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Petrified Wood September 2012 Mineral of the Month: Petrified Wood PETRIFIED WOOD (QUARTZ var. CHALCEDONY) This month we are featuring quartz from Madagascar in the beautiful and unusual form of petrified wood. Our specimens are actually 225-million-year-old fossils, and our write-up explains the formation, history, and lore of petrified wood. OVERVIEW PHYSICAL PROPERTIES Chemistry: SiO2 Silicon Dioxide, containing small amounts of iron and manganese. Class: Silicates Subclass: Tectosilicates Group: Quartz Subgroup: Microcrystalline Quartz (Chalcedony) Crystal System: Hexagonal Crystal Habits: Massive and compact with no visible crystal structure; petrified wood consists of microcrystalline quartz (chalcedony) that has replaced wood. Color: White, gray, tan, brown, and black to red, pink, brownish-red, yellow, green, and blue; many specimens are varicolored. Luster: Waxy and vitreous to dull Transparency: Translucent to opaque Streak: White Refractive Index: 1.55 Cleavage: None Fracture: Conchoidal to subconchoidal and irregular, brittle to tough. Hardness: Mohs 6.0-7.0 Specific Gravity: 2.62-2.66 Luminescence: Impurities occasionally produce weak green and white fluorescence. Distinctive Features and Tests: Best field marks are hardness, massive or compact structure, relatively low specific gravity, varied colors, occurrence in sedimentary environments, and recognizable ring, bark, and other tree-trunk features. Dana Classification Number: 75.1.3.1 NAME: The name “quartz,” pronounced “kworts,” is derived from the German Quarz, which is derived from the Slavic kwardy, meaning “hard.” “Chalcedony,” pronounced “cal-SED-do-nie,” stems from Chalcedon, an ancient Greek city on the Strait of Bosphorus in Asia Minor. In the term “petrified wood,” also known as “silicified wood” or “agatized wood,” the word “petrified” comes from the Greek petros, meaning “stone.” In European mineralogical literature, petrified wood appears as bois pétrifié, versteinetes Holz, and legno agatizatto. COMPOSITION: Most petrified wood, including our specimens, consists of chalcedony, the microcrystalline variety of quartz [silicon dioxide, SiO2]. Quartz or silica consists of 46.74 percent silicon and 53.26 percent oxygen. Quartz is a member of the silicates, the largest of all 1 September 2012 Mineral of the Month: Petrified Wood mineral classes, in which silicon and oxygen are combined with one or more metals. The basic 4- building block of the silicates is the silica radical (SiO4) , in which a silicon ion is surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron). Microcrystalline quartz or chalcedony consists of microscopic silica grains or fibrous silica crystals and forms in the low temperatures and pressures of shallow environments from the solidification of colloidal silica suspensions in restricted growth spaces. In petrified wood, chalcedony has replaced the original wood structures. Quartz is an allochromatic mineral, meaning that its color is caused not by its essential elemental components or the nature of its crystal structure, but by traces of accessory, color-producing elements called chromophores. Iron and manganese oxides are the primary chromophores in petrified wood. COLLECTING LOCALITIES: Petrified wood is found in Madagascar, Libya, Namibia, the Czech Republic, Greece, Italy, Belgium, France, Romania, Germany, Austria, Russia, Bolivia, Indonesia, Mongolia, Cambodia, China, Japan, and Australia. The American West has many sources, notably in Arizona, California, Colorado, South Dakota, North Dakota, Washington, Nebraska, Texas, Wyoming, Oklahoma, Oregon, New Mexico, and Nevada. Other localities are in Ohio, Louisiana, and Mississippi. HISTORY, LORE, & GEMSTONE/TECHNOLOGICAL USES: Since the stone ages, petrified wood has served as a source of chalcedony for the manufacture of tools and weapons. In medieval times, amulets of petrified wood, because of their age and resemblance to tree wood, were thought to impart longevity to those who wore them. Because trees have circulatory systems, as well as bark and leaves that seemed to correspond to human skin and hair, medieval physicians prescribed tonics of powdered petrified wood to protect health, increase longevity, and alleviate ailments of the circulation system, skin, and hair. Because of its abundance, attractive colors, workability, fine polishing characteristics, and fascinating preservation of wood structures and shapes, petrified wood has long been a popular gemstone and decorative stone. Petrified wood is fashioned into beads for necklaces and bracelets, cabochons, ring stones, cameos, tie tacks and cuff links. As a decorative stone, petrified wood is made into spheres, snuffboxes, bowls, paperweights, figurines, bookends, polished slabs, countertops, tabletops, and clock faces. Since the late 1800s, petrified wood has been a popular souvenir of the American West. Because of its connection to trees, petrified wood has a prominent place in modern metaphysical lore. With branches reaching high toward the heavens and roots set firmly in the Earth, trees are thought to be particularly wise and balanced. Petrified wood is considered a source of ancient and stable energy that connects with nature, imparts longevity, and provides the power, wisdom, and patience needed to maintain spirituality in the everyday, physical world. ABOUT OUR SPECIMENS: Our petrified-wood specimens were collected near Morondava in the Menabe Region on Madagascar’s southwest coast. The Republic of Madagascar, which occupies an island in the Indian Ocean 400 miles off Africa’s east coast, is nearly the size of the state of Texas. During the Triassic Period some 225 million years ago, the Morondava area was covered with dense forests of giant coniferous trees of the gymnosperm genus Aruacaria. Sediments from the surrounding highlands quickly buried many Aruacaria forests in an anaerobic (oxygen-free or oxygen-deficient) environment that inhibited normal, aerobic decay and thus preserved many tree features. Groundwater rich in silica then circulated through these buried tree trunks, dissolving and carrying away cellulose and other organic matter and replacing 2 September 2012 Mineral of the Month: Petrified Wood it with chalcedony. Relatively recent erosion has exposed these petrified trees on the semiarid, desert grasslands east of present-day Morondava. While some fragments of petrified tree trunks appear on the surface, most of the petrified wood is buried in the iron-rich, reddish soil and is recovered by manually digging shallow trenches. Our petrified-wood specimens were collected by Madagascar Minerals, a company that mines, processes, and markets Madagascar gemstones, ornamental stones, and mineral specimens. COMPREHENSIVE WRITE-UP COMPOSITION The term “petrified wood” is something of a misnomer because it incorrectly implies that the wood has turned to stone. Technically, petrified wood is a pseudomorph, a word derived from the Greek pseudēs, “to lie,” and morphē, or “form,” and literally meaning “false form.” In its mineralogical context, pseudomorphs are minerals that have replaced an original mineral (or material) while retaining the shape of that mineral or material. Our specimens of pseudomorphic, silicified wood can therefore be described as “silica-after-wood.” Most petrified wood, including our specimens, consists of chalcedony, the microcrystalline variety of quartz [silicon dioxide, SiO2]. Quartz occurs in two basic forms: macrocrystalline quartz (hereinafter referred to as “quartz”), which consists of visible, well-developed crystals; and microcrystalline quartz or chalcedony, the compact or massive form of silica. Familiar forms of quartz include amethyst, rock crystal, and milky, smoky, and rose varieties, which form visible, transparent-to-translucent crystals. Chalcedony, however, consists of microscopic silica grains or fibers, occurs in many colors, and includes such types as jasper, agate, chert, and flint. Quartz or silica, SiO2, consists of two elemental components: the semimetal silicon (Si) and oxygen (O). Quartz’s molecular weight is made up of 46.74 percent silicon and 53.26 percent oxygen. The cation, or positively charged ion, in the quartz molecule is the silicon ion Si4+ with its +4 charge. The anion, or negatively charged ion, consists of two oxygen ions 2O2- with their collective -4 charge. The balance of these cationic and anionic charges provides the quartz molecule with electrical stability. Quartz is a member of the silicates, the largest mineral class, in which silicon and oxygen are combined with one or more metals. (Quartz, which consists of a semimetal combined with oxygen, is the sole exception.) The silicate minerals make up 93 percent of the total weight of the Earth’s crust. Quartz is the most abundant silicate mineral and is found in virtually all igneous, metamorphic, and sedimentary rocks. The basic building block of the silicates is the 4- silica radical (SiO4) , in which a silicon ion is surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron). In the silicates, silica anions join together with metallic or semimetallic cations in repeating chains to form seven types of structures: independent tetrahedral silicates (nesosilicates); double tetrahedral silicates (sorosilicates); single- and double-chain silicates (inosilicates); ring silicates (cyclosilicates); sheet silicates (phyllosilicates); and framework silicates (tectosilicates). Quartz
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