Mineral of the Month Club October 2015

Transcription

1 TINCALCONITE This month our featured mineral is tincalconite, a rare, pseudomorphic borate from the huge evaporite-mineral deposit at Searles Lake, California. Our write-up explains tincalconite s mineralogy, the uses of borate minerals, and the origin and different types of mineral pseudomorphs. PHYSICAL PROPERTIES Chemistry: Na 2 B 4 O 5 (OH) 4 3H 2 O Basic Hydrous Sodium Borate (Hydrous Sodium Borate Hydroxide) Class: Borates Subclass: Hydrated Borates with Hydroxyl or Halogen Ions Group: Tincalconite Crystal System: Trigonal (often considered a subdivision of the hexagonal system) Crystal Habits: Usually microcrystalline; occurs as aggregates, masses, crusts, and powder; often forms pseudomorphs, especially after borax. Crystals, which are rare, occur in di-rhombohedral and pseudo-octagonal habits. Color: Almost always white; rarely, pale shades of gray or yellow. Luster: Earthy, dull. Transparency: Opaque Streak: White Refractive Index: Cleavage: None Fracture/Tenacity: Hackly, irregular; brittle, crumbly. Hardness: Specific Gravity: Luminescence: Fluoresces pale white under shortwave and long-wave ultraviolet light. Distinctive Features and Tests: Best field marks are its occurrence in borate-rich, evaporite deposits; association with other borate minerals, especially borax; very low density; softness; solubility in water; and frequent pseudomorphic form. Tincalconite, which is nontoxic, has a sweet, metallic taste. Dana Classification Number: NAME: The name tincalconite, pronounced TIN-kahl-con-ite, stems from the Sanskrit word tincal, meaning borax, and the Greek word konis, or powder, the latter alluding to the mineral s frequently powdery form. Alternative names have included mohavite, dehydrated borax, three-hydration borax, and pseudoborax. In European mineralogical literature, tincalconite appears as tincalconit and tincalconita. COMPOSITION & STRUCTURE: Tincalconite, which consists of percent sodium, percent boron, percent oxygen, and 3.30 percent hydrogen, is a hydrous mineral with three attached molecules of water. It is a member of the borate class of minerals, a complex Page1

2 class of about 160 members in which boron and oxygen are combined with one or more metals. Tincalconite is technically an allochromatic or other-colored mineral. However, because it forms as an alteration product of preexisting borate minerals or as a direct precipitate, it is usually quite pure and white in color. Although crystals are rare, tincalconite crystallizes in the trigonal subsystem (often considered a division of the hexagonal system) in di-rhombohedral and pseudo-octagonal habits. Weak atomic bonding explains tincalconite s softness, its poorly structured crystal lattice that prevents the development of large crystals, and its solubility in water. Tincalconite is a low-temperature, low-pressure mineral that occurs in sedimentary layers that underlie playas (intermittent lakes with no outlets) in arid regions. It most often forms from the dehydration of borax and frequently forms pseudomorphs after borax. COLLECTING LOCALITIES: California is the most abundant source of tincalconite specimens, with notable localities in San Bernadino, Kern, Inyo, and Los Angeles counties. Tincalconite also occurs in Nevada. Worldwide, tincalconite is found in China, Ukraine, Argentina, Italy, and Turkey. HISTORY, LORE, & GEMSTONE/TECHNOLOGICAL USES: In the mid-1800s, researchers began studying a basic hydrous sodium borate with three attached water molecules that seemed to represent a previously unidentified mineral. But mineralogists initially did not recognize it as such because it seemed to occur only as an alteration product of borax where mining had artificially created dehydrating conditions, and thus could not be considered natural. But in 1876, drill-core samples taken from 100 feet below the dry surface of California s Searles Lake revealed crystals of this three-hydration borate that had formed by natural precipitation from aqueous solution. Mineralogists then discovered the same three-hydration borate on the Searles Lake shoreline as pseudomorphs after borax, confirming that this borate was a natural mineral compound. In 1877, American mineralogist Charles Upham Shepard ( ) formally named the new mineral tincalconite. Because of its rarity, softness, and oftenpowdery form, tincalconite has no use in jewelry. Collectors value tincalconite specimens for their rarity and their frequent occurrence as pseudomorphs after borax. No metaphysical properties have been assigned to tincalconite. Tincalconite serves as a minor borate ore. Borate minerals are converted to industrial borax, which is used to manufacture heat-resistant glassware, fiberglass, fiber-optic cable, fiber-composite materials, detergents, soaps, paints, motor oils, enamels, ceramic glazes, and coated papers. Borates are obtained by both open-pit and brineextraction mining methods. The United States, which is self-sufficient in borate minerals, has led the world in borate production since the 1880s and now accounts for one-quarter of the 4.1 million metric tons of borates mined worldwide each year. Purified borates now sell for about $400 per metric ton. ABOUT OUR SPECIMENS: Our tincalconite specimens were collected at Searles Lake at Trona in San Bernadino County in southeastern California. Located 25 miles east of Ridgecrest and 25 miles west of the southern limit of Death Valley National Park, Searles Lake is one of a chain of large Pleistocene Epoch sinks (intermittent lakes with no outlets) that originated with glacial runoff from the southern Sierra Nevada. The cyclical evaporation of mineral-rich water from sinks creates lacustrine deposits of sedimentary rocks and evaporite minerals. At Searles Lake, 2.5 million years of evaporite deposition has created a huge, bedded deposit of sodium and Page2

3 potassium carbonates, sulfates, borates, and halides more than 600 feet thick. The lake, which is dry most of the year, covers 50 square miles; its beds contain more than 25 different evaporite minerals. Solution mining is a major industry at Searles Lake. Over the past 140 years, the lake has yielded evaporite minerals valued at $1.5 billion. Searles Valley Minerals, Inc. currently mines the lake bed and extracts 1.7 million tons of evaporite minerals each year. The lake s evaporite-mineral reserves, estimated at four billion tons, are sufficient to support mining at the current rate of production for centuries to come. Searles Lake is the site of one of the world s most popular mineral-collecting field trip the Searles Lake Gem-O-Rama, an October event sponsored by the Searles Lake Gem & Mineral Society. Searles Valley Minerals, Inc. generously provides equipment and personnel to trench the lake bed and pump up slurries of subterranean brines and crystals to make various evaporite minerals accessible to collectors. Nearly 1,000 mineral collectors and visitors attend this annual event. COMPREHENSIVE WRITE-UP COMPOSITION & STRUCTURE Evaporite-type, borate minerals have been featured twice previously as Minerals of the Month. Colemanite [hydrous calcium borate, Ca 2 B 6 O 11 5H 2 0] was featured in February 2003; probertite [basic hydrous sodium calcium borate, NaCaB 5 O 7 (OH) 4 3H 2 O] was the Mineral of the Month in August This month we are featuring another borate mineral tincalconite [basic hydrous sodium borate, Na 2 B 4 O 5 (OH) 4 3H 2 O], a rare pseudomorph with an interesting history. Tincalconite s chemical formula Na 2 B 4 O 5 (OH) 4 3H 2 O shows that it contains four elements: sodium (Na), boron (B), oxygen (O), and hydrogen (H). Its molecular weight is made up of percent sodium, percent boron, percent oxygen, and 3.30 percent hydrogen. Like all molecules, those of tincalconite consist of positively charged cations and negatively charged anions. Anions (and occasionally cations) often occur as radicals, which are groups of ions of different elements that function as entities in chemical reactions. Tincalconite s compound cation consists of two sodium ions 2Na 1+ with a collective +2 charge, along with the borate radical (B 4 O 5 ) 2+, also having a +2 charge. In this borate radical, the four boron ions 4B 3+ have a collective +12 charge, while the five oxygen ions 5O 2- have a collective -10 charge. This provides the sodium-borate cation (Na 2 B 4 O 5 ) 4+ with a total +4 cationic charge. Tincalconite s anion consists of four hydroxyl radicals 4(OH) 1- with a collective -4 charge. This -4 anionic charge balances the +4 cationic charge to provide the tincalconite molecule with electrical stability. The 3H 2 O in tincalconite s chemical formula indicates that it is a hydrous (or hydrated) mineral with three molecules of water (3H 2 O) attached to each parent molecule. Attached water molecules, known as water of hydration, are electrically neutral and do not affect the electrical balance of the parent molecule. Water molecules have an unusual atomic configuration, with two hydrogen ions grouped together on one side of a large oxygen ion. These hydrogen ions retain a small, residual positive charge, while the opposite side of the molecule, dominated by the large oxygen ion, retains a small negative charge. The resulting polarity enables water molecules Page3

4 to behave as tiny dipole magnets that can attach themselves to other molecules by a weak attraction called hydrogen bonding. Tincalconite is a member of the borates, a complex class of about 160 minerals in which boron and oxygen are combined with one or more metals. Many borate minerals also contain water molecules; some contain hydroxyl and halogen ions. Still others are compound boratephosphates, borate-sulfates, and borate-arsenates. The basic building block of borate minerals is the borate radical (BO 3 ) 3-, a triangular structure in which three oxygen ions 3O 2- surround a single boron ion B 3+. These triangular borate structures link together in infinite ring and chain arrangements. By sharing varying numbers of their electrons, boron ions and oxygen ions can join together in many combinations, such as (B 3 O 5 ) 1-, (B 4 O 5 ) 2+, (B 5 O 7 ) 1+, (B 5 O 9 ) 3-, and (B 6 O 11 ) 4-, making possible the large number of borate minerals. The two general types of borate minerals are anhydrous (without attached water molecules) and hydrous (with attached water molecules). Anhydrous borates are rare, chemically stable, and occur in igneous and metamorphic environments; they are dark in color, opaque, and relatively heavy. Hydrous borates, such as tincalconite, are much more abundant and occur as secondary minerals in sedimentary environments, usually in the beds of playas (intermittent lakes with no outlets). Hydrous borates tend to be brittle, soft, low in density, colorless or white, and transparent. Although tincalconite rarely forms macrocrystals, it crystallizes in the trigonal subsystem (often considered a division of the hexagonal system), which is characterized by three axes of equal length with angles between them of other than 90 degrees. The basic trigonal shape is the rhombohedron, a six-sided polygon with parallelogram sides; the pseudo-hexagonal habit is the most common. The tincalconite molecule exhibits three different types of atomic bonding: covalent, ionic, and hydrogen. Strong covalent bonding exists within the borate radical, where boron ions share electrons with oxygen ions. Much weaker ionic bonding joins the sodium ions and hydroxyl ions to the borate radicals. The water molecules are attached by extremely weak hydrogen bonding. The tincalconite lattice consists of infinite, stacked, flat sheets of the borate radicals (B 4 O 5 ) 2+, with inter-sheet spaces accommodating both the attached water molecules and the ionically bonded sodium ions and hydroxyl ions. Weak ionic and hydrogen inter-sheet bonding account for tincalconite s very low hardness (Mohs ), its poorly structured crystal lattice that prevents the growth of well-developed crystals, and its solubility in water. When tincalconite is placed in water, its ionic and hydrogen bonds slowly part, freeing sodium ions and hydroxyl ions that impart a diagnostic, sweet, metallic taste to the mineral (tincalconite is nontoxic). Tincalconite s very low density (specific gravity 2.14) is explained by the low atomic weights of its constituent elements (sodium, 22.99; boron, 10.81; oxygen, 16.00; hydrogen, 1.01) and also by the large inter-ionic distances within its crystal lattice that result from weak ionic bonding. The Dana mineral-classification number first identifies tincalconite as a hydrated borate containing hydroxyl or halogen ions (26). It is then grouped into the fourth (4) of six subdivisions and assigned to the tincalconite group (2) as the first (1) and only member. Page4

5 Tincalconite is a low-temperature, low-pressure, secondary mineral that occurs in sedimentary layers that underlie playas in arid regions. The geochemical process that creates borate-rich, sedimentary deposits begins when deep magmas move upward through volcanic conduits to emplace basalt and other igneous rocks containing small amounts of dark, dense, insoluble, anhydrous borate minerals. Upon contact with water at elevated temperatures, these anhydrous borates metamorphose into soluble hydrous borates that dissolve in groundwater and are transported to the surface. In arid regions, this borate-rich water sometimes collects in playas. During lengthy and repeated cycles of water replenishment and evaporation, borates crystallize on the lake bottoms to form bedded deposits of borax [basic hydrous sodium borate, Na 2 B 4 O 5 (OH) 4 8H 2 O] and other heavily hydrated borates. Subsequent low-temperature metamorphism and dehydration alters these minerals into rarer, less-hydrated borates such as tincalconite. Small amounts of tincalconite also form from direct precipitation of borate-rich solutions. Tincalconite is technically an allochromatic or other-colored mineral, in which colors are derived from traces of accessory chromophoric (color-causing) elements. However, because of the nature of tincalconite s formation either by the alteration of preexisting borate minerals or by direct precipitation it is usually quite pure and almost always white, a color that is due to the reflection of all wavelengths of incident white light from the adjoining faces of surface microcrystals. Due to this high level of surface reflectivity, microcrystalline tincalconite is opaque, with very little, if any, light penetrating into its interior. The key element in all borate minerals is boron, a hard, brittle, light, semimetallic element with a low atomic weight of 10.81, higher than that of beryllium, but below that of carbon. Because of its strong chemical affinity for oxygen, boron does not occur free in nature. Ranking 38 th among the elements in crustal abundance, boron is about as common as lead. Although boron-bearing minerals are widely distributed, concentrated deposits large and rich enough to justify mining are unusual. Boron s properties are similar to those of silicon and carbon. It has an extremely high melting point of 3956 F. (2180 C.) Because of its excellent neutron-absorbing capability, elemental boron has specialized applications in the nuclear fields, mainly in neutron-detecting instruments and neutron-shielding materials, and as radiation-control absorbers in nuclear reactors. COLLECTING LOCALITIES Our tincalconite specimens were collected at Searles Lake at Trona in San Bernadino County, California, which is the type locality for the species. California s other tincalconite localities include the Boron Pit near Boron in the Kramer district, Kern County; the Tick Canyon borate deposit near Lang in Los Angeles County; and the Ryan borate deposit near Ryan, the Furnace Creek borate district, and Eagle Borax Spring in Death Valley, all in Inyo County. Tincalconite also occurs at Fish Lake Marsh in Esmeralda County, Nevada. Page5

6 In China, tincalconite occurs at the Juhangtu borate deposit at Yashatu in Delingha County and the Da Quidam salt lake in Da Quidam County, both in Haixi Autonomous Prefecture, Quinghai Province. Other Chinese localities are in the Tibet Autonomous Region and include Guojialin and Duliali lakes in Nyima County, and the Bangkog salt lake in Baingoin County, both in Nagchu Prefecture; and Qia Chaka and Chalaka, Gê gyai County in Ngari Prefecture. In Ukraine, tincalconite occurs at Vulkonovka, Novoselovka, and Bondarenkovo on the Crimean Peninsula near Kerch, Krym. Argentina s sources are the Tincalayu Mine at Salar del Hombre Muerto, Salta; and the Loma Blanca borate deposit at Coranzuli, Jujuy. Tincalconite also occurs at Larderello, Pomerance, Picsa Province, Tuscany, Italy; and the Sankaya borate deposit at Kirka, Eskişehir Province, Central Anatolia Region, Turkey. JEWELRY AND DECORATIVE USES Because of its rarity, softness (Mohs ), dull luster, and often-powdery form, tincalconite has no use in jewelry. Collectors value tincalconite specimens for their rarity and their frequent occurrence as pseudomorphs after borax. HISTORY & LORE Borate minerals and compounds were known in antiquity. Babylonian gold workers were employing borate minerals as metallurgical fluxes before 2000 B.C. By the 8 th century A.D., Arabian metallurgists had adopted borates as their standard soldering fluxes, while the Chinese were utilizing borate-based ceramic glazes. Borate minerals from Tibet were introduced to Europe in the late 13 th century by Venetian traveler Marco Polo ( ). Tibet was the only source of borate minerals until the early 1800s, when small deposits of sassolite [natural boric acid, hydrogen borate, H 3 BO 3 ] were discovered at volcanic steam vents in Tuscany, Italy. By 1850, borates, despite being costly and obtainable only from limited sources in Tibet, Turkey, and Italy, were popular medicinal remedies for everything from acne and dandruff to bunions and epilepsy. Borates become widely affordable and available in quantity only after large sedimentary deposits were discovered in California s Mojave Desert in the late 1800s. The word borax which refers generally to any borate mineral, entered the English language in the 14 th century. The word stems from the Arabic būraq, meaning white and alluding to the color of borate salts. Boron (see Composition & Structure ) was not recognized as an element until 1808, when French chemist Joseph Louis Gay-Lussac ( ) and British chemist Sir Humphry Davy ( ), working independently, isolated it as a gray, amorphous powder. Davy named the new element boron after borax, the most abundant boron-containing mineral. Scientists began to differentiate the borates in the mid-1700s by recognizing borax as a distinct mineral species. In the mid-1800s, mineralogists discovered a basic hydrous sodium borate with a borax-like chemistry, but containing three attached water molecules, rather than the eight water molecules of borax. This three-hydration compound, which occurred as pseudomorphs after borax, was initially thought to have formed only after the original borax crystals had been mined Page6

7 and thus artificially exposed to arid, dehydrating conditions. Because mineralogists were uncertain if it was natural, this three-hydration compound, variously known as mohavite, dehydrated borax, and pseudoborax, was not immediately recognized as a new mineral. Finally in 1876, drill-core samples taken from 100 feet below the dry surface of California s borate-rich Searles Lake revealed di-rhombohedral and pseudo-octagonal crystals of the threehydration compound that had developed by natural precipitation from aqueous solution. Months later, mineralogists also discovered the three-hydration compound along the Searles Lake shoreline as natural pseudomorphs after borax. After these discoveries proved that the three-hydration borate compound did indeed occur naturally, mineralogists recognized it as a new mineral. In 1877, American mineralogist Charles Upham Shepard ( ) named the new mineral tincalconite after the Sanskrit word tincal, meaning borax, and the Greek word konis, meaning powder, and alluding to the mineral s often-powdery form. No metaphysical properties have been assigned to tincalconite. TECHNOLOGICAL USES As a very minor borate ore, tincalconite is mined along with the four major borate ores: borax; colemanite [hydrous calcium borate, Ca 2 B 6 O 11 5H 2 0]; ulexite [basic hydrous sodium calcium borate, NaCaB 5 O 6 (OH) 6 5H 2 O]; and kernite [basic hydrous sodium borate, Na 2 B 4 O 6 (OH) 2 3H 2 O]. The world s largest borate mine is the Boron Pit, an open-pit mine in California s Mojave Desert. Borates are also recovered by brine-extraction and solutionextraction at other localities, including Searles Lake. The mixed, crude borates that are recovered are dissolved in water, purified, separated by fractional crystallization into individual borate compounds, then converted to synthetic, anhydrous sodium borate (Na 3 BO 3 ), which is known by the industrial term borax. About 60 percent of all industrial borax is used to manufacture fiberglass, fiber-optic cable, and heat-resistant, Pyrex -type glassware. Another 10 percent is used in ceramics, ceramic glazes, and advanced fiber-composite materials. Smaller amounts go into agricultural fertilizers, detergents, soaps, paints, motor oils, enamels, welding-rod flux coatings, and coated papers. Borates are among the few minerals in which the United States is self-sufficient. The United States, the world s leading source of borates since the 1880s, accounts for one-quarter of the 4.1 million metric tons of borates mined globally each year. Turkey ranks second in borate production, followed by Argentina and Chile. Purified borates currently sell for about $400 per metric ton. MINERAL PSEUDOMORPHS Your tincalconite specimen, which is technically described as tincalconite-after-borax, is an example of a pseudomorph a mineral that has replaced, altered, or coated a previously deposited mineral, while retaining the exterior shape of the original mineral. Derived from the Page7

8 Greek terms pseudēs, to lie, and morphē, meaning form, the word pseudomorph literally means false form. Pseudomorphic relationships among minerals are conventionally expressed in two ways. With A representing the original mineral and B representing the replacement mineral, the relationship can be written as A>B, or as B-after-A. Our specimens may thus be described as borax>tincalconite, meaning borax to tincalconite or more commonly as tincalconite-after-borax. Our tincalconite-after-borax specimens formed when tincalconite replaced borax. Replacement or substitution is one of several types of pseudomorphism. Pseudomorphism can be chemical or structural in nature; it results from changes in the chemical or physical environment after the crystallization of the original mineral. The ability of one mineral to replace another while retaining the original mineral s external shape is divided into two general processes: paramorphism and pseudomorphism. Paramorphism: Paramorphism (literally creation of closely related forms ) involves structural, but not chemical, change. It occurs when a mineral alters its internal crystal structure while retaining both its original chemical composition and its external crystal shape. One example is calcite-after-aragonite. Both calcite and aragonite are polymorphic forms of calcium carbonate (CaCO 3 ); they share the same chemistry, but have different crystal structures, with calcite crystallizing in the rhombohedral system and aragonite in the orthorhombic system. Because it is structurally unstable, aragonite eventually converts to calcite to form the paramorph calciteafter-aragonite, which has calcite s internal rhombohedral structure, but aragonite s external orthorhombic shape. Pseudomorphism: Pseudomorphism involves chemical change and occurs by replacement, alteration, or incrustation. In pseudomorphism by replacement, a mineral with a different chemistry and a different crystal structure completely replaces the original mineral. The two minerals involved must differ substantially in solubility, enabling the original mineral to dissolve before, or as, it is being replaced by the second mineral. An example of pseudomorphism by replacement is quartz-after-aragonite. Quartz [silicon dioxide, SiO 2 ] is insoluble and crystallizes in the hexagonal system, while aragonite is soluble and crystallizes in the orthorhombic system. The resulting pseudomorph, quartz-after-aragonite, has the internal, hexagonal structure and chemical composition of quartz, but exhibits the external, orthorhombic shape of aragonite. Another familiar example of pseudomorphism by replacement is petrified wood, in which quartz has replaced organic wood on a cell-by-cell basis, while retaining the external form, and even certain internal structures, of the original wood. In pseudomorphism by alteration, the original mineral is altered by chemical oxidation, chemical reduction, or dehydration into a secondary mineral, while retaining the external shape of the original mineral. The original mineral can be completely or partially altered, with changes to both its chemistry and its crystal structure. An example of pseudomorphism by alteration is malachite-after-azurite, specimens of which are usually only partially altered and thus consist of both species. Malachite [basic copper carbonate, Cu 2 (CO 3 )(OH) 2 ] and azurite [basic copper carbonate, Cu 3 (CO 3 ) 2 (OH) 2 ] have similar but not identical chemistries. Both crystallize in the monoclinic system, but in different crystal habits. In the pseudomorph malachite-afterazurite, azurite has chemically oxidized into malachite, while retaining the exterior shape of the original azurite. Our current Mineral of the Month, tincalconite-after-borax, is an example of Page8

9 pseudomorphism by alteration in which dehydration has altered borax into tincalconite, while retaining the exterior shape of the original borax. In pseudomorphism by incrustation, a new mineral coats an original mineral with a thin film. An example is chalcocite-after-pyrite, which forms when chalcocite [copper sulfide, Cu 2 S, orthorhombic] coats pyrite [iron disulfide, FeS 2, cubic]. In chalcocite-after-pyrite, dark-gray chalcocite assumes the shape of the pyrite cubes and pyritohedrons. In pseudomorphism by incrustation, the incrusting minerals are called epimorphs or outer forms ; the enclosed minerals are called endomorphs or inner forms. Endomorphs can sometimes dissolve away, leaving hollow cavities or negative molds called perimorphs or enclosed forms. Epimorphs, endomorphs, and perimorphs can have complex relationships. Consider, for example, a calcite crystal (an endomorph) that becomes coated with dolomite [calcium magnesium carbonate, CaMg(CO 3 ) 2 ]. The calcite can dissolve away, leaving a hollow, epimorphic, dolomite shell in the shape of the original calcite crystal. This void could then fill with another mineral such as halite [sodium chloride, NaCl], which crystallizes in the isometric system. The final result is halite-after-calcite, in which halite has the rhombohedral exterior shape of the original calcite crystal. This example would be correctly described as a pseudomorph of perimorphic origin. Many minerals have pseudomorphic relationships. Quartz alone takes part in more than 100. Because of its chemical stability, durability, and very low solubility, quartz itself is rarely replaced, but it is almost always the secondary mineral that replaces an original mineral or material. Some common quartz pseudomorphs are quartz-after-anhydrite [calcium sulfate, CaSO 4 ], quartz-after-barite [barium sulfate, BaSO 4 ], quartz-after-calcite [calcium carbonate, CaCO 3 ], quartz-after-enargite [copper arsenic sulfide, Cu 3 AsS 4 ], quartz-after-epidote [basic calcium aluminum iron silicate, Ca 2 Al 2 (Fe,Al)Si 3 O 12 (OH)], quartz-after-fluorite [calcium fluoride, CaF 2 ], and quartz-after-gypsum [hydrous calcium sulfate, CaSO 4 2H 2 O]. In recent years, mineral pseudomorphs have been appearing more frequently in museums and at gem-and-mineral-show exhibits and have become quite popular among mineral collectors, some of whom have built extensive collections that focus exclusively on pseudomorphs. ABOUT OUR SPECIMENS Our tincalconite specimens were collected at Searles Lake at Trona in San Bernadino County, California. Searles Lake, a leading source of both commercial evaporite minerals and evaporitemineral specimens, is located in southeastern California s San Bernadino County, 25 miles east of the city of Ridgecrest and 25 miles west of the southern limit of Death Valley National Park. It is one of a chain of large Pleistocene Epoch sinks (intermittent lakes with no outlets) that includes Owens, China, Panamint, and Manly lakes, all of which were created some 2.5 million years ago by glacial runoff from the eastern slopes of the southern Sierra Nevada. Lacustrine deposits of evaporite minerals form when surface water from rain or snowmelt drains into sinks. Sinks are created in arid climates where incoming water flows only seasonally. This incoming water, which can originate nearby or hundreds of miles away, often leaches mineral Page9

10 salts from soil and rock and becomes highly saline. Dissolved minerals typically include the halides, sulfates, carbonates, bicarbonates, and borates of such alkali and alkaline-earth elements as lithium, sodium, potassium, calcium, and magnesium. Because sinks have no outlets, their saline water is lost only through evaporation, which concentrates the dissolved salts. When the resulting brines become saturated, the dissolved minerals precipitate out of solution to crystallize on the lake bed. Sink water often evaporates entirely during seasonal cycles, causing complete precipitation and deposition of all the dissolved minerals. Sinks range in size from small ponds to large lakes many miles across. The resulting evaporite deposits also vary greatly in size; some are only a few feet thick, but others, given optimum conditions of sink size, incoming water volume, climatic aridity, and water salinity, can be hundreds of feet thick. The high salinity of the Searles Lake feed water is attributed to large numbers of regional hot springs and to Pleistocene Epoch mountain glaciers that ground rock into fine powder, both of which increased mineral solubility. The stratigraphic record indicates that Searles Lake once contained brackish water more than 600 feet deep. Lake levels fluctuated in rhythm with the advances and retreats of the Sierra Nevada glaciers. Geologists have identified a sequence of salt beds and mud beds deposited during the last 150,000 years that represent more than 30 major lake levels. Evaporite precipitation is a complex process that depends upon a combination of temperature, ph, and qualitative and quantitative water salinity. Because each evaporite mineral has its own solubility level, precipitation occurs in specific sequences with carbonates first, followed by borates, sulfates, and halides. During the cyclical deposition process, crystallized evaporite minerals sometimes redissolve and precipitate again later. Evaporite beds are usually interspersed with beds of mud and silt that formed when particulate and organic matter was carried into the sink along with the saline water. Today, Searles Lake is a huge, bedded deposit of sodium and potassium carbonates, sulfates, borates, and halides that is at least 600 feet thick. The lake itself covers 50 square miles and contains more than 25 different evaporite minerals. Searles Lake and other regional playas provided salt for Native Americans. By the mid-1800s, Searles Lake, first named Slate Range Playa after a nearby mountain range, had also become familiar to American prospectors and explorers. At this site in 1862, a frustrated American gold prospector named John Wemple Searles ( ) collected surface encrustations of a white salt which chemists subsequently identified as borax [basic hydrous sodium borate, Na 2 B 4 O 5 (OH) 4 8H 2 O]. Searles did nothing about his discovery until 1872, when he learned that a Nevada dry lake was being mined for borax. Now aware of the potential value of his own discovery, Searles, along with his brother Dennis, returned to Slate Range Playa to stake a section (640 acres) of land. The following year, the Searles brothers established the San Bernadino Borax Mining Company and began shipping borax by mule-drawn wagons 175 miles west to the harbor at San Pedro, California. Although Searles limited his mining operations to surface digging, he also drilled exploratory wells that revealed continuous beds of evaporite minerals to a depth more than 600 feet. After John Searles died in 1897, Slate Range Playa was renamed Searles Lake in his honor. Although Searles company then ceased operations, other mining companies arrived to employ solution-mining methods, drilling wells and pumping subterranean brines to the surface to recover the mineral salts. Today, solution mining at Searles Lake remains a major industry that Page10

11 is now controlled by Searles Valley Minerals, Inc., which extracts 1.7 million tons of evaporite minerals each year. Over the past 140 years, Searles Lake has yielded evaporite minerals worth $1.5 billion. The lake s evaporite-mineral reserves, estimated at four billion tons, are sufficient to support mining at the current rate of production for centuries to come. In recognition of its rich mining history, Searles Lake was declared a California Registered Historic Landmark in Searles Lake is the site of one of the world s most popular, annual, mineral-collecting field trip the Searles Lake Gem-O-Rama, held each October and sponsored by the Searles Lake Gem & Mineral Society. Searles Valley Minerals, Inc., generously provides equipment and personnel to trench the lake bed and pump up slurries of subterranean brines and crystals to make various evaporite minerals accessible to collectors. This event attracts more than 1,000 mineral collectors and visitors annually. Your tincalconite specimen is a pseudomorph of tincalconite-after-borax. Borax crystallizes in the monoclinic system, usually as clusters of well-formed, stubby, prismatic crystals. Your specimen consists of white, opaque tincalconite with a dull, earthy luster and a powdery feel that retains the faces and edges of the original borax crystals. Hefting the specimen in your hand reveals a distinct lightness that reflects its very low density (specific gravity ). Your specimen is very soft at Mohs and is easily scratched with a fingernail. Some specimens may show small sections of a colorless, transparent-to-translucent, crystalline material. These are remnants of the original borax that has not yet altered to tincalconite. The sections of finegrained, gray sediments are the lithified remains of the strata of mud and silt that were originally deposited with the borate minerals. Your tincalconite specimen is a textbook example of a mineral pseudomorph and a souvenir of California s Searles Lake, one of the world s greatest evaporite deposits. References: Dana s New Mineralogy, Eighth Edition; Encyclopedia of Minerals, Second Edition, Roberts, et al, Van Nostrand Reinhold Company; 2008 Glossary of Mineralogical Species, J. A. Mandarino and Malcolm E. Back, The Mineralogical Record, Inc.; Mineralogy, John Sinkankas, Van Nostrand Reinhold Company, 1993; Rocks and Minerals, Joel Arem, Geoscience Press, 1991; Economics of Boron, 9 th Edition, Roskill Information Services, 1998; Borates, D. E. Garrett, Academic Press, 1998; Basic Geology and Chemistry of Borates, J. D. Smith, Ceramic Engineering and Science Proceedings, Volume 22, 2001; Borax, Borates, and Boron, Steve Voynick, Rock & Gem, August 2003; The Crystal Structure of Tincalconite, Carmello Giacovazzo, Silvio Mancuetti, and Fernando Scordari, American Mineralogist, Volume 58, 1973; Salines in the Owens, Searles, and Panamint Basins, Southeastern California, Hoyt S. Gale, USGS Bulletin 580, United States Geological Survey, 1915; Tincalconite Crystals from Searles Lake, San Bernadino County, California, A. Pabst and D. L. Sawyer, Bulletin of the Mineralogical Society of France, Volume 1, 1878; Borax from California, Charles Upham Shepard, Bulletin of the Society of Mineralogists, 1877; On the Nature of Tincalconite, Rudy L. Luck and Ge Wang, American Mineralogist, Volume 87, 2002; Boron, Marc A. Angulo and Robert D. Crangle, Jr., 2013 Minerals Yearbook, United States Geological Survey. Steve Voynick (C) copyright Celestial Earth Minerals Page11

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