R. Peter Richards

The 2H and 3R polytypes of sabieite, NH4Fe3+(SO4)2, from a natural fire in an oil-bearing shale near Milan, Ohio

Abstract The mineral sabieite, NH4Fe3+(SO4)2, was found in 2011 along the banks of the Huron River near Milan, Ohio, where it formed as the result of a natural fire in an oil-bearing shale. The mineral is directly associated with pyracmonite, tschermigite, and voltaite and occurs as colorless, pale pink, tan, and yellow hexagonal tablets. The streak is white. Crystals are transparent with vitreous luster. Mohs hardness is 2½, tenacity is brittle, fracture is irregular, and cleavage is perfect on {001}. The measured density is 2.65(2) g/cm3. The mineral is optically uniaxial (-) with indices of refraction ω = 1.657(3) and ε = 1.621(5) (white light). The empirical formula (based on 2 S apfu) is [(NH4)0.73(H3O)0.22K0.04Na0.01]Σ1.00(Fe3+0.95Al0.02Mg0.01)Σ0.98(SO4)2. Powder diffraction showed crystals to be combinations of the 2H and 3R polytypes. The structure of the 2H polytype was solved and refined from single-crystal data yielding R1 = 0.0694 for 509 Fo > 4σ(F) reflections. The 2H polytype has space group P63 and cell parameters a = 4.83380(17), c = 16.4362(9) Å, V = 332.59(2) Å3, and Z = 2 and the 3R polytype has space group R3̄ and cell parameters a = 4.835(2), c = 24.496(15) Å, V = 495.9(5) Å3, and Z = 3. The sabieite polytypes (including the original sabieite from Sabie, South Africa, which is the 1T polytype) have glaserite-like structures with layers consisting of Fe3+O6 octahedra that share each of their vertices with SO4 tetrahedra. NH4 groups occupy 12-coordinated sites in the interlayer region, bonding to 6 O atoms in each of the adjacent layers. In the 1T polytype, successive layers have identical configuration and orientation, providing a one-layer repeat sequence. In the 2H polytype, alternate layers are flipped (or rotated), in a two-layer repeat sequence. In the 3R polytype, successive layers are shifted relative to one another, in a three-layer repeat sequence. The different orientations of adjacent layers in the structures of the 2H and 3R polytypes result in significant changes in the linkages between the (NH4)O12 and Fe3+O6 polyhedra.

The Four Twin Laws of Calcite and How To Recognize Them

Abstract Calcite collectors often ask, “How many kinds of calcite twins are there?” The answer is hard to give because the word kinds is ambiguous. The question can be given an unambiguous answer, however, if the question is rephrased: “How many twin laws are there for calcite?” In this case, the answer is “four.” I will go further and assert that there are four, and only four, twin laws for calcite. The rest of this article will be spent explaining my answer and providing enough information that an observant collector should be able to tell whether a calcite crystal is twinned or not and to determine which of the four twin laws is represented. The basics of calcite crystallography are well covered in the excellent article by Brock (1993), and basic aspects of twinning are covered by Werner Lieber elsewhere in this issue.

Almeidaite, Pb(Mn,Y)Zn2(Ti,Fe3+)18O36(O,OH)2, a new crichtonite-group mineral, from Novo Horizonte, Bahia, Brazil

Abstract Almeidaite (IMA 2013-020), ideally Pb(Mn,Y)Zn2(Ti,Fe3+)18O36(O,OH)2, from Novo Horizonte, Bahia, Brazil, occurs in association with quartz, rutile, anatase, hematite, kaolinite, muscovite, xenotime-(Y) and bastnaesite-(La). Almeidaite forms isolated, black, opaque, sub-metallic, platy crystals flattened on [0001], measuring up to 30 mm × 30 mm × 6 mm in size, dominated by the basal pinacoid {0001}, which is bounded by various, mostly steep, rhombohedra and the hexagonal prism {112̅0}. Most of the crystals are multiply twinned, with non-planar contact surfaces that are approximately parallel to the c axis. The streak is brown. Reflectance values are [(RO, Re) λ (nm)]: (12.78, 15.39) 470; (12.86, 15.43) 546; (12.91, 15.55) 589; (13.04, 15.75) 650. The empirical formula is (Pb0.59Sr0.12Ca0.04La0.03)∑0.78(Mn0.54Y0.46)∑1.00Zn1.43(Ti13.02Fe3+4.98)∑18.00(Fe3+0.32Mn0.15)∑0.47 [O37.18(OH)0.82]∑38.00. It is trigonal, space group R3̅, with the unit-cell parameters a = 10.4359(2), c = 21.0471(4) Å, V = 1985.10(7) Å3 and Z = 3. The crystal structure was solved (R1 = 0.039) using 2110 unique reflections with I > 3σ(I). Almeidaite is a member of the crichtonite group with Pb dominant in the A site (with 12-fold coordination) and Zn dominant in the T site (with 4-fold coordination). It is a Zn analogue of senaite and a Pb analogue of landauite. The mineral is named after Professor Fernando Flávio Marques de Almeida (1916 - 2013).

Morphology of Salammoniac From the Huron River Shale Fire, Huron County, Ohio

H County, in north-central Ohio, is not a place one would expect to find an oil shale fire. However, a surface outcrop of Late Devonian Ohio Shale located on the Huron River caught fire in September 2009 and burned until March 2011 (fig. 1). The minerals that formed from the gasses of the fire are similar to those from other oil shale fires, coal mine fires, and volcanic vents, but they are quite unusual for Ohio. Twenty-seven fire-generated species were identified, twenty-one of which are new to Ohio and four of which are new to science. They have been described in a related paper (Richards et al. 2017). In this article I describe the habits of the most common and morphologically most complex mineral in the shale fire assemblage, salammoniac. The great array of habits of salammoniac reflects the highly variable conditions at the edge of the fire zone where it was precipitated by deposition from a gas and, at the same time, provides a remarkably comprehensive example of the relationships between conditions during crystal growth and the resulting crystal form. Morphology of Salammoniac

Hematite Twins from the Kalahari Manganese Field, Northern Cape Province, South Africa

Dr. R. Peter Richards is a consulting editor of Rocks & Minerals and an avid mineral collector with special interests in crystal morphology, twinning, and epitaxy. Dr. Mirjan Žorž is a passionate mineral collector who specializes in crystal morphology of elongated, twisted, and twinned minerals with a special emphasis on their symmetry properties. R. PETER RICHARDS Department of Geology Oberlin College Oberlin, Ohio 44074 peterichards@oberlin.net

Connoisseur's Choice: Distinctive Twinned Calcite from the Palmarejo Mine, Chihuahua, Mexico

Calcite comes from such a seemingly infinite number of worldwide localities and shows such a bewilderingly wide range of colors and associations, forms, and habits, including twins, that new finds ...

Connoisseur's Choice: Calcite from the American Midwest, Gordonsville Mine, Carthage, Smith County, Tennessee

Calcite is ubiquitous in the American Midwest where it occurs in a wide range of geological environments and frequently forms crystals of high quality, some with great complexity of form. It is the major constituent of limestone, most cave formations, and many fossil shells. North America’s finest calcites are predominantly found in the American Midwest where large crystals occur in aesthetic combinations with such other well-crystallized minerals as barite, fluorite, galena, and sphalerite. Calcite crystals are classic components of Mississippi Valley–type (MVT) deposits, frequent occupants of open spaces within sedimentary rocks, and treasured finds in the very different, and much older, rocks that host the well-known copper mines and iron deposits of the upper Midwest. No other region on the continent can boast the same diversity of calcite. Yet, connoisseurship of fine minerals is not just about appreciation of specimen size and abundance; it is also about appreciating the variety of habits, color, luster and clarity, twinning, and interesting associations that may occur. With this in mind, we have nominated calcite from the interconnected Elmwood/Cumberland/Gordonsville mines in the Central Gordonsville Mine, Carthage, Smith County, Tennessee TERRY E. HUIZING 5341 Thrasher Drive Cincinnati, Ohio 45247 tehuizing@fuse.net

Mineral Mysteries: Skunks: A Mystery Solved?

In his Mineral Mysteries column in the July/August 2013 issue of Rocks & Minerals (pages 368–372), John White discussed “skunks,” calcite crystals with stripes of other minerals (usually) aligned a...

Minerals of the Beekmantown Group, Southeastern Ontario, Southern Quebec, and Northwestern New York

Overshadowed by such famous localities as Mont Saint-Hilaire and the Francon quarry in southern Quebec, or classics such as Pierrepont, DeKalb, Rossie, and Antwerp in upstate New York, it is little...

Calcite: Pseudo-Octahedral from The Minesota Mine Ontonagon County, Michigan

after a high-temperature isometric polymorph of boracite; at surface temperatures boracite is orthorhombic, and crystals with isometric morphology are composed of complexly twinned orthorhombic domains (Palache, Berman, and Frondel 1951). The calcite crystals that are the subject of this article are examples of the first kind of deceptive symmetry—although they appear to be octahedra, they are not, as we will show. Many crystals that possess pseudosymmetry are not very good imitations. When viewed in the right way, their deviations from the apparent higher symmetry are easily seen. These calcites, however, do an excellent job of mimicking the form of Many mineral collectors who are interested in crystals and their shapes are particularly intrigued by crystals that have an apparent symmetry that is different from (and usually more regular than) their true symmetry. Pseudocubic quartz is often mentioned (e.g., Frondel 1962); one well-known locality is the Pecos River in New Mexico. Pseudo-octahedral dolomite is found at Teruel, Spain, and other places. Palache (1898) describes pseudocubic calcite from the Ridge mine and other copper mines in the Keweenaw Peninsula. Several articles on pseudo-octahedral calcite appeared in the calcite collectors’ newsletter, The Spar Box, during its fifteen-year lifespan. Hematite sometimes occurs as cube-shaped crystals. In all these cases, the crystals only approximate isometric symmetry, and careful examination will usually reveal deviations of a few degrees from the apparent symmetry. There are also similar examples of minerals that mimic hexagonal or tetragonal symmetry, also more regular than their true symmetry. One example is brookite from Magnet Cove, Arkansas, which is orthorhombic but has pseudohexagonal symmetry. An example of a pseudosymmetry that is lower than the true symmetry is offered by the grossular garnets from the Vilui River in Siberia. Some of these isometric crystals are elongated along one of the a-axes and have a definite tetragonal appearance. They might easily be mistaken for vesuvianite. In other instances, the symmetry is what it appears to be, but it is inappropriate for the mineral that displays it. Boracite occurs as beautiful and often interestingly crystallized isometric crystals, but these are actually pseudomorphs R. PETER RICHARDS Morphogenesis Inc. 154 Morgan Street Oberlin, Ohio 44074 peterichards@oberlin.net