Fluoride is one of the most consumed pharmaceuticals in the world, and its facility in preventing dental caries is recognized as one of the top 10 public health achievements of the 20th century. Although hydroxylapatite is often used as an analog of dental enamel, the details of the substitution of F for OH in the apatite anion column are not well known. Using new synthesis techniques, this study extends the structure work on P63/m apatites along the middle portion of the F-OH apatite join to compositions near that of fluoridated human teeth. The first F substituent in hydroxylapatite, near fluoridated dental enamel compositions, is dramatically underbonded by the surrounding Ca2 atoms (0.72 v.u.) in a hydroxylapatite matrix. However, the hydroxyl hydrogen can contribute 0.20 or 0.10 v.u. in hydrogen bonding to the F, depending on whether the substitution creates a reversal site in the anion column; this hydrogen bonding alleviates the bonding requirements of the substituent F. As F concentrations increase along the join, the average hydroxyl contributes increasing amounts of hydrogen bonding to the F column anions; to mitigate the loss of its hydrogen bonding, the hydroxyl oxygen migrates toward the adjacent mirror plane that contains the bonded Ca2 atoms, and the triangle of bonded Ca2 ions concomitantly contracts. These two mechanisms increase bonding to the column hydroxyl oxygen from the adjoining Ca2 atoms to balance the loss of hydrogen bonding that stabilizes the substituent F column anion and the increasing concentration of underbonded F.

Polarized Fourier transform infrared (FTIR) microspectroscopy of the OH-stretching region of hydroxylapatite-chlorapatite solid solutions presents novel problems for the assignment of peaks to specific OH-Cl pairs. Crystal structure refinements of Hughes et al. (2016) identified new positions for column anions in synthetic mixed Cl-OH apatites, with three different column anion arrangements depending on composition. These structural refinements, combined with bond-valence calculations, allow for interpretation of the OH-stretching region.A peak at 3574 cm-1 is identified as that from end-member hydroxylapatite. A second major peak at 3548 cm-1 is only found in mixed chlorapatite-hydroxylapatite solid solutions, as is a third peak at 3592 cm-1. Both represent perturbations of the OH-stretching vibration as compared to hydroxylapatite, to lower and higher frequency, respectively. Both of the new peaks are the result of a Clb-OH sequence, with adjacent anions in crystallographically similar positions, both above or both below adjacent mirror planes. One configuration has the hydrogen atom pointed toward the chlorine atom. The second has the hydrogen of the OH group pointed away from the chlorine atom.Both configurations present novel problems. The shift to lower wavenumber at 3548 cm-1 is characteristic of hydrogen bonding in fluorapatite-hydroxylapatite mixtures, yet the distance between O(H) and Clb is too great to allow it. The shift of OH-stretching vibrations to lower wavenumber is produced through changes in polarization of intervening Cl-Ca2 ' (or Ca2) and Ca2(')-O3 bonds, which are affected by the presence of the large chlorine atom. Lowering the OH-stretching vibration mimics the expected effect of chlorine on a neighboring OH group in the apatite c-axis column, though without hydrogen bonding. The shift to higher wavenumbers, i.e., higher frequency at 3592 cm-1, is the opposite of that expected for hydrogen bonding between column anions in the apatite mineral group. It is ascribed to the interaction between an adjacent Clb and the oxygen end of an adjacent OH dipole. This pairing places an oxygen and a chlorine atom in close proximity. Possible means of accommodation are discussed.A ubiquitous peak at 3498 cm-1 represents hydrogen bonding between an OH and the OHa site, with an interoxygen distance of about 2.9 & Aring;. Published modeling supports the hypothesis that the OHa site is occupied by an O rather than an OH. However, no clear counterpart to this pairing is observed in crystal structure refinements for specimens lacking OHa, although the infrared absorbance is present. The existence of oxyapatite is inferred from studies of plasma-sprayed biomaterials, but the crystallographic details of the substitution have remained elusive.A minor shoulder at 3517 cm-1 does not have a clear counterpart in the structural refinements. Sequences of three columnar anions (e.g., OH-Cl-OH or Cl-OH-OH) can be ruled out, but an unequivocal assignment awaits further research.

Abstract Rare earth elements (REE) in calcium apatite have been widely described in the literature. Based on the investigations of minerals and their synthetic analogs, the mechanism of substitution of REE3+ for Ca2+ and their structural positions are well established. Although the presence of REE in natural pyromorphite has been reported, the structural response of substitution of REE3+ for Pb2+ is not established. A better understanding of REE-rich Pb-apatite may facilitate the potential use of this mineral in industrial processes. Two La-doped pyromorphite analogs [Pb5(PO4)3Cl] and two control pyromorphite analogs (with the absence of La) were synthesized from aqueous solutions at 25 °C. Na+ and K+ were used as charge-compensating ions to facilitate the incorporation of trivalent REE cations (La3+ + Na+ ↔ 2Pb2+ and La3+ + K+ ↔ 2Pb2+). Microprobe analysis, scanning electron microscopy, and Raman spectroscopy were used to confirm the purity of obtained phases. High-precision crystal structure refinements (R1 = 0.0140–0.0225) of all four compounds were performed from single-crystal X-ray diffraction data. The La content varied from 0.12(1) to 0.19(1) atoms per formula unit with the counter ions of K+ and Na+, respectively. Both substituting ions were accommodated at the Pb1 site only. By comparing the La-doped pyromorphite analogs with their control samples, it was possible to detect small changes in bond distances and polyhedral volumes caused by the La substitution. Variations in individual and mean interatomic distances reflected the cumulative effect of both the amount of substitution and ionic radii of substituting ions (La3+, Na+, and K+).

The annual New Mexico Mineral Symposium provides a forum for both professionals and amateurs interested in mineralogy.The meeting allows all to share their cumulative knowledge of mineral occurrences and provides stimulus for mineralogical studies and new mineral discoveries.In addition, the informal atmosphere encourages intimate discussions among all interested in mineralogy and associated fields.

Llallagua is one of the world's biggest tin deposits and part of the metallogenic Bolivian Tin Belt, which occurs in the inner arc of the Central Andes. The Llallagua deposit formed through emplacement of a subvolcanic porphyry stock, of intermediate dacitic to rhyodacitic composition, metasomatism, and hydrothermal mineralization. The deposit is the subject of a well-established geochronologic controversy that includes contradictory ages (approximate to 40 and 20 Ma) from multiple geochronometers. Geochronological characterization of phosphate minerals from the metasomatized igneous porphyry and hydrothermal vein assemblages, along with detailed petrography and chemical analyses, are used to reconcile the age controversy. The new interpretation is further supported by existing textural and geochemical data. The U-Pb ages of unaltered igneous fluorapatite (e.g. 21.2 +/- 2.9 Ma), monazite (e.g. 21.22 + 0.80/-0.66 Ma), and zircon (e.g. 21.15 +/- 0.39 Ma) from the Llallagua porphyry are all equivalent within error at approximate to 20 Ma, and are interpreted to represent the age of porphyry stock emplacement. Ages determined from altered portions of these minerals (e.g. apatite, 18.8 +/- 8.0 Ma) are within error the same as unaltered portions, suggesting alteration of the porphyry soon after emplacement. U-Pb ages from unaltered fluorapatite (e.g. 21.4 +/- 6.7 Ma), altered and unaltered monazite (e.g. 19.4 +/- 1.4 Ma and 20.29 +/- 0.30 Ma respectively), and unaltered xenotime (e.g. 19.32 +/- 0.67 Ma) from the hydrothermal veins are also approximate to 20 Ma, indicating a very short hiatus or temporal continuity between stock emplacement and hydrothermal vein formation. The early Miocene age for tin mineralization at Llallagua is consistent with neighboring mineralization centers at the Morococala, Colquechaca, Japo, and Santa Fe mines, and with regional trends observed across the Bolivian Tin Belt, including stratigraphy, magmatic, and tectonic history of the Andean Eastern Cordillera.

One of the fundamental aspects of crystal structures, on the local scale, is the coordination of individual atoms or ions. Coordination is the number of neighboring atoms that are bonded to the atom of interest. Because the arrangement of these coordinating or bonded atoms is regular throughout a structure, the geometry of coordination is also an important feature. The centers of the coordinating atoms can be used to define the geometry of a coordination polyhedron. The focus of the lead article in this issue of Rocks & Minerals is silicates, or silicate minerals (Heaney 2023), and the fundamental structural unit of all silicates is the coordination polyhedron composed of four oxygen ions bonded to a central silicon, the geometry of which can be described by a tetrahedron. Thus, the coordination number (of the Si) is four, and the coordination geometry is tetrahedral. Let’s use the structure of the most abundant silicate mineral in Earth’s crust, quartz, to illustrate the concepts behind coordination. Figure 1 is a rendering of the quartz structure in which the Si and O ions are drawn to scale for their measured ionic radii. The O ions are dark blue; the Si ions (not seen in the drawing) are light blue. In this image the much larger O ions block your view of Si ions below them. This type of rendering is often called space-filling and is the most accurate depiction of the structure if you could image it by some type of microscopy. Every Si ion in the structure is bonded to (coordinated by) four O ions. Because we are using drawings, rather than microscopic images of a real crystal, we can create any type of rendering CO O R D I N AT I O N

s were presented at the eRMS, and seven were accepted for publication only. Four were carried over until next year and will be presented and published during the symposium in 2021. All of the abstracts were reviewed by a committee consisting of Dr. Steven Chamberlain, New York State Museum; Alexander Falster and Dr. Carl Francis, Maine Mineral & Gem Museum; Dr. Sarah Hanson, Adrian College; Dr. Marian Lupulescu, New York State Museum; and Dr. George Robinson, St. Lawrence University. We warmly welcome the newest member of the review panel, Alexander Falster. The abstracts that were presented at the 2020 eRMS are noted with an asterisk. *OPTICAL ANOMALIES IN FLATTENED INCLUSIONS FROM MUSCOVITE. M. Murchland, M. Rutherford, J. Fink, and J. Rakovan, Dept. of Geology and Environmental Earth Science, 250 S. Patterson Ave., Miami University, Oxford, OH 45056. The topic of flattened mineral inclusions within micas received a recent exposé in an article by John S. White in a 2018 issue of Rocks & Minerals. The article reported on the appearance and characteristics of flattened crystal inclusions in muscovite, specifically almandine and gahnite from the Spruce Pine district of North Carolina and Delaware County, Pennsylvania. White included the results of optical analyses performed by our research group on a flattened gahnite, which showed that the crystal exhibited a distinctly biaxial interference figure despite being an isometric mineral. In isometric crystals, the index of refraction is uniform in all directions and therefore should not exhibit any birefringence nor be capable of producing a coherent interference figure. Further research has indicated that the particularly strong interference figure (an acute bisectrix figure) observed in our analysis was the result of residual muscovite on the underside of the crystal. However, after completely exposing a flattened gahnite crystal from another Spruce Pine sample, the same optically anomalous properties were observed, although the biaxial interference figure was much less intense and defined. Not only is the flattened gahnite birefringent, but it also shows undulatory extinction and an acute bisectrix figure when viewed normal to the direction of flattening. By experimenting with the addition of mica of varying thickness to the optical path when viewing a flattened gahnite and garnet in crossed polars, we found that the observed interference figure was the result of the mica even when exceedingly thin. We believe that this is the result of the very weak birefringence in the gahnite and garnet. Thus, light transmitted through the mica is only weakly disturbed when passing through the flattened inclusions. Multiple flattened almandine crystals have also been extracted from muscovite books from Avondale, Pennsylvania. Like gahnite, garnet has an isometric crystal structure and therefore should exhibit isotropic properties when viewed in cross-polarized light. However, these crystals also exhibit birefringence. Double refraction in flattened almandine was noted as early as 1895 in crystals from North Carolina; however, further details are lacking from this early work (Matthews 1895). When viewed in cross-polarized light, our flattened garnets exhibit undulatory extinction across the grain, suggesting that these are possible stress-induced optical anomalies. Strain-induced birefringence has been recorded in multiple isometric minerals. Stress from plastic deformation, inclusions, impurities, fractures, and other defects can result in dislocations and lattice modifications that hold strain in the interior of the crystal. This alters the refractive indices in the strained zone and leads to anomalous optical characteristics. The studied gahnites and garnets most likely experienced initial stress from constrained growth within the muscovite sheets, leading to defects within the structure that Contributed Papers in Specimen Mineralogy: Part 3 47th Rochester Mineralogical