RockHoundBlog

Oregon Sunstone

Filed under: Mineral of the day,Rare Rocks!,Video — Gary October 31, 2010 @ 10:49 am

world’s largest  Oregon Sunstone:

A variety known as Oregon sunstone is found in Harney County, Oregon and in eastern Lake County north of Plush. Only Oregon sunstone contains inclusions of copper crystals. Oregon sunstone can be found as large as three inches across. The copper leads to varying color within some stones, where turning one stone will result in multiple colors. The more copper within the stone, the darker the complexion.

On August 4, 1987, Oregon State Legislature designated Oregon sunstone as its state gemstone by joint resolution.

Oregon sunstone

Oregon Sunstone

Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities. It is the official gemstone of Oregon.

The optical effect appears to be due to reflections from enclosures of red haematite, in the form of minute scales, which are hexagonal, rhombic or irregular in shape, and are disposed parallel to the principal cleavage-plane. These enclosures give the stone an appearance something like that of aventurine, whence sunstone is known also as “aventurine-feldspar.” The optical effect called shiller and the color in Oregon Sunstone is due to copper. In the middle part of this crystal, it sparks a lot, and usually has a dark color in the middle, and the color becomes lighter as it becomes the outer part.

Sunstone Mining

Sunstone Mining

Sunstone

Sunstone

The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as “oligoclase sunstone” and “orthoclase sunstone.”

Distribution

Sunstone was not common until recently. Previously the best-known locality being Tvedestrand, near Arendal, in south Norway, where masses of the sunstone occur embedded in a vein of quartz running through gneiss. Due to the discovery of large deposits in Oregon, Sunstone is now readily available.

Other locations include near Lake Baikal in Siberia, and several United States localities—notably at Middletown Township, Delaware County, Pennsylvania, Lakeview, Oregon, and Statesville, North Carolina.

unpolished_sunstone

Unpolished Sunstone

The “orthoclase sunstone” variant has been found near Crown Point and at several other localities in New York, as also at Glen Riddle in Delaware County, Pennsylvania, and at Amelia Courthouse, Amelia County, Virginia.

Sunstone is also found in Pleistocene basalt flows at Sunstone Knoll in Millard County, Utah.

n the short video below you will see the process of prospecting for Oregon sunstone with the use of a drill. As drilling begins we watch the cuttings coming out of the hole. With experience you can tell when it is time to check the cuttings more closely. The drilling penetration rate will vary depending on the type of material being drilled. The color and size of the particles will also vary.The bore hole is cleaned out by compressed air. Compressed air is pumped down the inside of the drill pipe and through the bit. The cuttings are blown to the surface and caught by hand for examination. If you are drilling in a potential mining site there will be ground up particles of feldspar in the cuttings. When you find feldspar in the cuttings you note how much drill pipe is in the hole being drilled. This will tell you how deep to dig with heavy equipment. The video will give you a better understanding and appreciation of what it is like to prospect for sunstone.

Thanks Wikipedia

Sodalite

Filed under: Mineral of the day,Video — Gary October 18, 2010 @ 4:53 pm
Sodalite

Sodalite

Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group and—together with hauyne, nosean, and lazurite—is a common constituent of lapis lazuli. Discovered in 1806 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.

Properties

A light, relatively hard yet fragile mineral, sodalite is named after its sodium content; in mineralogy it may be classed as a feldspathoid. Well known for its blue color, sodalite may also be grey, yellow, green, or pink and is often mottled with white veins or patches. The more uniformly blue material is used in jewellery, where it is fashioned into cabochons and beads. Lesser material is more often seen as facing or inlay in various applications.

Sodalite

Sodalite

Although not similar to lazurite and lapis lazuli, sodalite is never quite comparable, being a royal blue rather than ultramarine. Sodalite also rarely contains pyrite, a common inclusion in lapis. It is further distinguished from similar minerals by its white (rather than blue) streak. Sodalite’s six directions of poor cleavage may be seen as incipient cracks running through the stone. Hackmanite is an important variety of sodalite exhibiting tenebrescence. When hackmanite from Mont Saint-Hilaire (Quebec) or Ilímaussaq (Greenland) is freshly quarried, it is generally pale to deep violet but the colour fades quickly to greyish or greenish white. Conversely, hackmanite from Afghanistan and the Myanmar Republic (Burma) starts off creamy white but develops a violet to pink-red colour in sunlight. If left in a dark environment for some time, the violet will fade again. Tenebrescence is accelerated by the use of longwave or, particularly, shortwave ultraviolet light. Much sodalite will also fluoresce a patchy orange under UV light.

Hackmanite
Tenebrescent sodalite from Greenland – Upon exposure to SW UV (UVC) the sodalite (also known as hackmanite) changes color to a dark purple. This is tenebrescence and is a reversible effect. The color can be faded by a bright light, and the effect can be repeated over and over. The sodalite is also fluorescent a bright orange under LW UV, and under SW UV the glow gradually darkens to a rusty color due to this tenebrescence.

Occurrence

Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks such as nepheline syenites. It is associated with other minerals typical of undersaturated environments, namely leucite, cancrinite and natrolite. Significant deposits of fine material are restricted to but a few locales: Bancroft, Ontario, and Mont-Saint-Hilaire, Quebec, in Canada; and Litchfield, Maine, and Magnet Cove, Arkansas, in the USA. The Ice River complex, near Golden, British Columbia, is being investigated for sodalite recovery. Smaller deposits are found in South America (Brazil and Bolivia), Portugal, Romania, Burma and Russia. Hackmanite is found principally in Mont. Saint-Hilare and Greenland, the latter locale producing a green specimen nicknamed “chameleon sodalite.” Euhedral, transparent crystals are found in northern Namibia and in the lavas of Vesuvius, Italy.

Silver

Filed under: Mineral of the day — Gary October 17, 2010 @ 10:27 pm

Silver

Silver (play /ˈsɪlvər/) is a metallic chemical element with the chemical symbol Ag (Latin: argentum, from the Indo-European root *arg- for “grey” or “shining”) and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc refining.

Silver has long been valued as a precious metal, and it is used to make ornaments, jewelry, high-value tableware, utensils (hence the term silverware), and currency coins. Today, silver metal is also used in electrical contacts and conductors, in mirrors and in catalysis of chemical reactions. Its compounds are used in photographic film and dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides. While many medical antimicrobial uses of silver have been supplanted by antibiotics, further research into clinical potential continues.

Silver bullion bar 1000oz bottom view / view from underneath

Silver bullion bar 1000oz bottom view / view from underneath

Silver is a very ductile and malleable (slightly harder than gold) monovalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost has prevented it from being widely used in place of copper for electrical purposes. Despite this, 13,540 tons were used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper). Another notable exception is in high-end audio cables.

Among metals, pure silver has the highest thermal conductivity (the non-metal diamond and superfluid helium II are higher) and one of the highest optical reflectivity. (Aluminium slightly outdoes silver in parts of the visible spectrum, and silver is a poor reflector of ultraviolet light). Silver also has the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide to form a black layer of silver sulfide which can be cleaned off with dilute hydrochloric acid. The most common oxidation state of silver is +1 (for example, silver nitrate: AgNO3); in addition, +2 compounds (for example, silver(II) fluoride: AgF2) and the less common +3 compounds (for example, potassium tetrafluoroargentate: K[AgF4] ) are known.

Isotopes

Naturally occurring silver is composed of two stable isotopes, 107Ag and 109Ag, with 107Ag being the most abundant (51.839% natural abundance). Silver’s isotopes are almost equal in abundance, something which is rare in the periodic table. Silver’s atomic weight is 107.8682(2) g/mol.[7][8] Twenty-eight radioisotopes have been characterized, the most stable being 105Ag with a half-life of 41.29 days, 111Ag with a half-life of 7.45 days, and 112Ag with a half-life of 3.13 hours. This element has numerous meta states, the most stable being 108mAg (t1/2 = 418 years), 110mAg (t1/2 = 249.79 days) and 106mAg (t1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 3 minutes.

Isotopes of silver range in relative atomic mass from 93.943 (94Ag) to 126.936 (127Ag); the primary decay mode before the most abundant stable isotope, 107Ag, is electron capture and the primary mode after is beta decay. The primary decay products before 107Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.

The palladium isotope 107Pd decays by beta emission to 107Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in 107Ag abundance. Radiogenic 107Ag was first discovered in the Santa Clara meteorite in 1978. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. 107Pd–107Ag correlations observed in bodies that have clearly been melted since the accretion of the solar system must reflect the presence of unstable nuclides in the early solar system.

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Utah Ice

Filed under: Mineral of the day,Video — Gary October 13, 2010 @ 8:38 pm

Someone asked me if I ever heard of the mineral “Utah Ice”.  Hmph I said…  After some digging (no pun intended) this is what I came up with.

Question:

This stuff looks almost like glass, specially when it gets into the water it looks alot like glass, but its not.
My warning is, that you shouldn’t buy it for the aquarium.
Why?
Well I bought a load of it for my 29gal, and guess what? Its all gone!
Yes, thats right.. it slowly dissolved away, over about 5 months some very large pieces are down to tiny little slivers.
Just a heads up, don’t buy it unless you want to have to KEEP buying it.

Selenite, a crystalline form of gypsum

Selenite, satin spar, desert rose, and gypsum flower are four varieties of gypsum; all four varieties show obvious crystalline structure. The four “crystalline” varieties of gypsum are sometimes grouped together and called selenite.

All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO4·2H2O.

Identification of gypsum

All varieties of gypsum are very soft minerals (hardness: 2 on Mohs Scale). This is the most important identifying characteristic of gypsum, as any variety of gypsum can be easily scratched with a fingernail. Also, because gypsum has natural insulating properties, all varieties feel warm to the touch.

Varieties

Though sometimes grouped together as “selenite”, the four crystalline varieties have differences. General identifying descriptions of the related crystalline varieties are:

Selenite

Selenite

Selenite

  • most often transparent and colorless: it is named after Greek σεληνη= “the moon”.
  • if selenite crystals show translucency, opacity, and/or color, it is caused by the presence of other minerals including druse (a coating of small crystal points)
  • druse is the crust of tiny, minute, or micro crystals that form or fuse either within or upon the surface of a rock vug, geode, or another crystal

Satin spar

  • most often silky, fibrous, and translucent (pearly, milky) – can exhibit some coloration
  • the satin spar name can also be applied to fibrous calcite (a related calcium mineral) – calcite is a harder mineral – and feels greasier, waxier, or oilier to the touch.

Desert rose

  • rosette shaped gypsum with outer druse of sand or with sand throughout – most often sand colored (in all the colors that sand can exhibit)
  • the desert rose name can also be applied to barite desert roses (another related sulfate mineral) – barite is a harder mineral with higher density

Gypsum flower

  • rosette shaped gypsum with spreading fibers – can include outer druse
  • the difference between desert roses and gypsum flowers is that desert roses look like roses, whereas gypsum flowers form a myriad of shapes

Use and history

Selenite mine

Selenite mine

Because of the long history of the commercial value and use of both gypsum and alabaster, the four crystalline varieties have been somewhat ignored, except as a curiosity or as rock collectibles.

Crystal habit and properties

Crystal habit refers to the shapes that crystals exhibit.

Selenite crystals commonly occur as tabular, reticular, and columnar crystals, often with no imperfections or inclusions, and thereby can appear water or glass-like. Many collectible selenite crystals have interesting inclusions such as, accompanying related minerals, interior druse, dendrites, and fossils. In some rare instances, water was encased as a fluid inclusion when the crystal formed.

Selenite mine

Selenite

Selenite crystals sometimes form in thin tabular or mica-like sheets and have been used as glass panes.

Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent and like desert roses) often with accompanying transparent, columnar crystals. Selenite crystals can be found both attached to a matrix or base rock, but can commonly be found as entire free-floating crystals, often in clay beds (and as can desert roses).

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Lake Superior agate

Filed under: Mineral of the day,Video — Gary September 26, 2010 @ 8:49 pm
Lake Superior agate -  R.Weller/Cochise College

Lake Superior agate - R.Weller/Cochise College

The Lake Superior agate is a type of agate stained by iron and found on the shores of Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone’s geologic history in Minnesota. In 1969 the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.

The Lake Superior agate was selected because the agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred in Minnesota about a billion years ago. The stone’s predominant red color comes from iron, a major Minnesota industrial mineral found extensively throughout the Iron Range region. Finally, the Lake Superior agate can be found in many regions of Minnesota as it was distributed by glacial movement across Minnesota 10,000 to 15,000 years ago.

Geologic history

More than a billion years ago, the North American continent began to split apart along plate boundaries. Molten magma upwelled into iron-rich lava flows throughout the Midcontinent Rift System, including what is now the Minnesota Iron Range region. These flows are now exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough, a depressed region that became the basin of Lake Superior.

Lake Superior agate

Lake Superior agate

The lava flows formed the conditions for creation of Lake Superior agates. As the lava solidified, water vapor and carbon dioxide trapped within the solidified flows formed a vesicular texture (literally millions of small bubbles). Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions deposited concentric bands of fine-grained quartz called chalcedony, or embedded agates.

Over the next billion years, erosion exposed a number of the quartz-filled, banded vesicles—agates—were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.

During the ensuing ice ages a lobe of glacial ice, the Superior lobe, moved into Minnesota through the agate-filled Superior trough. The glacier picked up surface agates and transported them south. Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.

Description

The Lake Superior agate is noted for its rich red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. Iron leached from rocks provided the pigment that gives the gemstone its beautiful array of color. The concentration of iron and the amount of oxidation determine the color within or between an agate’s bands.

The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about 1 cm in diameter. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms. Very large agates are extremely rare.

The most common type of Lake Superior agate is the fortification agate with its eye-catching banding patterns. Each band, when traced around an exposed pattern or “face,” connects with itself like the walls of a fort, hence the name fortification agate.

A common subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. Perfectly straight, parallel bands occur over all or part of these stones. The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure. The parallel nature of the bands also indicates the agate’s position inside the lava flow.

Probably the most popular Lake Superior agate is also one of the rarest. The highly treasured eye agate has perfectly round bands or “eyes” dotting the surface of the stone.

Occasionally, collectors find a gemstone with an almost perfectly smooth natural surface. These rare agates are believed to have spent a long time tumbling back and forth in the waves along some long-vanished, wave-battered rocky beach. They are called, appropriately enough, “water-washed” agates.

Cutting and polishing

A gemstone can be used as a jewel when cut and polished. Only a fraction of the Lake Superior agate are of the quality needed for lapidary. Three lapidary techniques are used on Lake Superior agates:

  • Tumbling—Small gemstones are rotated in drums with progressively finer polishing grit for several days until they are smooth and reflective.
  • Saw-cut and polish—Stones up to 1/2 kg are cut with diamond saws into thin slabs, which then are cut into various shapes. One side of the shaped slab is polished producing fine jewelry pieces and collectible gems called cabochons.
  • Face polishing—Polishing a curved surface on a portion of the stone and leaving the major portion in its natural state is called face polishing.

Distribution of Lake Superior agate

One of the most appealing reasons for naming the Lake Superior agate as the Minnesota state gemstone is its general availability. Glacial activity spread agates throughout northeastern and central Minnesota, extreme northwestern Wisconsin and Michigan’s Upper Peninsula in the United States and the area around Thunder Bay in Northwestern Ontario, Canada.

Finding the gem

Typically the richly colored banding pattern is not well exposed and prospectors must look for other clues to the presence of agates.

The following characteristics are used to identify agates in the field.

  • Band planes along which the agate has broken are sometimes visible, giving the rock a peeled texture. It appears as though the bands were partially peeled off like a banana skin.
  • Iron-oxide staining is found on nearly all agates to some degree, and generally covers much of the rock. Such staining can be many different colors, but the most common are shades of rust-red and yellow.
  • Translucence is an optical feature produced by chalcedony quartz, the principal constituent of agates. The quartz allows light to penetrate, producing a glow. Sunny days are best for observing translucence.
  • A glossy, waxy appearance, especially on a chipped or broken surface, is another clue.
  • A pitted texture often covers the rock surface. The pits are the result of knobs or projections from an initial layer of softer mineral matter deposited on the wall of the cavity in which the agate formed. Later, when the quartz that formed the agate was deposited in the cavity, these projections left impressions on the exterior.

Thanks Wikipedia

Bornite – Peacock Ore / Peacock Copper

Filed under: Mineral of the day,Reader submissions- Rockhound stores,Video — Gary September 21, 2010 @ 7:51 pm

Bornite is a sulfide mineral with chemical composition Cu5FeS4 that crystallizes in the orthorhombic system.  Also called ‘the stone of happiness‘.

Peacock Ore

Peacock Ore

Appearance

Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places. Its striking iridescence gives it the nickname peacock copper or peacock ore. As this appearance can not always be naturally found, many sellers of peacock ore dip the mineral in acid to accentuate the colors.

Mineralogy

Bornite is an important copper ore mineral and occurs widely in porphyry copper deposits along with the more common chalcopyrite. Chalcopyrite and bornite are both typically replaced by chalcocite and covellite in the supergene enrichment zone of copper deposits. Bornite is also found as disseminations in mafic igneous rocks, in contact metamorphic skarn deposits, in pegmatites and in sedimentary cupriferous shales. It is important as an ore for its copper content of about 63 percent by mass.

Bornite

Bornite / Peacock Copper

Occurrence

It occurs globally in copper ores with notable crystal localities in Butte, Montana and at Bristol, Connecticut in the U. S. It is also collected from the Carn Brea mine, Illogan, and elsewhere in Cornwall, England. Large crystals are found from the Frossnitz Alps, eastern Tirol, Austria; the Mangula mine, Lomagundi district, Zimbabwe; from the N’ouva mine, Talate, Morocco and in Dzhezkazgan, Kazakhstan.

History and etymology

It was first described in 1725 for an occurrence in the Krušné Hory Mountains (Erzgebirge), Karlovy Vary Region, Bohemia in what is now the Czech Republic. It was named in 1845 for Austrian mineralogist Ignaz von Born (1742–1791).

Interesting Video about Bornite-

A reader submitted a question to me and this was the first thing that came to mind (Peacock Ore).  Anyone want to try and answer (story and question below)-

So I live in Utah and spend much time in the mountains and also work on a mountain range.
I came across an old miners bouillon. It looked out of place so I exposed the rest of it. The outside appeared to be shaped like a bowl ( I later found out it was a cauldron) so of course, ya keep it.
It sparked an interest, I had heard of stories of an old sheepherder from Spain that spent his summers there on the hill.
The man was rich back in Spain.
Sparked an interest….what was he doin in Utah herding sheep for 20 years.
So I was on a mission.
I came across a spot on the mountain with rock that was a rhyolite that I had not seen before, so I looked around. Turns out there is a vein of rhyolit that was inside some quartzite rocks that someone has been chipping and taking the vein, replacing the outcropping rocks with the rocks that were around the vein to make it look as if noone was there.
By the look of things someone had been doing this for some time.
I happened to grab some of this vein and it is beautiful multi colored and heavy as hell.
I need info on how to identify the already cooked bouillon that I found. Any help!!

Pegmatite

Filed under: Mineral of the day — Gary August 10, 2010 @ 3:17 am
pegmatite

pegmatite

A pegmatite is a very coarse-grained, intrusive igneous rock composed of interlocking grains usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic.

Most pegmatites are composed of quartz, feldspar and mica; in essence a granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 meters across have been found, and the world’s largest crystal was found within a pegmatite.

Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye.

Petrology

Crystal growth rates in pegmatite must be incredibly fast to allow gigantic crystals to grow within the confines and pressures of the Earth’s crust. For this reason, the consensus on pegmatitic growth mechanisms involves a combination of the following processes;

  • Low rates of nucleation of crystals coupled with high diffusivity to force growth of a few large crystals instead of many smaller crystals
  • High vapor and water pressure, to assist in the enhancement of conditions of diffusivity
  • High concentrations of fluxing elements such as boron and lithium which lower the temperature of solidification within the magma or vapor
  • Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metamorphic terranes

Despite this consensus on likely chemical, thermal and compositional conditions required to promote pegmatite growth there are three main theories behind pegmatite formation;

  1. Metamorphic; pegmatite fluids are created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss, to liberate the right constituents and water, at the right temperature
  2. Magmatic; pegmatites tend to occur in the aureoles of granites in most cases, and are usually granitic in character, often closely matching the compositions of nearby granites. Pegmatites thus represent exsolved granitic material which crystallises in the country rocks
  3. Metasomatic; pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and textural change.

Metasomatism is currently not well favored as a mechanism for pegmatite formation and it is likely that metamorphism and magmatism are both contributors toward the conditions necessary for pegmatite genesis.

Mineralogy

The mineralogy of a pegmatite is in all cases dominated by some form of feldspar, often with mica and usually with quartz, being altogether “granitic” in character. Beyond that, pegmatite may include most minerals associated with granite and granite-associated hydrothermal systems, granite-associated mineralisation styles, for example greisens, and somewhat with skarn associated mineralisation.

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Pyrite or Foolsgold

Filed under: Mineral of the day — Gary July 25, 2010 @ 9:16 pm
Pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral’s metallic lustre and pale-to-normal, brass-yellow hue have earned it the nickname fool’s gold because of its resemblance to gold. The color has also led to the nicknames brass, brazzle and Brazil, primarily used to refer to pyrite found in coal.

Pyrite is the most common of the sulfide minerals. The name pyrite is derived from the Greek πυρίτης (puritēs), “of fire” or “in fire”, from πύρ (pur), “fire”. In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel; Pliny the Elder described one of them as being brassy, almost certainly a reference to what we now call pyrite. By Georgius Agricola’s time, the term had become a generic term for all of the sulfide minerals.

Pyrite is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as a replacement mineral in fossils. Despite being nicknamed fool’s gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin, Nevada, gold deposit, arsenian pyrite contains up to 0.37 wt% gold. Auriferous pyrite is a valuable ore of gold.

Weathering and release of sulfate

Pyrite exposed to the atmosphere during mining and excavation reacts with oxygen and water to form sulfate, resulting in acid mine drainage. This acidity results from the action of Acidithiobacillus bacteria, which generate their energy by oxidizing ferrous iron (Fe2+) to ferric iron (Fe3+) using oxygen. The ferric iron in turn attacks the pyrite to produce ferrous iron and sulfate. The ferrous iron is then available for oxidation by the bacterium; this cycle continues until the pyrite is depleted.

Iron pyrite oxidation is sufficiently exothermic that underground coal mines in high-sulfur coal seams have occasionally had serious problems with spontaneous combustion in the mined-out areas of the mine. The solution is to hermetically seal the mined-out areas to exclude oxygen.

In modern coal mines, limestone dust is sprayed onto the exposed coal surfaces to reduce the hazard of dust explosions. This has the secondary benefit of neutralizing the acid released by pyrite oxidation and therefore slowing the oxidation cycle described above, thus reducing the likelihood of spontaneous combustion. In the long term, however, oxidation continues, and the hydrated sulfates formed may exert crystallization pressure that can expand cracks in the rock and lead eventually to roof fall.

Building stone containing pyrite tends to stain brown as the pyrite oxidizes. This problem appears to be significantly worse if any marcasite is also present. The presence of pyrite in the aggregate used to make concrete can lead to severe deterioration as the pyrite oxidizes. In early 2009, problems with Chinese drywall imported into the United States after Hurricane Katrina were attributed to oxidation of pyrite.

Pyrite_from_Ampliación_spain

Pyrite_from_Ampliación_spain

Uses

Pyrite enjoyed brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where the cock held a lump of pyrite against a circular file to strike the sparks needed to fire the gun.

Pyrite has been used since classical times to manufacture copperas, or iron sulfate. Iron pyrite was heaped up and allowed to weather as described above (an early form of heap leaching). The acidic runoff from the heap was then boiled with iron to produce iron sulfate. In the 15th century, oil of vitriol (sulfuric acid) was manufactured either from copperas or by burning sulfur to sulfur dioxide and then converting that to sulfuric acid. By the 19th century, the dominant method was to burn iron pyrite. Pyrite remains in commercial use for the production of sulfur dioxide, for use in such applications as the paper industry, and in the manufacture of sulfuric acid. Thermal decomposition of pyrite into FeS and elemental sulfur starts at 550 °C; at around 700 °C pS2 is about 1 atm.

Pyrite is a semiconductor material with band gap of 0.95 eV.

During the early years of the 20th century, pyrite was used as a mineral detector in radio receivers, and is still used by ‘crystal radio’ hobbyists. Until the vacuum tube matured, the crystal detector was the most sensitive and dependable detector available- with considerable variation between mineral types and even individual samples within a particular type of mineral. The most sensitive mineral was galena, which was very sensitive also to mechanical vibration, and easily knocked off the sensitive point; the most stable were perikon mineral pairs; and midway between was the pyrites detector, which is approximately as sensitive as a modern 1N34A diode detector.

Pyrite has been proposed as an abundant inexpensive material in low cost photovoltaic solar panels.Synthetic iron sulfide is used with copper sulfide to create the experimental photovoltaic material.[21]

Pyrite is used to make marcasite jewellery (incorrectly termed marcasite). Marcasite jewellery, made from small faceted pieces of pyrite, often set in silver, was popular in the Victorian era.

pyrite_cubes

pyrite_cubes

Color Pale brass-yellow, tarnishes darker and iridescent
Crystal habit Cubic, faces may be striated, but also frequently octahedral and pyritohedron. Often inter-grown, massive, radiated, granular, globular and stalactitic.
Crystal system Isometric Diploidal, Space group Pa3
Twinning Penetration and contact twinning
Cleavage Indistinct on {001}; partings on {011} and {111}
Fracture Very uneven, sometimes conchoidal
Tenacity Brittle
Mohs scale hardness 6–6.5
Luster Metallic, glistening
Streak Greenish-black to brownish-black; smells of sulfur
Diaphaneity Opaque
Specific gravity 4.95–5.10
Fusibility 2.5–3 to a magnetic globule
Solubility Insoluble in water
Other characteristics paramagnetic

Thanks wikipedia

Amphibole

Filed under: Mineral of the day — Gary July 15, 2010 @ 11:23 am
Amphibole (Hornblende)

Amphibole (Hornblende)

Amphibole (pronounced /ˈæmfɨboʊl/) defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. In chemical composition and general characteristics they are similar to the pyroxenes. The chief differences from pyroxenes are that (i) amphiboles contain essential hydroxyl (OH) or halogen (F, Cl) and (ii) the basic structure is a double chain of tetrahedra (as opposed to the single chain structure of pyroxene). Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxenes have cleavage angles of approximately 90 degrees. Amphiboles are also specifically less dense than the corresponding pyroxenes. In optical characteristics, many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction (Z angle c) on the plane of symmetry. Amphiboles are the primary constituent of amphibolites.

Amphiboles are minerals of either igneous or metamorphic origin; in the former case occurring as constituents (hornblende) of igneous rocks, such as granite, diorite, andesite and others. Those of metamorphic origin include examples such as those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (hornblende). Pseudomorphs of amphibole after pyroxene are known as uralite.

The name amphibole (Greek αμφιβολος – amphibolos meaning ‘ambiguous’) was used by RJ Haüy to include tremolite, actinolite, tourmaline and hornblende. The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its minerals. This term has since been applied to the whole group. Numerous sub-species and varieties are distinguished, the more important of which are tabulated below in two series. The formulae of each will be seen to be built on the general double-chain silicate formula RSi4O11.

Chemical formulae

Orthorhombic series

  • Anthophyllite (Mg,Fe)7Si8O22(OH)2

Monoclinic series

  • Tremolite Ca2Mg5Si8O22(OH)2
  • Actinolite Ca2(Mg,Fe)5Si8O22(OH)2
  • Cummingtonite Fe2Mg5Si8O22(OH)2
  • Grunerite Fe7Si8O22(OH)2
  • Hornblende Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2
  • Glaucophane Na2(Mg,Fe)3Al2Si8O22(OH)2
  • Riebeckite Na2Fe2+3Fe3+2Si8O22(OH)2
  • Arfvedsonite Na3Fe2+4Fe3+Si8O22(OH)2
  • Crocidolite Na2Fe2+3Fe3+2Si8O22(OH)2
  • Richterite Na2Ca(Mg,Fe)5Si8O22(OH)2
  • Pargasite NaCa2Mg3Fe2+Si6Al3O22(OH)2

Descriptions

On account of the wide variations in chemical composition, the different members vary considerably in properties and general appearance.

Anthophyllite occurs as brownish, fibrous or lamellar masses with hornblende in mica-schist at Kongsberg in Norway and some other localities. An aluminous related species is known as gedrite and a deep green Russian variety containing little iron as kupfferite.

Hornblende is an important constituent of many igneous rocks. It is also an important constituent of amphibolites formed by metamorphism of basalt.

Actinolite is an important and common member of the monoclinic series, forming radiating groups of acicular crystals of a bright green or greyish-green color. It occurs frequently as a constituent of greenschists. The name (from Greek ακτις/aktis, a ‘ray’ and λιθος/lithos, a ‘stone’) is a translation of the old German word Strahlstein (radiated stone).

Glaucophane, crocidolite, riebeckite and arfvedsonite form a somewhat special group of alkali-amphiboles. The first two are blue fibrous minerals, with glaucophane occurring in blueschists and crocidolite (blue asbestos) in ironstone formations, both resulting from dynamo-metamorphic processes. The latter two are dark green minerals, which occur as original constituents of igneous rocks rich in sodium, such as nepheline-syenite and phonolite.

Pargasite is a rare magnesium-rich amphibole with essential sodium, usually found in ultramafic rocks. For instance, it occurs in uncommon mantle xenoliths, carried up by kimberlite. It is hard, dense, black and usually idiomorphic, with a red-brown pleochroism in petrographic thin section.

Amphibole compositions

Amphibole compositions

Amphibole compositions in the system Mg7Si8O22(OH)2 (anthophyllite)–Fe7Si8O22(OH)2 (grunerite)–“Ca7Si8O22(OH)2.” The general compositional fields are outlined, and coexisting amphibole compositions are shown by tie lines between the actinolite field and the anthophyllite-grunerite field.

Feldspar- Mineral of the day.

Filed under: Mineral of the day — Gary April 13, 2010 @ 9:12 pm
feldspar

feldspar

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth’s crust.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.

Potassium_feldspar

Potassium_feldspar

Etymology

Feldspar is derived from the German Feld, “field”, and Spath, “a rock that does not contain ore”. “Feldspathic” refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.

feldspar

feldspar

Compositions

This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

Potassium-Feldspar (K-spar) endmember KAlSi3O8

Albite endmember NaAlSi3O8

Anorthite endmember CaAl2Si2O8

Solid solutions between K-feldspar and albite are called alkali feldspar.Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.

alkali_feldspar

alkali_feldspar

Alkali feldspars

The alkali feldspars are as follows:

  • orthoclase (monoclinic), — KAlSi3O8
  • sanidine (monoclinic)—(K,Na)AlSi3O8
  • microcline (triclinic) — KAlSi3O8
  • anorthoclase (triclinic) — (Na,K)AlSi3O8

Sanidine is stable at the highest temperatures, and microcline at the lowest.Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

feldspar_map

feldspar_map

Plagioclase feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

  • albite (0 to 10) — NaAlSi3O8
  • oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
  • andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
  • labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
  • bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
  • anorthite (90 to 100) — CaAl2Si2O8

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.

lunar_feldspar

lunar_feldspar

Barium feldspars

The barium feldspars are monoclinic and comprise the following:

  • celsian — BaAl2Si2O8
  • hyalophane — (K,Na,Ba)(Al,Si)4O8

Feldspars can form clay minerals through chemical weathering.

Uses

  • Feldspar is a common raw material in the production of ceramics and geopolymers.
  • Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
  • Feldspar is one of several abrasive ingredients in Bon Ami, a brand of household cleaner in the USA.

In 2005, Italy was the top producer of feldspar with almost one fifth of world share, followed by Turkey, China and Thailand—reports the International Monetary Fund.