Uses - as a source of valuable ores and minerals, including chromite, platinum, nickel and precious garnet; diamonds are obtained from mica-rich peridotite (kimberlite) in South Africa. New Zealand occurrences - North Cape, Dun Mountain (southeast of Nelson), Fiordland.
Serpentinites and Serpentine Formation
Ultramafic rocks are rare at Earth's surface but are abundant at the oceanic moho, the boundary between the base of the oceanic crust and the upper mantle. During hydrothermal metamorphism, olivine and pyroxene minerals are transformed into or are replaced by serpentine minerals.Serpentine is often confused with jade, but it differs from jadeite jade in having lower density and hardness. Bowenite is a hard variety of serpentine. A light green variety of bowenite, with the misleading name new jade, resembles nephrite, a variety of jade. Korean jade is a bowenite serpentine.
Olivine is usually green in color but can also be yellow-green, greenish yellow, or brown. It is transparent to translucent with a glassy luster and a hardness between 6.5 and 7.0. It is the only common igneous mineral with these properties. The properties of olivine are summarized in the table.
The serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)3Si2O5(OH)4) minerals, resulting from the metamorphism of the minerals that are contained in ultramafic rocks. They may contain minor amounts of other elements including chromium, manganese, cobalt or nickel.
Serpentine rock is apple-green to black and is often mottled with light and dark colored areas. Its surfaces often have a shiny or wax-like appearance and a slightly soapy feel. Serpentine is usually fine-grained and compact but may be granular, platy, or fibrous in appearance.
However, the green serpentine gemstones most often used in jewelry making are part of the serpentine subgroup of stones known as antigorite, as this form of serpentine is safe for drilling and shaping unlike other forms such as chrysotile, which is used in asbestos.
Amphibolite is a rock of convergent plate boundaries where heat and pressure cause regional metamorphism. It can be produced through the metamorphism of mafic igneous rocks such as basalt and gabbro, or from the metamorphism of clay-rich sedimentary rocks such as marl or graywacke.
Chlorite forms by the alteration of mafic minerals such as pyroxenes, amphiboles, biotite, staurolite, cordierite, garnet, and chloritoid. Chlorite can also occur as a result of hydrothermal alteration of any rock type, where recrystallization of clay minerals or alteration of mafic minerals produce chlorite.
The truth is that most serpentine does not contain asbestos. Serpentine is closely associated with gold deposits in the foothills, with the California Gold Rush, and California's history. Serpentine is formed by hydration of rocks (peridotite) that come from the Earth's mantle, the layer beneath the Earth's crust.
Serpentine minerals are made of tiny sheets of silica tetrahedrons that are loosely held together. The weak bonds between these sheets gives serpentine its greasy or scaly look, and slippery feel (like a snake skin).
Serpentine Gemstone Information. Serpentine is a gem-quality hydrated magnesium silicate, usually green, yellowish-green, or brownish-green in color. Serpentine is not just a gemstone, but rather, it is a group of minerals which includes up to 20 different related members.
Marble is a metamorphic rock formed when limestone is exposed to high temperatures and pressures. Marble forms under such conditions because the calcite forming the limestone recrystallises forming a denser rock consisting of roughly equigranular calcite crystals.
Hornfels is a metamorphic rock formed by the contact between mudstone / shale, or other clay-rich rock, and a hot igneous body, and represents a heat-altered equivalent of the original rock. This process is termed contact metamorphism.
Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals.
Serpentine is not a toxic rock. It sometimes contains the fibrous mineral chrysotile asbestos, but chrysotile asbestos is not the form of asbestos that is proven to cause mesothelioma and lung cancer. The rock is also quite pretty, to this geologist's eye. It ranges in color from black to intense jade-green.
Characteristics of Hornfels
The result is often a dense, hard, fine-grained rock that is generally homogenous and exhibits a semi-conchoidal fracture. Hornfels can be almost any color, but black, gray, brown, reddish and greenish rocks are common. A wide range of minerals and mineral groups are encountered in hornfels.The American geologist Sydney Hobart Ball, who wrote an article on "Luminous Gems, Mythical and Real", outlined the history of discoveries about luminescent and phosphorescent minerals. The phosphorescent quality of diamonds when heated by sunlight is usually believed to have been first revealed by Albertus Magnus (c.
Phyllite is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. The protolith (or parent rock) for phyllite is shale or pelite, or slate, which in turn came from a shale protolith.
Flint is a hard, tough chemical or biochemical sedimentary rock that breaks with a conchoidal fracture. It is a form of microcrystalline quartz that is typically called “chert” by geologists. Flint often forms as nodules in sedimentary rocks such as chalk and marine limestones.
Most serpentine rocks are translucent to opaque with a hardness score that can range from 2.5 to 5.5, depending on exact composition. Serpentine is fairly soft and light, with a specific gravity (density) ranging from 2.44 to 2.62, which is slightly lower than quartz. Its luster can be greasy, waxy or silky.
Mafic rock. Mafic rock, in geology, igneous rock that is dominated by the silicates pyroxene, amphibole, olivine, and mica. These minerals are high in magnesium and ferric oxides, and their presence gives mafic rock its characteristic dark colour.
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium
Formation. Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more.
Ultramafic rocks are primarily found in Earth's mantle. Since the mantle represents about 84% of the Earth's volume, most of the Earth is actually ultramafic.
Mafic minerals crystallize at higher temperatures, which Bowen's Reaction Series describes. Therefore, rocks composed of mafic minerals are formed at higher temperatures. They are found in locations deep within the earth and also in tectonic spreading environments, like the seafloor.
Finally, if the magma is quite silica-rich to begin with, there will still be some left at around 750° to 800°C, and from this last magma, potassium feldspar, quartz, and maybe muscovite mica will form.
Kimberlite is an igneous rock, which sometimes contains diamonds. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle.
Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro.
The two most notable are obsidian and dunite. Obsidian is volcanic glass which erupts as a lava flow. Most obsidian is felsic in composition, yet typically it will have a very dark color (dark brown to black). Dunite has an ultramafic composition yet is apple green to yellowish green in color.
