four- to six-rayed figure is displayed on the dome. This phenomenon of reflected light is called "asterism" and the gems are called star stones. The most commonly seen examples are star corundums; where there are inclusions of titanium oxide (rutile or "silk") parallel to three crystal faces giving a six-rayed star. In rare cases, a twinned crystal slightly offset with its own set of rutile needles can lead to the formation of a twelve-rayed star. Although rutile is an extremely common inclusion in sapphire, few good, natural, star sapphires are found. One of the major reasons is that the heating, which is almost universally done to sapphire rough, dissolves rutile needles; clarifying and sometimes color enhancing the stone, yes, but eliminating potential stars! [Rutile needles (silk) aligned in three directions in unheated corundum] [Star ruby, white star sapphire ring, rare bi-colored star sapphire] The only other gem which commonly forms stars is quartz, where the phenomenon tends to be more visible in transmitted than in reflected light. In this species, rose quartz is the most frequently asterated variety. Most citrine in commerce has been heated, which tends to dissolve the fine rutile inclusions necessary for star formation, so it is rather rare. In fine, near-transparent quartzes which have been cut to a spherical shape, multiple stars can form an intersecting pattern over the surface.
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List of Abbreviations
List of Abbreviations Ab albite Cbz chabazite Fa fayalite Acm acmite Cc chalcocite Fac ferroactinolite Act actinolite Ccl chrysocolla Fcp ferrocarpholite Adr andradite Ccn cancrinite Fed ferroedenite Agt aegirine-augite Ccp chalcopyrite Flt fluorite Ak akermanite Cel celadonite Fo forsterite Alm almandine Cen clinoenstatite Fpa ferropargasite Aln allanite Cfs clinoferrosilite Fs ferrosilite ( ortho) Als aluminosilicate Chl chlorite Fst fassite Am amphibole Chn chondrodite Fts ferrotscher- An anorthite Chr chromite makite And andalusite Chu clinohumite Gbs gibbsite Anh anhydrite Cld chloritoid Ged gedrite Ank ankerite Cls celestite Gh gehlenite Anl analcite Cp carpholite Gln glaucophane Ann annite Cpx Ca clinopyroxene Glt glauconite Ant anatase Crd cordierite Gn galena Ap apatite ern carnegieite Gp gypsum Apo apophyllite Crn corundum Gr graphite Apy arsenopyrite Crs cristroballite Grs grossular Arf arfvedsonite Cs coesite Grt garnet Arg aragonite Cst cassiterite Gru grunerite Atg antigorite Ctl chrysotile Gt goethite Ath anthophyllite Cum cummingtonite Hbl hornblende Aug augite Cv covellite He hercynite Ax axinite Czo clinozoisite Hd hedenbergite Bhm boehmite Dg diginite Hem hematite Bn bornite Di diopside Hl halite Brc brucite Dia diamond Hs hastingsite Brk brookite Dol dolomite Hu humite Brl beryl Drv dravite Hul heulandite Brt barite Dsp diaspore Hyn haiiyne Bst bustamite Eck eckermannite Ill illite Bt biotite Ed edenite Ilm ilmenite Cal calcite Elb elbaite Jd jadeite Cam Ca clinoamphi- En enstatite ( ortho) Jh johannsenite bole Ep epidote
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Mining, Geology, and Geological History of Garnet at the Barton Garnet Mine, Gore Mountain, New York William Kelly
6: RARE EARTH ELEMENT AND YTTRIUM MINERAL OCCURENCES IN THE ADIRONDACK MOUNTAINS 7 MINING, GEOLOGY, AND GEOLOGICAL HISTORY OF GARNET AT THE BARTON GARNET MINE, GORE MOUNTAIN, NEW YORK WILLIAM KELLY New York State Geologist, Emeritus, Division of Research and Collections, New York State Museum, Albany, NY 12230, [email protected] KEYWORDS: Garnet, Mining, Gore Mountain, Metamorphism, Lyon Mountain Granite ABSTRACT Garnet megacrysts commonly 30 centimters (cm) ranging up to 1 meter (m) in diameter occur at the summit of Gore Mountain, Adirondacks, NY and were mined there for abrasives for more than a century. The mine, owned by Barton Mines Co., LLC, is roughly 2 km x 150 m and is located in a hornblende-rich garnet amphibolite at the southern boundary of a metamorphosed olivine gabbro body that is in fault contact with charnockite. Barton supplies garnet, a chemically homogeneous pyrope-almandine, to the waterjet cutting, lapping, and abrasive coatings industries. The garnet megacrysts are reliably dated at 1049 ± 5 Ma. The growth of the garnet megacrysts was facilitated by an influx of hydrothermal fluid emanating from the ore body’s southern boundary fault. The fluids were most probably associated with the intrusion of the Lyon Mountain Granite (1049.9 ± 10 Ma) and/or associated pegmatitic rocks late in the tectonic history of the Adirondacks. INTRODUCTION The Adirondack Mountains in upstate New York are a small outlier of a larger body of rocks of similar age and geologic history that is located to the north in Canada. The Adirondack region can be loosely divided into amphibolite metamorphic facies Lowlands, in the northwest, and the granulite facies Central Highlands, which are 86 THE ADIRONDACK JOURNAL OF ENVIRONMENTAL STUDIES VOLUME 21 87 7: MINING, GEOLOGY, AND GEOLOGICAL HISTORY OF GARNET AT THE BARTON GARNET MINE, GORE MOUNTAIN, NEW YORK separated by a very large, northwest-dipping fault zone.
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Barite–Celestine Geochemistry and Environments of Formation Jeffrey S
Barite–Celestine Geochemistry and Environments of Formation Jeffrey S. Hanor Department of Geology and Geophysics Louisiana State University Baton Rouge, Louisiana 70803 INTRODUCTION Minerals in the barite (BaSO4)–celestine (SrSO4) solid solution series, (Ba,Sr)SO4, occur in a remarkably diverse range of sedimentary, metamorphic, and igneous geological environments which span geological time from the Early Archean (~3.5 Ga) to the present. The purpose of this chapter is to review: (1) the controls on the chemical and isotopic composition of barite and celestine and (2) the geological environments in which these minerals form. Some health risks are associated with barite, and these are discussed near the end of this chapter. Although complete solid solution exists between BaSO4 and SrSO4 most representa- tives of the series are either distinctly Ba-rich or Sr-rich. Hence, it is convenient to use the term barite to refer to not only the stochiometric BaSO4 endmember but also to those (Ba,Sr)SO4 solid solutions dominated by Ba. Similarly, the term celestine will refer here not only to the stoichiometric SrSO4 endmember but to solid solutions dominated by Sr. Such usage is in accord with standard mineral nomenclature. The Committee on Mineral Names and Nomenclature of the International Mineralogical Association recognizes “celestine” as the official name for SrSO4. However, the name “celestite” is still commonly used in the literature. Geological significance of barite and celestine Most of the barite which exists in the Earth’s crust has formed through the mixing of fluids, one containing Ba leached from silicate minerals, and the other an oxidized shallow fluid, such as seawater, which contains sulfate.
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UNITED J STATES PATENT OFFICE
Patented Nov. 15, 1949 ’ 2,488,507 UNITED j STATES PATENT OFFICE , SYNTHETIC STAR RUBIES AND STAR SAP PHIRES, AND PROCESS FOR‘ PRODUCING SAME John N. Burdick and John w. Glenn, Jr., Ken more, N. Y., assignors to The Linde Air Prod ucts Company, a corporation of Ohio No Drawing. Application August 27, 1947, Serial No. 770,942 19 Claims. (01. 63-82) 1 This invention relates to a process for develop Other-objects are to provide a process for ing asterism in natural and synthetic corundum developing asterism in single crystals of ruby. crystals, such as crystals of ruby and sapphire, and sapphire containing oxide of titanium; to which contain oxide of titanium dissolved in provide a process for obtaining star ruby and alumina. The invention is also concerned with .star sapphire gemstones from non-asteriated asteriated single crystals of synthetic corundum single crystals of ruby and sapphire containing as articles of manufacture, for example synthetic oxide of titanium; and to provide a process for star rubies and star sapphires. intensifying asterism inasteriated ruby and sap For more than thirty years massive nongran phire crystals. ular synthetic rubies and sapphires of gem qual 10 The term “massive non-granular,” as used ity have been produced commercially on a large herein, is intended to designate singlev crystals scale by the process of Verneuil, as disclosed in as distinguished from‘ a sintered or agglomerated U. S. Patents 988,230 and 1,004,505. In that proc mass of granules. Moreover, this term is further } ess powdered alumina, with or without small intended to designate crystals which are larger percentages of coloring oxides, drops through 15 than such tiny particles or granules of corundum an oxy-h'ydrogen ?ame, fuses, and accumulates as are commonly employed in abrasives, those on a refractory support until a long, narrow, crystals designated by the term “massive non approximately cylindrical boule of the desired granular” being, indeed, large enough to be used size is formed.
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Chemical Staining Methods Used in the Identification of Carbonate Minerals
CHEMICAL STAINING METHODS USED IN THE IDENTIFICATION OF CARBONATE MINERALS Tamer AYAN Mineral Research and Exploration Institute of Turkey INTRODUCTION Carbonate minerals are the major constituents of sedimentary rocks; however, they are also found in igneous and metamorphic rocks, either as primary or as secondary minerals. In carbonate rocks formed either by chemical or mechanical deposition, these minerals constitute the main rock-forming components. Sometimes they are also found in great abundance in pelitic, psammitic and psephitic rocks. According to their crystallographic characteristics, carbonate minerals may be subdivided into two main groups : 1) Calcite group : Hexagonal-rhombohedral system. 2) Aragonite group : Orthorhombic system «pseudohexagonal». Of the calcite group of minerals, calcite (CaCO3) and dolomite [CaMg (CO3)2] are the two minerals which are dominantly found in sediments. Depending on its purity, or iron and magnesium content, calcite may be found in states of pure calcite, ferro-calcite, or slightly to abundantly magnesian calcite. Similarly, according to the iron content percentage, dolomite may also be regarded as having been composed of two end members, pure dolomite and ferrous dolomite, in varying proportions. The other calcite group minerals are ankerite «ferroferric dolomite» [CaFe(CO3)2], magnesite (MgCO3), siderite (FeCO3), smithsonite (ZnCO3), rhodochrosite (MnCO3) and spherocobaltite (CoC03). The most important mineral of the aragonite group is aragonite (CaCO3); the others, in decreasing order of importance, are witherite (BaCO3), strontianite (SrCO3), cerussite (PbCO3) and alstonite [(Ba, Ca)CO3]. Apart from their refractive indexes (R.I.), the optical and crystallographic characteristics of carbonate minerals are very similar to each other. Therefore identification of carbonates on hand specimens or even in thin sections, is rather difficult.
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The Crystal Structure of Alstonite, Baca(CO3)2: an Extraordinary Example of ‘Hidden’ Complex Twinning in Large Single Crystals
Mineralogical Magazine (2020), 84, 699–704 doi:10.1180/mgm.2020.61 Article The crystal structure of alstonite, BaCa(CO3)2: an extraordinary example of ‘hidden’ complex twinning in large single crystals Luca Bindi1* , Andrew C. Roberts2 and Cristian Biagioni3 1Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy; 2Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada; and 3Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy Abstract Alstonite, BaCa(CO3)2, is a mineral described almost two centuries ago. It is widespread in Nature and forms magnificent cm-sized crystals. Notwithstanding, its crystal structure was still unknown. Here, we report the crystal-structure determination of the mineral and discuss it in relationship to other polymorphs of BaCa(CO3)2. Alstonite is trigonal, space group P31m, with unit-cell parameters 3 a = 17.4360(6), c = 6.1295(2) Å, V = 1613.80(9) Å and Z = 12. The crystal structure was solved and refined to R1 = 0.0727 on the σ c basis of 4515 reflections with Fo >4 (Fo) and 195 refined parameters. Alstonite is formed by the alternation, along , of Ba-dominant and Ca-dominant layers, separated by CO3 groups parallel to {0001}. The main take-home message is to show that not all structure determinations of minerals/compounds can be solved routinely. Some crystals, even large ones displaying excellent diffraction quality, can be twinned in complex ways, thus making their study a crystallographic challenge. Keywords: alstonite, carbonate, barium, calcium, crystal structure, twinning (Received 11 June 2020; accepted 25 July 2020; Accepted Manuscript published online: 29 July 2020; Associate Editor: Charles A Geiger) Introduction b = 17.413(5), c = 6.110(1) Å and β = 90.10(1)°, with a unit-cell volume 12 times larger than the orthorhombic cell proposed by Alstonite, BaCa(CO ) , is polymorphous with barytocalcite, para- 3 2 Gossner and Mussgnug (1930).
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