Thursday, August 18, 2011

Mineral F


 

 

 

THE MINERAL FAYALITE

 


Fayalite is named for the Island of Fayal of the Azores. It is one of two minerals that are simply known as olivine. The other mineral is forsterite. Fayalite is the iron rich member with a pure formula of Fe2SiO4. Forsterite is the magnesium rich member with a pure formula of Mg2SiO4. The two minerals form a series were the iron and magnesium are substitutable for each other without much effect to the crystal structure. Fayalite due to its iron content has a higher index of refraction, is heavier and has a darker color than forsterite. Otherwise they are difficult to distinguish and virtually all specimens of the two minerals contain iron and magnesium. For simplicity sake and general public recognition, they are often treated as one mineral, olivine. Olivine, however is actually not officially recognized as a mineral. Olivine's gemstone variety is known as peridot. Most peridot is the magnesium rich forsterite, not fayalite. Fayalite's higher iron content make for darker, less attractive specimens that are not generally used as gemstones.

Fayalite forms a second series with the mineral tephroite. Tephroite is the manganese rich member of the series with a formula of Mn2SiO4 and fayalite is the iron rich member. Tephroite is a member of the more inclusive Olivine Group. Sometimes the Olivine Group is considered limited to just fayalite and forsterite, but most mineralogist include all silicate minerals with the same olivine structure, such as tephroite.

Fayalite is found in ultramafic igneous rocks. Mafic is a word that is used to define igneous rocks with a high iron and magnesium content. The "MA" is for magnesium while the "F" is for ferrum, the latin word for iron. The olivine minerals have a high melting point and are the first minerals to crystallize from a mafic magma. Forsterite crystallizes first with fayalite crystallizing last when other minerals such as the pyroxenes are just beginning to form. The early crystallization of olivine is the reason that molten lavas can contain already crystallized grains of olivine. Some ultramafic rocks can be composed of almost all olivine and these are called dunites or peridotites. Olivine is also present in marbles that formed from metamorphosed impure limestones.

Fayalite is also found in many iron-nickel meteorites. Not just as small grains but as significantly sized crystals sometimes occupying over 50% of the meteorites volume. Thinly cut slices of these meteorites are extremely attractive with the polished steel gray of the iron and the embedded grains of gemmy green olivine. The effect produces the closest mineral equilalent to stained glass artwork.

 

PHYSICAL CHARACTERISTICS:

  • Color is a light near emerald green to the more common pale yellowish green, also found greenish brown to black.
  • Luster is vitreous.
  • Transparency crystals are transparent to translucent.
  • Crystal System is orthorhombic; 2/m 2/m 2/m.
  • Crystal Habits include flatten tabular to box shaped crystals, but good crystals are rare. More commonly found as grains in alluvial gravels and as granular pockets in highly magnesium and iron rich volcanic rock. Also massive.
  • Cleavage is distinct in two directions at 90 degrees.
  • Fracture is conchoidal.
  • Hardness is 6.5 - 7.
  • Specific Gravity is approximately 4.3 (above average for non-metallic minerals).
  • Streak is white.
  • Other Characteristics: Index of refraction is 1.87, moderate double refraction can usually be seen and crystals are typically striated.
  • Associated Minerals are diopside, spinel, plagioclase feldspars, chromite, anorthite, biotite, cristobalite, hornblende, serpentine, obsidian, iron-nickel meteorites and augite.
  • Notable Occurrences include the Salt Lake Crater, Oahu, Hawaii; Sugarloaf Mountain, Inyo County, California and Peridot, Gila County, Arizona, USA; Lipari Islands, Sicili and Mt. Vesuvius, Italy; France; Sweden and many other locations.
  • Best Field Indicators are environment of formation, color, crystal habit, hardness and density.

 

 

 

THE FELDSPAR GROUP

 

The feldspar group is a fairly large group with nearly 20 members recognized, but only nine are well known and common. Those few, however, make up the greatest percentage of minerals found in the Earth's crust. The following are some of the more common feldspar minerals:

 

The plagioclase feldspars:

 

The K-feldspars or alkali felspars:


The feldspars are a group of minerals that have similar characteristics due to a similar structure. All feldspars have low symmetry, being only monoclinic, 2/m, to triclinic, bar 1. They tend to twin easily and one crystal can even be multiply twinned on the same plane, producing parallel layers of twinned crystals. They are slightly hard at around 6, and have an average density at 2.55 to 2.76. They have a rather dull to rarely vitreous luster. Crystals tend to be blocky. Some feldspars may be triboluminescent. They have two directions of cleavage at nearly right angles. Feldspars also tend to crystallize in igneous enviroments, but are also present in many metamorphic rocks.

The general formula, for the common feldspars, is XAl(1-2) Si(3-2) O8 . The X in the formula can be sodium, Na and/or potassium, K and/or calcium, Ca. When the cation in the X position has a positive one (+1) charge such as with sodium or potassium, then the formula contains one aluminum and three silicons ions. If the formula contains the positive two (+2) cation calcium, then the formula will contain two aluminums and only two silicon ions. This substitution keeps the formula balanced, because aluminum has a charge of positive three (+3) and silicon has a charge of positive four (+4). Basically, the more calcium in the crystal, the more aluminum that will be needed to balance the charge.

The silicons and aluminums occupy the centers of interlinked tetrahedrons of SiO4 and AlO4. These tetrahedrons connect at each corner to other tetrahedrons forming an intricate, three dimensional, negatively charged framework. The cations that represent the X in the formula sit within the voids in this structure.

The different feldspars are distinguished by structure and chemistry. The potassium or K-feldspars are polymorphs, meaning they have the same chemistry, KAlSi3 O8 , but different structures and therefore are different minerals. The plagioclase feldspars are a set of minerals that are in a series from a sodium rich end member, albite, to a calcium rich end member, anorthite. The intermediate members of the series are given arbitrary boundries based on their percentage of sodium or calcium.

Often, feldspars are simply referred to as plagioclase and orthoclase (a K-feldspar) because identification to greater precision is difficult with ordinary methods. Once identified, however, some feldspar mineral varieties are found to have distinctive characteristics or originate from a classic locality and on these bases are recognized by mineral collectors as belonging to a specific feldspar mineral.

 

 

 

 

 

THE MINERAL FERBERITE

 

  • Chemistry: FeWO4, Iron tungstate
  • Class: Sulfates
  • Subclass: Tungstates
  • Uses: as a minor ore of tungsten (an important industrial element) and as a mineral specimen
  • Specimens

Ferberite belongs to a series with another mineral, Huebnerite , MnWO4. Huebnerite is the Manganese rich end member while ferberite is the iron rich end member. Wolframite is the name of the series and the name applied to indistinguishable specimens and specimens intermediate between the two end members. Most specimens found in nature fall within 20 - 80% range of the series and these are termed wolframites, (Fe, Mn)WO4. Only if they are 80% or more iron are they called ferberite. Ferberite's iron content causes the dramatic differences between it and hubnerite (wolframite is of course intermediate). Ferberite tends to be black colored, with a black streak, is opaque with a nearly submetallic luster, is denser, has crystals with a different elongation and can be weakly magnetic. Huebnerite, however, tends to be light in color, with a lighter streak, more transparent and less dense. Ferberite is the rarer member of the series.

 

PHYSICAL CHARACTERISTICS:

  • Color is Black.
  • Luster is submetallic.
  • Transparency crystals are opaque.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include the flat, heavily modified, tabular crystals. The crystals are elongated along the b axis and are generally flattened in the a axis direction. Also as columnar aggregates and lamellar masses.
  • Cleavage is perfect in one direction parallel to the a and c axes.
  • Fracture is uneven.
  • Hardness is 4 - 4.5.
  • Specific Gravity is approximately 7.6 (heavy even for metallic minerals)
  • Streak is black.
  • Associated Minerals are quartz, hematite, tourmalines, cassiterite, micas and pyrite.
  • Other Characteristics: crystals are often striated.
  • Notable Occurrences include Nanling Range, China; South Dakota and Colorado, USA; Russia; Korea; England and Bolivia.
  • Best Field Indicators are crystal habit, color, density, luster and cleavage.

 

 

 

 

THE MINERAL FERRO-EDENITE

 


Ferro-edenite is an uncommon amphibole mineral. It is almost exactly the same as the more common amphibole mineral edenite, but contains more iron than magnesium thus the name ferro-edenite (ferro is latin for iron). The two minerals form a solid solution series in which the iron and magnesium substitute for each other. Edenite is the magnesium rich member. The two are similar in properties except that ferro-edenite is generally darker and denser.
Ferro-edenite is related to the more well known amphibole, hornblende. Although hornblende is no longer an official mineral, it still serves as a general name for iron, magnesium, aluminum and calcium rich amphiboles of which ferro-edenite is one. In fact ferro-edenite had been referred to as "ferro-edenitic hornblende" before its adoption as an official and distinct mineral.
Amphiboles like ferro-edenite, edenite and hornblende serve as important petrographic minerals. Their presence allows petrologists (rock scientists) to accurately gauge the pressure of the rock during crystallization. From this information a rock's depth during crystallization can be deduced.

 

PHYSICAL CHARACTERISTICS:

  • Color is green to black.
  • Luster is vitreous to dull.
  • Transparency: Crystals are generally translucent to opaque.
  • Crystal System is Monoclinic; 2/m.
  • Crystal Habits include prismatic to bladed crystals with a nearly diamond shaped cross-section the points of which can be truncated by minor prism faces. The typical termination appears to be the two faces of a slightly slanted dome but is actually two of the four faces of a prism. The termination faces are not only slanted toward each other but the two faces are slanted with respect to the long axis of the crystal as well. Some terminations are rather complex and can make the crystal appear pseudo-orthorhombic. Twinning is commonly seen and results in a groove or notch running down the "spine" of the prismatic crystals.
  • Cleavage is imperfect in two directions at nearly 60 and 120 degrees.
  • Fracture is uneven.
  • Hardness is 5 - 6.
  • Specific Gravity is approximately 3.46 (above average for non-metallic minerals)
  • Streak is white.
  • Associated Minerals include edenite, tremolite, kaolinite, biotite, pyroxenes, spinel and calcite.
  • Notable Occurrences include Rio Grande de Norte, Brazil and other localities where edenite is found.
  • Best Field Indicators are crystal habit (especially cross-section), color, density and cleavage.



THE MINERAL FERROGLAUCOPHANE


Ferroglaucophane is an uncommon amphibole mineral. Its name comes from its increased iron (or ferrous) content over that of it close cousin glaucophane. Ferroglaucophane forms a solid solution series with the more common glaucophane; Na2 (Mg, Fe)3Al2Si8O22(OH)2. Ferroglaucophane and glaucophane have the same structure and the same chemistry except for their respective iron and magnesium content. Ferroglaucophane is similar to glaucophane but because of more iron in its formula it is slightly denser and generally darker in color with a duller luster. Both minerals are related to riebeckite.

Ferroglaucophane is formed in metamorphic zones known as blueschist facies. This facies forms from material caught under subduction zones in mountain belt regions. This material has undergone intense pressure and relatively low heat as it was subducted downward almost to the mantle.

 

PHYSICAL CHARACTERISTICS:

  • Color is blue-gray, gray to black.
  • Luster is vitreous to dull.
  • Transparency: Crystals are generally translucent to opaque.
  • Crystal System is Monoclinic; 2/m
  • Crystal Habits include scarce prismatic to acicular crystals, usually fibrous, granular or massive.
  • Cleavage is imperfect in two directions at nearly 58 and 122 degrees.
  • Fracture is conchoidal to uneven.
  • Hardness is 6.
  • Specific Gravity is approximately 3.2 - 3.3 (slightly above average for non-metallic minerals).
  • Streak is pale gray to blue.
  • Other Characteristics: Strongly pleochroic from violet to blue to colorless.
  • Associated Minerals include epidote, paragonite, almandine, glaucophane, lawsonite, omphacite, spessartine, albite and jadeite.
  • Notable Occurrences include Champ de Praz, Val d'Aosta, Italy; near Menai Bridge, Anglesey, Wales; Scotland; Ile de Groix, France and Syra Island, Cyclades Islands, Greece as well as numerous other localities where glaucophane is found.
  • Best Field Indicators are crystal habit (especially cross-section), color, pleochroism, environment of formation, associations and cleavage.

 

 

THE MINERAL FIEDLERITE

 

  • Chemistry: Pb3Cl4F(OH)2, Lead Chloride Fluoride Hydroxide.
  • Class: Halides
  • Uses: Only as mineral specimens.
  • Specimens

Fiedlerite forms tiny, platy, colorless crystals and is an extremely rare mineral. It is known from one location, its type locality of Lavrio (formerly Lavrion and Laurium), Greece. An ancient lead mining area. Fiedlerite is named for a nineteenth century director of the mines, K. G. Feidler. Other rare minerals from this locality include paralaurionite, zincaluminite, nealite, laurionite, penfieldite, agardite, serpierite, thorikosite, ktenasite, glaucocerinite, beudantite, georgiadesite and phosgenite to name just a few. There are easily over three hundred and seventy minerals known from this locality. This locality has been mined for centuries starting with the Greeks and then the Romans for the lead content of its ores. The left over rocks, that were judged too poor in the metals to be processed by the ancient miners, were dumped into the sea. Such mining dumps are called slag dumps. Today these dumps are being reprocessed for their valuable metals by modern ore processing techniques that are capable of extracting the metals from these ores. Analysis of these rocks have yielded some amazing new minerals. Some of these minerals were not there when the rocks were first mined centuries before. But they are there now! The sea water altered the low grade lead ores and produced a most unusual assortment of rare minerals of which fiedlerite is one of them. Many people do not consider these minerals to be true minerals because their creation was indirectly aided by the actions of humans and therefore not exactly natural. Minerals must have a natural origin in order to be minerals. However, these minerals were only indirectly affected and the study of their origins is best left to mineralogists.

Fiedlerite and others minerals with similar chemistries belong to a division in the Halide Class called the Oxyhalides and Hydroxyhalides. These minerals have either oxygen or hydroxide groups in their chemistries. The oxygen atom in their chemistries might require their classification in the Oxides Class of minerals except that their structures are more tied to the halide elements and the oxygens and hydroxides are kind of superfluous to the overall structure.

 

THE PHYSICAL CHARACTERISTICS OF FIEDLERITE:

  • Color is colorless and white.
  • Luster is adamantine.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is monoclinic; 2/m.
  • Crystal Habits include tiny tabular to platy crystals.
  • Cleavage is good, but not discernible.
  • Fracture is conchoidal.
  • Hardness is 3.5.
  • Specific Gravity is 5.88 (very heavy for translucent minerals)
  • Streak is white.
  • Associated Minerals include paralaurionite, laurionite and phosgenite
  • Notable Occurrence is limited to its type locality of Lavrio (formerly Lavrion and Laurium), Greece.
  • Best Field Indicators are crystal habit, density and locality.






















FLUORITE, "The Most Colorful
Mineral in the World"



  • Chemistry: CaF2, Calcium Fluoride
  • Class: Halides
  • Uses: As a flux (hence the name) in iron smelting, a rare gemstone, a source of fluorine, as special optical lenses and a popular mineral specimen.
  • Physical Properties.
  • Specimens

Fluorite is a mineral with a veritable bouquet of brilliant colors. Fluorite is well known and prized for its glassy luster and rich variety of colors. The range of common colors for fluorite starting from the hallmark color purple, then blue, green, yellow, colorless, brown, pink, black and reddish orange is amazing and is only rivaled in color range by quartz. Intermediate pastels between the previously mentioned colors are also possible. It is easy to see why fluorite earns the reputation as "The Most Colorful Mineral in the World".

The many colors of fluorite are truly wonderful. The rich purple color is by far fluorite's most famous and popular color. It easily competes with the beautiful purple of amethyst. Often specimens of fluorite and amethyst with similar shades of purple are used in mineral identification classes to illustrate the folly of using color as the sole means to identify minerals.

The blue, green and yellow varieties of fluorite are also deeply colored, popular and attractive. The colorless variety is not as well received as the colored varieties, but their rarity still makes them sought after by collectors. A brown variety found in Ohio and elsewhere has a distinctive iridescence that improves an otherwise poor color for fluorite. The rarer colors of pink, reddish orange (rose) and even black are usually very attractive and in demand.

Most specimens of fluorite have a single color, but a significant percentage of fluorites have multiple colors and the colors are arranged in bands or zones that correspond to the shapes of fluorite's crystals. In other words, the typical habit of fluorite is a cube and the color zones are often in cubic arrangement. The effect is similar to phantomed crystals that appear to have crystals within crystals that are of differing colors. A fluorite crystal could have a clear outer zone allowing a cube of purple fluorite to be seen inside. Sometimes the less common habits such as a colored octahedron are seen inside of a colorless cube. One crystal of fluorite could potentially have four or five different color zones or bands.

To top it all off, fluorite is frequently fluorescent and, like its normal light colors, its fluorescent colors are extremely variable. Typically it fluoresces blue but other fluorescent colors include yellow, green, red, white and purple. Some specimens have the added effect of simultaniously having a different color under longwave UV light from its color under shortwave UV light. And some will even demonstrate phosphorescence in a third color! That's four possible color luminescence in one specimen! If you count the normal light color too. The blue fluorescence has been attributed to the presence of europium ions (Eu +2). Yttrium is the activator for the yellow fluorescence. Green and red fluorescent activation is not exactly pinned down as of yet, but may be due to the elements already mentioned as well as other rare earth metals; also manganese, uranium or a combination of these. Even unbonded fluorine trapped in the structure has been suggested. The word fluorescent was derived from fluorite since specimens of fluorite were some of the first fluorescent specimens ever studied. The naming followed the naming precedence set by opalescence from opal; ergo fluorescence from fluorite.

Another unique luminescent property of fluorite is its thermoluminescence. Thermoluminescence is the ability to glow when heated. Not all fluorites do this, in fact it is quite a rare phenomenon. A variety of fluorite known as "chlorophane" can demonstrate this property very well and will even thermoluminesce while the specimen is held in a person's hand activated by the person's own body heat (of course in a dark room, as it is not bright enough to be seen in daylight). The thermoluminescence is green to blue-green and can be produced on the coils of a heater or electric stove top. Once seen, the glow will fade away and can no longer by seen in the same specimen again. It is a one shot deal. Chlorophane (which means to show green) is found in very limited quantities at Amelia Court House, Virginia; Franklin, New Jersey and the Bluebird Mine, Arizona, USA; Gilgit, Pakistan; Mont Saint-Hilaire, Quebec, Canada and at Nerchinsk in the Ural Mountains, Russia.

Fluorite has other qualities besides its great color assortments that make it a popular mineral. It has several different crystal habits that always produce well formed, good, clean crystals. The cube is by far the most recognized habit of fluorite followed by the octahedron which is believed to form at higher temperatures than the cube. Although the cleavage of fluorite can produce an octahedral shape and these cleaved octahedrons are popular in rock shops the world over, the natural (e.g. uncleaved) octahedrons are harder to find.

A rarer habit variety is the twelve sided dodecahedron however it is never seen by itself and usually modifies the cubic crystals by replacing the edges of the cube with one flat face of a dodecahedron. The tetrahexahedron is a twenty four sided habit that is also seen modifying the cubic habit. But instead of one face replacing each cubic edge, two faces modify the cube's edges. Occasionally combinations of a cube, dodecahedron and tetrahexahedron are seen producing an overall cubic crystal with no less that three minor parallel faces replacing each cubic edge. A fifth form is the hexoctahedron which modifies the cube by placing six very minor faces at each corner of the cube. Twinning is also common in fluorite and symmetrical penetration twins, especially from Cumberland England are much sought after by collectors.

Fluorite, as mention above, has octahedral cleavage. This means that it has four identical directions of cleavage and when cleaved in the right ways can produce a perfect octahedral shape. Many thousands of octahedrons are produced from massive or large undesirable crystals of fluorite (hopefully!) and are sold in rock shops and museum gift shops at a small cost. Fluorite mine workers are reported to sit down at lunch breaks and cleave the octahedrons for the extra cash. The octahedrons are very popular due to their attractive colors, clarity, "diamond-shaped" and low costs, but to a serious collector they are nothing more than "cleavage fragments".

Fluorite not only is attractive in its own right but is often associated with other attractive minerals. Fluorite crystals will frequently accompany specimens of silver gray galena, brassy yellow pyrite, chalcopyrite or marcasite, golden barite, black sparkling sphalerite, intricately crystallized calcite and crystal clear quartz, even amethyst.

The origin of the word fluorite comes from the use of fluorite as a flux in steel and aluminum processing. It was originally referred to as fluorospar by miners and is still called that today. Fluorite is also used as a source of fluorine for hydrofluoric acid and fluorinated water. The element fluorine also gets its name from fluorite, fluorines only common mineral. Other uses of fluorite include an uncommon use as a gemstone (low hardness and good cleavage reduce its desirability as a gemstone), ornamental carvings (sometimes misleadingly called Green Quartz) and special optical uses.

Fluorite is the most popular mineral for mineral collectors in the world, second only to quartz. Every mineral collection owned by even the newest and youngest of mineral collectors must have a specimen of fluorite. Fluorite is by far one of the most beautiful and interesting minerals available on the mineral markets.

 

THE PHYSICAL CHARACTERISTICS OF FLUORITE:

  • Color is extremely variable and many times can be an intense purple, blue, green or yellow; also colorless, reddish orange, pink, white and brown. A single crystal can be multi-colored.
  • Luster is vitreous.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System: Isometric; 4/m bar 3 2/m
  • Crystal Habits include the typical cube and to a lesser extent, the octahedron as well as combinations of these two and other rarer isometric habits. Always with equant crystals; less common are crusts and botryoidal forms. Twinning also produces penetration twins that look like two cubes grown together.
  • Cleavage is perfect in 4 directions forming octahedrons.
  • Fracture is irregular and brittle.
  • Hardness is 4
  • Specific Gravity is 3.1+ (average)
  • Streak is white.
  • Other Characteristics: Often fluorescent blue or more rarely green, white, red or violet and may be thermoluminescent, phosphorescent and triboluminescent.
  • Associated Minerals are many and include calcite, quartz, willemite, barite, witherite, apatite, chalcopyrite, galena, sphalerite, pyrite and other sulfides.
  • Notable Occurrences include in addition to those mentioned above Cumberland, England; Spain; China; Brazil; Morocco; Bancroft, Ontario, Canada; Naica, Chihuahua, Mexico; Germany; Elmwood, Tennessee; Rosiclare, Illinois; Fort Wayne, Indiana; Pugh Quarry and Wood County, Ohio; Nancy Hanks Mine, Colorado and many other USA localities as well as many other localities from around the world.
  • Best Field Indicators are crystal habit, color zoning, hardness (harder than calcite, but softer than quartz or apatite), fluorescence and especially the octahedral cleavage.





 

THE MINERAL FLUORRICHTERITE

 


Fluorrichterite is a rare mineral, known from only one location. It is found near Wilberforce, Ontario in a skarn formation. The skarn is a term for a rock that is the product of a chemically unusual magma body that has intruded into and recrystallized a "dirty" limestone. The "dirty" limestone is not composed of just calcite, CaCO3, like "clean" limestone but is mixed with silicates and/or phosphates, etc. This mixing of the hot chemically unusual liquids and volatiles of the magma with the different minerals of the "dirty" limestones produces some interesting and rare minerals after all the recrystallizing is done. One of these is of course, fluorrichterite.

Fluorrichterite forms well shaped crystals that can have a pearly luster. Its luster is higher than most members of the amphibole group possibly because of the unusual presence of fluorine which comes from the unusual magma chemistry. Mixed with fluorrichterite in the white recrystallized calcite of the skarn are well formed crystals of biotite making specimens extra special.

Fluorrichterite is a brand new mineral, only receiving official recognition in 1996, although it has been collected for many years by many collectors. During this time specimens have been called various names in addition to fluorrichterite such as fluor-silicic-edenite, richterite (another amphibole mineral), augite (a pyroxene!) and many others. Its good to get this straightened out finally.

 

PHYSICAL CHARACTERISTICS:

  • Color is dark green to blue green or less commonly brown, gray and black.
  • Luster is vitreous to pearly to dull.
  • Transparency: Crystals are generally translucent to opaque.
  • Crystal System is Monoclinic; 2/m
  • Crystal Habits include prismatic crystals with an overall nearly diamond shaped cross-section with the four points usually cut by minor prism faces. The typical termination appears to be two faces of a slightly slanted dome but is actually two of the four faces of a prism. The termination faces are not only slanted toward each other but the two faces are slanted with respect to the long axis of the crystal as well. Some terminations are rather complex and can make the crystal appear pseudo-orthorhombic. Due to dissolution reactions after initial crystallization, the crystal faces can appear pitted and the terminations can be rounded. Twinning is commonly seen and results in a groove or notch running down the "spine" of the prismatic crystals.
  • Cleavage is imperfect in two directions at nearly 60 and 120 degrees.
  • Fracture is uneven.
  • Hardness is 6.
  • Specific Gravity is approximately 3 (average for translucent minerals)
  • Streak is white.
  • Other Characteristics: Crystals can have a iridescent shimer and associated minerals can be seen intergrown in some of the crystals filling pits and voids on their surface.
  • Associated Minerals are biotite, molybdenite, cancrinite and calcite
  • Notable Occurrence is several sites near Wilberforce, Ontario, Canada.
  • Best Field Indicators are locality, crystal habit (especially cross-section), color, pearly luster and cleavage.

 

 

 

 

 

 

THE MINERAL FORSTERITE

 


Forsterite is named for the German naturalist, Johann Forster. It is one of two minerals that are simply known as olivine. The other mineral is fayalite. Fayalite is the iron rich member with a pure formula of Fe2SiO4. Forsterite is the magnesium rich member with a pure formula of Mg2SiO4. The two minerals form a series were the iron and magnesium are substitutable for each other without much effect to the crystal structure. Fayalite due to its iron content has a higher index of refraction, is heavier and has a darker color than forsterite. Otherwise they are difficult to distinguish and virtually all specimens of the two minerals contain iron and magnesium. For intermediate specimens or those difficult to differentiate as well as simplicity sake and general public recognition, they are often treated as one mineral, olivine, although it is not officially recognized as a mineral.

Forsterite's gemstone variety known as peridot, is one of the most mispronounced of gemstone names. The correct pronouciation has peridot rhyming with doe or depot. But peridot is often pronouced incorrectly so that it rhymes with dot. Peridot is the birthstone of August and is usually a very affordable colored gemstone. Unfortunately it is often compared to the rich dark green of emerald and in this comparison it is often found lacking. But peridot has its own unique green-yellow color that is different from emerald and this comparison is rather unfair. Most peridot is forsterite and its color is caused by the presence of iron ions. Fayalite's higher iron content make for darker, less attractive specimens that are not generally used as gemstones. The best colored peridot has an iron percentage less than 15% and includes nickel and chromium as trace elements that may also contribute to the best peridot color. Most gem grade forsterite comes from volcanic basalts, but the bem gravels of Mogok, Myanmar (formerly known as Burma) are thought to have weathered out of peridot-containing metamorphosed marbles.

Forsterite is found in ultramafic igneous rocks or in marbles made from the metamorphism of dolomitic limestones. Mafic is a word that is used to define igneous rocks with a high iron and magnesium content. The "MA" is for magnesium while the "F" is for ferrum, the latin word for iron. The olivine minerals have a high melting point and are the first minerals to crystallize from a mafic magma. Forsterite crystallizes first with fayalite crystallizing last when other minerals such as the pyroxenes are just beginning to form. This early crystallization is the reason that molten lavas will contain already crystallized grains of olivine before they are ejected from volcanoes. Some ultramafic intrusive rocks can be composed of almost all olivine and these are called dunites or peridotites. Forsterite is never found with quartz, SiO2, as these two common minerals are unstable in each others proximity (during crystallization of course). If quartz were present, the two would react and form the mineral enstatite by the following formula:
Mg2SiO4 (forsterite) + SiO2 (quartz) = 2MgSiO3 (enstatite)

Forsterite is also found in many iron-nickel meteorites. Not just as small grains but as significantly sized crystals sometimes occupying over 50% of the meteorites volume. Thinly cut slices of these meteorites are extremely attractive with the polished steel gray of the iron and the embedded grains of gemmy green forsterite crystals. The effect produces the closest mineral equilalent to stained glass artwork. Some meteoritic forsterite crystals have been cut into gems.

 

 

PHYSICAL CHARACTERISTICS:

  • Color is a light green yellow to the more common yellowish green; also known to be colorless.
  • Luster is vitreous.
  • Transparency crystals are transparent to translucent.
  • Crystal System is orthorhombic; 2/m 2/m 2/m.
  • Crystal Habits include flatten tabular to box shaped crystals, but good crystals are rare. More commonly found as grains in alluvial gravels and as granular xenoliths in magnesium rich volcanic rock. Also massive. Twinning is rare, but has produced star shaped trillings.
  • Cleavage is poor in two directions at 90 degrees, rather indistinct and rarely noticed.
  • Fracture is conchoidal.
  • Hardness is 6.5 - 7.
  • Specific Gravity is approximately 3.2 when pure Mg2SiO4 and rises with increasing iron content (somewhat above average for non-metallic minerals).
  • Streak is white.
  • Other Characteristics: Index of refraction is 1.64 - 1.71 and has double refraction.
  • Associated Minerals are diopside, spinel, plagioclase feldspars, chromite, magnetite, talc, hornblende, serpentine, iron-nickel meteorites and augite.
  • Notable Occurrences are numerous and include the ancient source of Zagbargad (Zebirget) Island in the Red Sea off the coast of Egypt and Mogok, Myanmar (formerly known as Burma). The type locality is Mt. Vesuvius, Italy and other localities include Kohistan, Pakistan; South Africa; Ural Mountains, Russia; Norway; Sweden; France; Minas Gerais, Brazil; Eifel, Germany; Chihuahua, Mexico; Ethiopia; Mt. Franklin, Victoria, Australia; Notre-Dame-du-Laus, Quebec Canada; China and Salt Lake Crater, Oahu, Hawaii; North Carolina; Bolton, Massachusetts; Crestmore, California; New Mexico and the most productive source of peridot to date from Peridot Mesa, San Carlos Apache Reservation, Gila County, Arizona, USA.
  • Best Field Indicators are color, hardness, mafic or metamorphic environment, lack of good cleavage and density.






  

THE MINERAL FRANCKEITE

 

  • Chemistry: (Pb, Sn)6FeSn2Sb2S14, lead Tin Iron Antimony Sulfide.
  • Class: Sulfides
  • Subclass: Sulfosalts
  • Uses: A very minor ore of lead and tin and as a mineral specimen.
  • Specimens

Franckeite is a rare sulfide mineral. It is found in the sulfide ores in Bolivia and Peru usually with other rare sulfides such as cylindrite, stannite, incaite and potosiite. Franckeite is a sulfosalt. Normally semi-metal ions such as antimony will substitute for the sulfur atoms, but in the sulfosalts they substitute for the metal ions and bond with the sulfurs.
The formula of franceite points out the difference between the two different tin ions in the mineral. If the formula was written with the oxidation states, it would look like this:
(Pb+2,Sn+2)6Fe+2Sn+42Sb+32S-214.

As can be seen there are two different tin ions. One is in a positive two (+2) oxidation state and the other is in a positive four (+4) oxidation state. The oxidation states also show the positive charge on the antimonies, a condition found in sulfosalts.

 

PHYSICAL CHARACTERISTICS:

  • Color is gray-black to gray-white.
  • Luster is metallic.
  • Transparency crystals are opaque.
  • Crystal System is triclinic; bar 1
  • Crystal Habits include accicular sprays and crusts.
  • Hardness is 2.5 - 3
  • Specific Gravity is approximately 5.8 - 6.0 (above average for metallic minerals)
  • Streak is gray-black.
  • Associated Minerals include quartz, galena and pyrite as well as rare sulfides such as cylindrite, stannite, incaite and potosiite.
  • Notable Occurrences are limited to Las Animas District, Bolivia and Hulcani Mine, Peru.
  • Best Field Indicators are crystal habit, locality, color and density.





  

 THE MINERAL FRANKLINITE

 

  • Chemical Formula: (Zn, Fe, Mn)(Fe, Mn)2O4, Zinc Iron Manganese Oxide
  • Class: Oxides and Hydroxides
  • Group: Spinel
  • Uses: Important ore of zinc and manganese and as mineral specimens.
  • Specimens

Franklinite is one of the minerals found at Franklin, New Jersey, a world famous locality that has produced many formerly unknown and exotic mineral species. It is found in large enough quantity to serve as a ore of zinc and manganese, two important strategic and industrial metals. It forms octahedral crystals that are typical of the spinel group of minerals. Specimens from Franklin often contain the rounded black grains of franklinite surrounded by white calcite and/or greenish willemite with a sprinkling of red zincite. Specimens of this exotic and interesting mineral are truly valued by mineral collectors.

 

PHYSICAL CHARACTERISTICS:

  • Color is dark black.
  • Luster is metallic.
  • Transparency: Crystals are opaque.
  • Crystal System is isometric; 4/m bar 3 2/m
  • Crystal Habits include octahedrons often with dodecahedral faces modifing the edges of the octahedron. The modifing can lead to an overall rounding of the crystal. Also as massive and granular.
  • Cleavage is absent.
  • Fracture is conchoidal.
  • Hardness is 6
  • Specific Gravity is 5.0 - 5.2 (slightly above average for metallic minerals)
  • Streak is reddish brown.
  • Other characteristics: Slightly magnetic.
  • Associated Minerals include willemite, zincite, calcite, rhodonite and other minerals found at Franklin, New Jersey.
  • Notable Occurrences are limited to the world famous mines at Franklin, New Jersey, USA from where franklinite gets its name.
  • Best Field Indicators are crystal habit, streak, associations with other zinc minerals and locality.

 



 

  

FUCHSITE,

the green variety of muscovite

 

VARIETY INFORMATION:

  • VARIETY OF: Muscovite , KAl2(AlSi3O10)(F, OH)2, Potassium aluminum silicate hydroxide fluoride.
  • USES: mineral specimen and ornamental stone.
  • COLOR: emerald green.
  • HARDNESS: 2 - 3
  • CLEAVAGE: perfect in one direction, forming sheets.
  • CRYSTAL SYSTEM: monoclinic
  • Specimens

Fuchsite is a dark green variety of muscovite. It can have a wonderfully dark emerald green color that can have a beautiful sparkly shine if the crystals are small. It is often found in compact masses with small crystals. The green color is the result of chromium impurities.

 

 

 

 

 

 

 

 

 

 

 

THE MINERALOID LECHATELIERITE

 

  • Chemistry: SiO2; Fused Silicon Dioxide.
  • Class: Mineraloids
  • Group: Some minerologists place lechatelierite in the Quartz Group.
  • Uses: Only as mineral specimens.
  • Specimens

Lechatelierite is considered a mineraloid because it lacks a crystal structure. It is actually made of fused silica; SiO2. Quartz is also made of silica, but it has a crystalline structure. Basically lechatelierite is glass! However, it differs from regular glass by virtue of its natural origins. Lechatelierite is also not the same as obsidian. Although obsidian is considered a natural glass it is not pure SiO2. And although lechatelierite is sometimes formed as a result of meteorite impacts like tektites, they are not the same. Unless the tektites are very pure silica as in the case of a variety called Libyan Glass (see above). To sum all of this up, lechatelierite is natural, pure or nearly pure, silica glass.

Lechatelierite has three different origins: Meteorite impacts, volcanism and lightning strikes. The meteorite impacts simply fused quartz grains in the impacted rocks with the incredible pressures that they generate. The volcanism origin is the same as for obsidian, where molten rock cools too quickly for a crystalline structure to form. But as was explained before, obsidian is generally a mix of several chemistries besides SiO2. The lightning strike origin is perhaps the most interesting.

When lightning strikes the earth in sandy areas such as deserts or sandy beaches, the intense heat and energy of the bolt will fuse the sand grains in a milli-second. A branching, hollow, glassy tube is sometimes the result. These "fossil" lightning strikes are composed of lechatelierite and occassionally fused biotite or other minerals. They are referred to as fulgarites and are certainly a natural history oddity. Fulgarites have been found that were several feet in length, although most are a few inches long by a half inch thick.

 

PHYSICAL CHARACTERISTICS:

  • Color is white, colorless or pale yellow; some fulgarites can be pale brown, gray or black.
  • Luster is vitreous.
  • Transparency: Specimens are translucent.
  • Crystal System: Does not apply because lechatelierite is amorphous.
  • Habits include disseminated grains in volcanic rocks and shatter zones of meteorite impacts and as tube shaped fulgarites (see above).
  • Cleavage is absent.
  • Fracture is conchoidal.
  • Hardness is 6.5
  • Specific Gravity is approximately 2.5 - 2.7 (average)
  • Streak is white.
  • Other Characteristics: Non-fluorescent.
  • Associated rocks are shocked quartz, volcanic rocks and sandy soils.
  • Best Field Indicators are shape, color, density, non-fluorescence and hardness.
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