Robert B. Cook

Geochemistry of biogenic pyrite and ferromanganese coatings from a small watershed: A bacterial connection?

Present‐day groundwater in an alluvial aquifer in Holocene floodplain deposits in east‐central Alabama contains 0.1–4 mg/L Fe, 0.1–0.7 mg/L Mn, ∼1–10 μg/L each of Co, Ni, As, Zn, La, and Ce, and 40–175 μ/L Ba. There is a distinct correspondence between trace elements present in groundwater and those concentrated on ferromanganese coatings on present‐day stream alluvium in the study area. This indicates that the reduction and dissolution of such coatings in the alluvial aquifer, probably mediated by Fe‐ and Mn‐reducing bacteria, has been a major control on groundwater chemistry. Authigenic euhedral pyrite crystals up to 1.5 cm in diameter replace lig‐nitic macro wood fragments near the base of the alluvial aquifer, and sulfur isotope data (δ34S values from +3 to ‐40‰CDT) indicate that pyrite precipitated as a consequence of bacterial sulfate reduction in and adjacent to the irregularly distributed wood fragments. The authigenic pyrite contains several hundred parts per million of As, Co, and Ni, indicating...

Arsenopyrite in the bank deposits of the Whitewood Creek-Belle Fourche-Cheyenne River-Lake Oahe system, South Dakota, U.S.A.

Mining, milling, and processing wastes containing quantities of arsenopyrite were produced around Lead, South Dakota, from 1875 to 1977. Much of this material was discharged into Whitewood Creek, and from there portions of the waste were transported to the Belle Fourche River, thence to the Cheyenne River, and finally to the Missouri River. In 1958, the Missouri River was dammed at Pierre, forming Lake Oahe. Analyses of cores collected from the lake bottom showed the presence of arsenic-rich layers in the bed sediments; substantial portions of the arsenic are due to arsenopyrite in the 8–16 and 16–32 μm size fractions of the sediments. In addition, suspended-sediment samples collected from the Cheyenne River above Lake Oahe contain detectable quantities of arsenopyrite in the 8–16 and 16–32 μm fractions. Solid material collected from the banks and floodplains of the Belle Fourche River and Whitewood Creek contains reduced and oxidized phases. The reduced phases have arsenic maxima in the 16–32 and 32–63 μm size ranges. These fractions also contribute the most arsenic to the samples; the major source being arsenopyrite. The oxidized segments have arsenic maxima in the 63 μm size ranges. The 63 μm fractions contribute the most arsenic to the oxidized samples. This arsenic, despite the oxidized nature of the samples, is associated with arsenopyrite coated with thin iron oxide rinds. It has been calculated that 80% of the arsenic in these deposits is associated with sulfides (in the form of arsenopyrite), while 20% is associated with iron oxides. The arsenopyrite found in the banks and floodplains of Whitewood Creek and the Belle Fourche River are the likely source of the arsenopyrite found in the suspended sediments of the Cheyenne River and in the bed sediment of Lake Oahe.

Connoisseur's Choice: Spessartine Marienfluss, Northern Namibia

member of the garnet family, almandine, was described (Cook 2009). Garnet forms under a wide range of geologic conditions and bulk compositions, resulting in its being one of the most locally abundant group of rock-forming minerals and certainly one of the most popular with collectors. In addition, the pleasing array of orange and red hues of both almandine and the closely related species spessartine have made them a historically important and relatively inexpensive gem material. Consequently, for completeness of our coverage of the garnet-group minerals, and in keeping with the gemstone theme of this year’s Tucson Gem and Mineral Show, spessartine has been chosen as this issue’s Connoisseur’s Choice mineral. To this end, a fine spessartine specimen from Marienfluss, northern Namibia, has been chosen. Spessartine, because of its compositional variability, occurs in a wide array of colors that range from almost black through various shades of brown, yellow, orange, and red; almost pure end-member spessartine is bright orange. It is typically transparent to translucent and has a vitreous to resinous luster. It has no cleavage but may exhibit parting on {110}. It is hard (7–7.5) and brittle, with an uneven to conchoidal fracture. The calculated density for the pure mineral is 4.179. Spessartine is cubic (4/m32/m) and is often seen in well-formed dodecahedral or trapezohedral crystals, or a combination of both. Individual crystals to 10 cm and weighing several kilograms have been reported. Some crystals appear to have been severely etched, resulting in irregularly shaped masses bounded by sharp, stepped {110} faces. Spessartine is one of fifteen minerals in the garnet group, each member of which is defined by its end-member chemical composition. The end-member spessartine composition is manganese aluminum silicate, Mn 3 Al 2 (SiO 4 ) 3 . However, crystals approaching “pure” or end-member spessartine are rare in nature, as essentially all analyses show at least some magnesium, calcium, and iron. It forms a complete solid-solution series with almandine. Many unanalyzed garnets are only provisionally named based on assumptions keyed to their mode of occurrence, accompanying minerals, and color. When compositions are determined analytically, however, the formula can be calculated and its chemistry expressed in molecular percentages of particular end members. We name the garnet according to the dominant end member. For example, a content determined to be Sp 61 Alm 31 Pyr 8 (spessartine 61%, almandine 31%, and pyrope 8%) represents a spessartine garnet, as that is the dominant component; this example is from Leiper’s quarry at Springfield, Delaware County, Pennsylvania (Peters 1984). When the spessartine and almandine components are present in nearly equal amounts, both names are often used (almandine-spessartine), and in fact it is not unusual for garnets from a single location to be slightly dominated by either component from one specimen to another. To further complicate matters, concentric compositional growth zoning within individual garnet crystals is common, so more than one species may be present in an individual crystal. For ROBERT B. COOK Department of Geology and Geography Auburn University Auburn, Alabama 36849 cookrob@auburn.edu

Connoisseur's choice

a professor of geology and head of the Department of Geology and Geography at Auburn University, Auburn, Alabama. He welcomes suggestions for this column and can be contacted at the addresses above. T here has been a dramatic increase recently in the awareness of the environmental impact of minor amounts of arsenic, including that from natural sources. Consequently, arsenic minerals that have been of interest to exploration and economic geologists for many decades are only now coming into their own in the geologic and environmental community as a whole. Although the list of arsenic-rich minerals is not particularly long, their individual geochemistries and geologic occurrences are quite varied. Orpiment (As 2 S 3 ) has already been covered in this column (Cook 2000), and it is now appropriate to discuss its sister species, realgar, one of the most intensely red of all minerals and one currently available from several unusual sources. When occurring in good specimens, realgar is relatively easy to recognize because of its bright red to orange-yellow color coupled with a resinous to greasy luster. It is soft with a hardness of 1.5–2, is sectile to brittle, has a specific gravity of about 3.6, and has good cleavage on {010} and less welldeveloped cleavage on {101}, {100}, {120}, and {110}. Realgar is monoclinic (2/m) and occurs in variably striated prismatic crystals that are reported to reach lengths of 12 cm. It forms contact twins on {100}. It is commonly seen in massive or fine granular forms or as incrustations and inclusions in other minerals. Realgar is simple arsenic sulfide (AsS). Published chemical analyses typically show no other constituents are so close to the ideal composition. The mineral is dimorphous with pararealgar, which is also thought to be monoclinic. On long exposure to light it disintegrates to a yellow powder composed of pararealgar (a polymorph of realgar) commonly together with another poorly described arsenic sulfide. Yet another red polymorph known as beta-AsS is believed to be Jiepaiyu Mine, Shimen, Hunan Province, China

Connoisseur's Choice: Afghanite, Sar-e-Sang, Badakhsham Province, Afghanistan

eldspathoids are peculiar minerals. Compositionally, most F appear to be complex chemical repositories whose major purpose seems to be the accommodation of some ions that might best go elsewhere (such as into a feldspar in the case of aluminum) if there were only enough silica in the system to get things reorganized. Familiar examples include nepheline, leucite, and members of the sodalite and cancrinite groups. Most collectors are familiar with the vibrant blue of sodalite and the yellow of cancrinite; alas, both are typically in massive, generally interesting, though not particularly desirable, specimens. Recently, however, the legendary lapis lazuli (lazurite) occurrence at Sar-e-Sang, Afghanistan, has produced peculiar specimens containing large, relatively sharp lazurite and sodalite crystals in white marble. Associated with these, though much rarer, are exceptional crystals of afghanite, a mineral that was only formally described as a new species in the late 1960s (Bariand 1968). An unusually fine afghanite specimen from this find has been nominated for this issue’s Connoisseur’s Choice (fig. 1). Afghanite is typically blue, though it tends toward colorless in smaller grains and is colorless in thin section. It exhibits {lOTO} cleavage and a conchoidal fracture. Its hardness is 5.5-6, and its density is about 2.6. It has a vitreous luster and is optically positive. It crystallizes in the hexagonal system; its point group is thought to be 6/m 2/m 2/m, although there is debate on its structure and symmetry. Most crystals are tabular and range from stout to thin (slender laths are described in thin sections). Some grains are rounded, exhibiting only poorly developed faces. Afghanite is a complex sodium-calcium-potassium aluminosilicate with the formula (Na,Ca, K)8(Si,Al)lzOz4 (SO,, Cl,CO,),-H,O. It is a member of the cancrinite group (Hogarth 1979), sharing this dlstinction with other interesting species that include bystrite, davyne, franzinite, liottite, and vishnevite. Most afghanite occurrences are related to metasomatized (silicified in part), varyingly dolomitic rocks. These range from skarns resulting from proximity to felsic intrusive igneous activity to altered limestone volcanic ejecta associated with pumice and other pyroclastic materials. One occurrence seems unrelated to igneous activity and may be the result of regional metamorphism of dolomiteand anhydrite-rich evaporites. Associated minerals include various pyroxenes and zeolites in addition to the more familiar lazurite, sodalite, pyrite, and carbonates. There are two North American afghanite occurrences. The first is described as the D4 orebody of the Edwards zinc mine, Edwards, Saint Lawrence County, New York. Here it was found sparsely as rounded grains associated with pargasite, dlopside, oligoclase and lazurite (Hogarth 1979). The other is a small lazurite deposit at the southern end of Baffin Island, Nunavut, Canada, where it is found as rounded, isolated grains in nepheline and is associated with diopside (Hogarth 1979). Afghanite occurs as small, typically colorless transparent crystals associated with a wide variety of feldspathoids in volcanic ejecta collected in the general Mount Vesuvius area, Italy (Parodi, Ballirona, and Maras 1996). Specific localities include Pitigliano (well-formed crystals with vesuvianite, nepheline, apatite, hauyne, sanidine, diopside, wollastonite, and others), Vetralla (crystals with nosean, vonsenite, sanidine, and garnet), Bassano, Sacrofano, and Mount Somma (well-formed crystals with hauyne, phlogopite, and calcite). Afghanite from at least one of these localities was recognized as a potentially new

Connoisseur's Choice: Silver, Uchucchacua, Peru

S be used by humans, with records of silver mines dating back at least six thousand years. For centuries it was far less available than gold, and from that scarcity, coupled with the metal’s general attractiveness and utility, grew its great monetary value. The need of dominant European countries for power-sustaining wealth demanded the acquisition of larger and larger quantities of precious metals, and the impact of this seemingly unquenchable thirst on human actions and destiny molded the development of large geographic areas, including much of western South America, specifically that portion dominated by the great Andes Mountains. Consequently, in keeping with the Andean mineral theme of the 2003 Tbcson Gem and Mineral Show, an exceptional Peruvian silver has been selected as this issue’s Connoisseur’s Choice. Additional information on the intricate relationship between the production of silver and the exploration and developROBEKT H. COOK Department of Geology

Fluorite Occurrences in the Southeastern United States

the southeastern United States, occurring in a variety of geological environments that range from Paleozoic limestones through metal-associated veins and plutonic rocks of the Piedmont and Blue Ridge provinces. Such a broad distribution is not surprising in light of fluorine’s high mobility under a wide range of temperatures and pressures and the universal abundance of calcium. When one eliminates the well-known occurrences such as those of the Elmwood-Gordonsville and Kentucky-Illinois districts and less-important though similar traditional Mississippi Valley–type deposits, there are still dozens of potential specimen-producing localities that are either well known to modern collectors or SOUTHEASTERN UNITED STATES

Connoisseur's Choice: Liddicoatite, Fluor-liddicoatite, and Liddicoatitic Tourmalines, Anjanabonoina District, Madagascar

The tourmaline database, like that of most other families of essential and accessory rock-forming minerals, has expanded dramatically during the past several decades. This has resulted in a better understanding of the structural and chemical complexities of the various members, increased species numbers, and a requirement for hierarchal reorganization. One early outcome of this explosion of knowledge was the identification and description of liddicoatite from Anjanabonoina, Madagascar, as the calcium analogue of the well-known gem and specimen species elbaite (Dunn, Appleman, and Nelen 1977). The relationship between liddicoatite and elbaite seemed clear, and liddicoatite’s fundamental place in the tourmaline family appeared to be established. Here things languished for some three decades. However, the devil is in the details, especially so in mineralogy. In the original description of liddicoatite, one site in the crystal structure was considered to be predominantly occupied by hydroxyl (OH). Recent work, however, has shown that holotype material studied by Dunn, Appleman, and Nelen (1977) actually was fluorine (F)-dominant at this site. This required changing the name of the type material from liddicoatite to fluor-liddicoatite (Henry et al. 2011). To further complicate matters, study of other supposed liddicoatite specimens (Aurisicchio et al. 1999; Breaks, Tindle, and Selway 2008) has reportedly found OH-dominant sites (as Dunn, Appleman, and Nelen described) from other localities, suggesting that liddicoatite as originally defined does indeed exist, perhaps just not at its type Madagascar locality. So, determining that a crystal is actually liddicoatite requires compositional verification, and at least some specimens— perhaps most—need renaming because they are actually fluor-liddicoatite. In their article on tourmaline nomenclature, Henry et al. (2011) suggest the name liddicoatitic tourmalines for compositionally and structurally unrefined “liddicoatite.” However, for purposes of this short review, if a particular tourmaline locality (including the type locality) has been described as producing liddicoatite, this terminology will be used herein as a convenient general name, with the understanding that the distinction between true liddicoatite as originally defined and fluor-liddicoatite may have to await specimen-by-specimen analysis. Liddicoatite is especially important to collectors because of its tendency to occur in complexly color-zoned crystals at its premier Anjanabonoina, Madagascar, type locality, a source that has produced wonderful crystals that lend themselves to the production of sets of thin polished slices Liddicoatite, Fluor-liddicoatite, and Liddicoatitic Tourmalines

Connoisseur's Choice: Wavellite, Mauldin Mountain, Montgomery County, Arkansas

Phosphate minerals are a traditional favorite of many mineral collectors. They are colorful and of widespread geologic and geographic occurrence. Clearly, minerals such as pyromorphite, mimetite, o...

Connoisseur's Choice: Beryl, Variety Morganite San Diego County, California

One of the most popular of all collectible minerals is beryl. Its wide variation in color, particularly in the gem varieties, its tendency to form exceptionally perfect crystals in pocket environments, and its unusually broad distribution make it ubiquitous in dealer stocks and most collections. Its several varieties are seen today from many currently producing localities in countries such as Pakistan, Afghanistan, Brazil, Colombia, Madagascar, Zambia, and the United States. High-quality specimens of the beryl gems, such as emerald and aquamarine, and to a lesser extent heliodor, morganite, bixbite (red beryl), and goshenite, are particularly prized, as are specimens from classic or unusual localities. A few collectors specialize in beryl, assembling magnificent collections that have been displayed at national gem and mineral shows in recent years. From a historical perspective, the United States has had its share of exceptional beryl localities including the emeralds of North Carolina; aquamarines from northeastern pegmatites, Idaho, and Colorado; red beryl from Utah; and, of course, phenomenal specimens from the pegmatites in Southern California. Consequently, in keeping with the California theme of the 2011 Tucson Gem and Mineral Show, morganite, the pink variety of beryl, so well known from the pegmatites of San Diego County, has been nominated for this issue’s Connoisseur’s Choice. It is the second beryl variety to be covered in this column, the first being emerald (Cook 1993). Morganite is, of course, typically described as pink in color. However, it is often a somewhat peach tint when found, Beryl, Variety Morganite