Sometimes water-rich hot fluids generated from the cooling molten material and rock separate from the main magma and bring with them the exotic elements that do not easily fit into the common silicate mineral crystal structures that includes ferromagnesian silicates, quartz, feldspar, mica, etc.
Quartz is one of the most abundant minerals in the crust. It is the most abundant in "gem" so all varieties are considered "semiprecious. Pure, clear quartz is called "rock crystal" Figure The combination of trace impurities and exposure to radiation give other varieties of quartz their color. Smoky quartz gets its dark color from silicon atoms set free in the crystalline structure of quartz Figure Amethyst , citrine , and rose quartz have traces of iron and other metals in the crustal structure and are also discolored from natural radiation exposure in the environments where they form Figures and Heating colored varieties of quartz can undue the effects of radiation exposure and alter the color of the mineral.
Most commercially sold citrine gems are actual heat-treated amethyst or smoky quartz. Quartz has different crystal structures. Silica Si02 has a polymorph an alternative crystal structure called moganite. Quartz and moganite combine in microcrystalline size to form the mineral chalcedony Figure Different varieties of quartz combine to form different kinds of stones that show contrasting bands or patches of color.
These include agate, sard, onyx, carnelian, heliotrope, and jasper. However, most of these forms also occur in sedimentary or metamorphic settings, not entirely igneous.
For instance, fossil wood is frequently preserved as brightly colored varieties of quartz and is abundant in deposits rich in volcanic ash Figure Chemical interactions between the volcanic ash, groundwater, and wood buried in the volcanic ash replace the organic tissue of the wood with silica in various forms of silica and other minerals. Quartz varieties with different trace impurities Fig. Rock crystal is pure quartz. Quartz is one form of silica SiO 2 Fig.
Smoky quartz is dark from radiation exposure Fig. Amethyst gets it color from both traces of iron and radiation exposure over time. Rose quartz gets its color from traces of titanium, iron, or manganese. Chalcedony is a variety of microcrystalline quartz with a botryoidal appearance. Fossil wood can be wood replaced by silica and other minerals derived from volcanic ash. Volcanoes and Volcanic Features A volcanic cone is a hill or mountain formed by the accumulation of volcanic material around a vent where magma reaches the surface Figure The size and shape of a volcanic cone depends on the volume of material ejected over time, the composition and temperature of the material lava, rock and gases vented from the volcano, and the nature of the eruption explosive or otherwise.
There are a variety of different kinds of volcanoes. As discussed above, different plate-tectonic settings produce different kinds of volcanoes Figure Most volcanoes of the world are associated with plate boundaries, either divergent boundaries spreading centers or convergent boundaries volcanic arcs associated with subduction zones see Figure in Chapter 5.
Volcanic eruptions release a variety of material including lava, tephra ash, cinders and rock fragments , gases, and water. Volcanic eruptions can be gentle venting of fluid lava to catastrophic explosions that blow volcanoes apart and scatter ash and material over large areas Figures and In general, very hot magmas produce more gentle eruptions that vent gases continuously where they erupt and pour lava on the surface that can flow under the influence of gravity over long distances.
Hot magmas occur in association with spreading centers and hotspots. These hot magmas are mafic to ultramafic in composition. Cooler magmas associated with subduction zones tend to be rich in dissolved water and gases.
As this magma approaches the surface, the water and gas trapped in the cooling magma escapes producing great pressure that expands explosively causing tremendous eruptions of clouds of steam and ash Figure Most of the material falls to the earth in the vicinity of the eruption piling up to build up tall volcanic cones.
These eruptions can occur suddenly and be tremendous, blowing entire mountain-size volcanoes apart, only to be rebuilt by later eruptions. Explosive volcanoes tend to be felsic to intermediate in composition. Features Associated with Volcanoes and Volcanism Lava flows are lava flowing on the surface under the influence of gravity.
The term lava flow also applies to a deposit of volcanic rock formed from lava flowing and cooling on the land's surface Figure A cinder cone is a cone-shaped hill formed around a volcanic vent by fragments of lava blocks, cinder, ash blown out during eruptions Figure Cinder cones and lava flows can for from a vent or series of vents associated with an eruption Figure A shield volcano is broad, domed volcano with gently sloping sides, characteristic of the eruption of fluid, basaltic lava.
The large volcanoes on Hawaii are shield cones Figure Shield cones are formed from very hot lavas. A composite cone also called stratovolcano is a typically tall and large, steep volcanic cone built up of many layers of both lava and pyroclastics tephra, pumice, and volcanic ash , often created by a series of cyclic eruptions in which pyroclastics are created by explosive eruptions until the vent is open, then lava flows occur Figure Most large continental volcanic cones are this type.
A dome volcano is a volcano composed of lava domes; a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Lava domes can vary from basalt to rhyolite in composition although most preserved domes tend to have high silica content Figure A fissure eruption is a volcanic eruptions along rift fault zones that can flood large area with basalt flows Figure In prehistoric times, great fissure eruptions have occurred.
F lood basalt s are the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Another older name is trap basalt. A crater is a large bowl-shaped vent or collapsed top of a volcano created by explosive eruptions Figure A crater may also be a large bowl-shaped hole created by an meteor or asteroid impact and explosion. A caldera is a very large volcanic crater, typically one formed by a major eruption explosion or the inward collapse of a volcanic cone following an eruption Figures and Large scale plutonic features: these features are typically extensive, measurable or mapable in the range of tens to hundreds of miles in size.
A batholith is a great mass of igneous rock, extending to great depths, formed from extensive magmatic intrusions plutons over a long period of time and throughout a region, typically associated with volcanic arcs. For example, the core of the Sierra Nevada Range in California is the exposed remnant of a great batholith Figure It is nearly miles long and up to 70 miles wide.
A volcanic field is an area of the Earth's crust that is prone to localized volcanic activity. They usually contain tens to hundreds of volcanoes, vents and lava flows. Volcanic fields may have hundreds of vents forming volcanoes and lava flows intermittently over long periods of time millions of years. Volcanic fields may be associated with plate boundary regions or hotspots in the mantle being overridden by continental crust as in Yellowstone , Figure or oceanic crust as in Hawaii , Figure A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust oceanic or continental usually in the interior of a tectonic plate to form volcanic features on the surface.
Examples include the Hawaii or Yellowstone hotspots. Figure A volcanic arc is a generally curved or linear belt of volcanoes above a subduction zone, including the volcanic and plutonic rocks formed there. When magma reaches the surface it becomes lava. Types of volcanoes are based on shape of their cones.
Gas and lava venting at Pu'u'o'o Volcano on Hawaii. Hawaii volcanoes produce some of the hottest lava on Earth. Explosive volcanic eruptions like this one in the Aleutian volcanic chain are associated with relatively "cool" and "wet" magmas. When magma containing dissolved water and gases is released in a volcanic eruption it expands hundreds of times in volume creating ash-filled clouds.
Hot flowing lava on Hawaii. The lava behaves predictably enough for visitors to approach close enough to view basalt rocks and landforms forming. The cinder cone formed around a vent, lava flowed from the base of the cone. Shield Cone Halualai Volcano, Hawaii Gentle slope formed from numerous hot fluid basaltic lava flows over time.
Magnetism in Gemstones. An Effective Tool for Gem Identification. Green Paraibas tend to have higher concentrations of manganese than Paraibas of other colors, and consequently green Paraibas as a whole show the strongest magnetic attraction. The green gems pictured below both show a Drag response, and the gem on the left has the highest measure magnetic susceptibility of any Paraiba Tourmaline tested for this study.
Paraiba Tourmalines. Overview of Paraiba Tourmalines. Elbaite Tourmalines whose colors are influenced by copper impurities are referred to in the trade as Paraiba Tourmalines, cuprian Tourmalines or copper-bearing Tourmalines. Copper imparts blue color.
Paraibas are rare. They were discovered in the state of Paraiba, Brazil in , but since they have also been mined in Nigeria and Mozambique. Paraiba Tourmaline Rough Crystals. Total,Weak to Strong. Color Varieties. Blue : Copper imparts the blue color that is associated with Paraiba Tourmalines.
Pure blue Paraibas are colored entirely by copper ions, which exist in concentrations too low to be magnetically detectable. Color intensities range from pale blue to deep blue. An Inert response by any blue Tourmaline can be considered diagnostic for Paraiba Tourmaline colored entirely by copper. The graph below shows the measured magnetic susceptibilities of 25 copper-bearing Tourmalines tested in this study. Each individual colored dot represents a single gem.
Light blue Tourmalines colored by iron and iron-iron charge transfer can look identical to blue Paraiba Tourmalines colored by copper see photos below , but magnetic susceptibilities can range much lower for Paraiba Tourmalines. The diamagnetic light blue gem shown below left is colored entirely by copper, and the strongly magnetic pale blue gem on the right is colored entirely by iron.
This concludes our discussion of magnetism in Tourmalines. For a comprehensive look at magnetism in the Garnet Group, which is even more diverse than Tourmaline, see our 12 page report on Garnets. Magnetic testing may be useful in separating light green Paraiba Tourmalines colored by copper and manganese from light green Tourmalines colored by iron, which may appear identical in color. Magnetic testing cannot be used to separate medium green to dark green Paraibas from medium to dark green "Verdelite" Tourmalines colored by iron, as both types can show a Drag response and have similar magnetic susceptibilities.
Pictured below are a light green Paraiba pear and a light green cuprian Tourmaline oval with an unusual yellow-green hue. Both gems show high magnetic susceptibility. In the yellowish green copper-bearing Tourmaline on the right, manganese and probably vanadium rather than copper are primarily responsible for the unusual color, and hence we do not refer to it as a true Paraiba-type Tourmaline.
These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder. Blue Tourmaline Colored by Copper. The amount of copper impurities in allochromatic Paraiba Tourmalines can be very small.
Researchers have found that copper oxide concentrations as low as 0. At these levels, copper in Paraiba Tourmalines cannot be detected with a magnet, as copper oxides are 30 times less paramagnetic than iron oxides. Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Elbaite crystals have a rounded, triangular cross-section.
Seldom in tabular crystals. Aggregates include columnar , radiating , botryoidal , in dense groups of tiny, elongated needles, and in compact masses. Crystals may be curved or warped, and in some cases may have their growth interrupted by host rock with the same crystal appearing in two parts of a matrix. Transparent to translucent. Specific Gravity. Conchoidal to uneven. In Group. Silicates ; Cyclosilicates ; Tourmaline Group.
Striking Features. Color, crystal form, hardness, and deep vertical striation s. Almost exclusively in granite pegmatite s. Rock Type. Popularity Prevalence Demand Achroite - Colorless variety of Elbaite Tourmaline. Blue Cap Tourmaline - Form of Elbaite Tourmaline with its top portion having a dark blue color, with the rest of the body having a red or pink color. Chrome Tourmaline - Elbaite Tourmaline with a deep green color caused by chromium impurities s. Fluor-elbaite - Form of Elbaite where part of the hydroxyl is replaced with fluorine.
Moor's Head Tourmaline - Lightly tinted, transparent Elbaite Tourmaline with a black top that is well known from the island of Elba, Italy. Mushroom Tourmaline - Elbaite Tourmaline from Burma Mynamar with an odd-shaped fan-like protruding botryoidal growth off the top of a crystal that resembles the formation of a mushroom. Rubellite - Pink to red variety of Elbaite Tourmaline. Siberite - Purple variety of Elbaite Tourmaline. Verdelite - Green variety of Elbaite Tourmaline.
Watermelon Tourmaline - Variety of Elbaite Tourmaline that is green on the outside and red on the inside. Elbaite is form of Tourmaline used as a gemstone. Its popularity has greatly increased since the 's, and this has also been reflected in the astronomical prices demanded from the finest Elbaite. Elbaite Tourmaline is cut into all forms and styles for jewelry use. All the colors of Elbaite, especially multicolored gems, are used in jewelry.
0コメント