THE MINERAL GASPEITE
- Chemistry: (Ni, Mg, Fe)CO3, Nickel Magnesium Iron Carbonate
- Class: Carbonate
- Group: Calcite
- Uses: As an ornamental stone and as mineral specimens.
- Specimens
Gaspeite is considered a rare mineral, being found in only a few localities, but it is becoming a popular mineral in the semi-precious stone markets. It is being used as a cabochon in sterling silver jewelry with such notable minerals and material as turquoise, malachite, azurite, onyx and coral. Gaspeite's light green, almost apple green color is quite unique and not hard to mistake. It real accents the banded greens of malachite and the blue green of turquoise. Carved material is generally homogenous, but it may contain portions of its brownish host rock which may give it a distinctive character.
Gaspeite is found as a secondary mineral around nickel sulfide deposits. It was thought of as just a gangue mineral by miners when it was encountered and usually placed in the mines dumps or tailings. A gangue mineral is a mineral that probably contains the ore metal, but its chemistry is such that processing it is either impossible or unprofitable. Such was the case with gaspeite despite being rich in nickel. Of course lapidary craftsmen have a different view of gaspeite's profitability! Gaspeite is named for its type locality of Gaspe' Peninsula, Lemieux Township, Gaspe'-ouest County, Quebec, Canada, but the best material of late is coming from North of Perth, Australia.
PHYSICAL CHARACTERISTICS:
- Color is a pale green to apple green.
- Luster is vitreous to dull.
- Transparency: Crystals are usually translucent, massive material is opaque.
- Crystal System is trigonal; bar 3 2/m.
- Crystal Habits include rhombohedrons and scalenohedrons, but crystals are very rare. More commonly found massive.
- Cleavage is perfect in 3 directions forming rhombohedrons.
- Fracture is uneven.
- Hardness is 4.5 - 5.
- Specific Gravity is 3.7 (somewhat heavy for a translucent mineral).
- Streak is yellowish green.
- Other Characteristics: Effervesces slightly in hydrochloric acid.
- Associated Minerals include millerite, pentlandite, skutterudite, annabergite and other nickel minerals.
- Notable Occurrences include the type locality of Gaspe' Peninsula, Lemieux Township, Gaspe'-ouest County, Quebec and Sudbury, Ontario, Canada and a new source of carvable material from Kambalda and Widgie Mooltha which is North of Perth, Australia.
- Best Field Indicators are color, locality, reaction to acids, cleavage and density.
THE MINERAL GAHNITE
- Chemistry: ZnAl2O4 , Zinc Aluminum Oxide
- Class: Oxides
- Group: Spinel
- Uses: Only as a mineral specimens.
- Specimens
Gahnite is one of the rarer members of the Spinel Group of minerals, an important group of oxides. It is named after Swedish chemist L. G. Gahn, who discovered manganese. Gahnite forms in some granitic pegmatites, zinc deposits and in skarns, a special type of contact metamorphic rock.
Gahnite produces crystals showing well formed octahedrons. The crystals may show a type of twinning called the Spinel Twin Law. It is common among members of the spinel group and is made famous by it namesake member, spinel.
This type of twinning produces a twin plane that is parallel to one of the octahedral faces. The plane acts as a mirror plane and produces a left and right side that are mirror images of each other. This may not sound all that spectacular for a very symmetrical mineral like spinel which is loaded with mirror planes. However this mirror plane is not parallel to any of the others and lowers the outward symmetry of the crystal. The crystal will now have a trigonal appearance with a flat triangular top and bottom.
PHYSICAL CHARACTERISTICS:
- Color is commonly green but also known to be blue, yellow, brown or gray.
- Luster is vitreous.
- Transparency: Crystals are generally just translucent to almost opaque, but some slightly transparent specimens are known.
- Crystal System: Isometric; 4/m bar 3 2/m
- Crystal Habits include the typical octahedron, but can be found as dodecahedrons and cubes. Octahedrons may be truncated by dodecahedral faces and commonly show striations parallel to the octahedron-dodecahedron boundary. Twinning by the Spinel Twin Law can produce trigonal appearing crystals with three reentrant angles on the sides of the crystal.
- Cleavage: None
- Fracture is conchoidal.
- Hardness is 7.5 - 8.0
- Specific Gravity is 4.55 - 4.60
- Streak is gray.
- Associated Minerals include calcite, spinel, franklinite and several types of garnets.
- Notable Occurrences include Rowe and Charlemont, Massachusetts; Spruce Pine, North Carolina; White Picacho district, Arizona; Topsham, Maine and Franklin New Jersey, USA and Falun, Sweden.
- Best Field Indicators are crystal habit, color, hardness, density and locality.
THE MINERAL GALENA
- Chemistry: PbS, Lead Sulfide
- Class: Sulfides
- Group: Galena
- Uses: Major ore of lead and silver
- Specimens
Galena is a common and popular mineral for rock hounds. Its characteristic cubes, distinctive cleavage and high density make it easy to identify and a favorite in high school geology labs. The structure of Galena is identical to that of halite, NaCl. The two minerals have the same crystal shapes, symmetry and cleavage. Some Galena may contain up to 1% silver in place of lead. The large volume of Galena that is processed for lead produces enough Silver as a by product to make Galena the leading ore of Silver.
To quote Allen N Wollscheidt, "Galena, back 75 years ago, was the stuff -- the crystal -- of crystal radio sets. Surely you have heard of these -- possibly your grand or greatgrandparents built radio receivers out of round Quaker Oats boxes, a few feet of copper wire, a pair of headphones and a little "cats-whisker-with crystal" gadget from the hardware store. Some worked so well, they could be heard across the room. Galena is a natural semiconductor and so the forerunner, the enabler, of all the electronic gadgets we have today, from telephones to TVs to GPS navigating systems as well as all sorts of medical equipment -- in short, modern life as we know it.”
PHYSICAL CHARACTERISTICS:
- Color is lead to silver gray sometimes with a bluish tint.
- Luster is metallic to dull in weathered faces.
- Transparency crystals are opaque.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits include the cube, octahedron and combinations of the two. Spinel twinning is possible forming flattened crystals. Also massive and granular.
- Cleavage is perfect in four direction forming cubes.
- Fracture is uneven and rarely seen because of the perfect cleavage.
- Hardness is 2.5+
- Specific Gravity is approximately 7.5+ (heavy even for metallic minerals)
- Streak is lead gray
- Associated Minerals are calcite, dolomite, sphalerite, pyrite and other sulfide minerals, also lead oxidation minerals such as cerussite and anglesite.
- Other Characteristics: brighter metallic luster on cleavage surfaces than on crystal faces.
- Notable Occurances include Texas-Oklahoma-Missouri area, USA; Germany, Peru, Mexico, Zambia, and England.
- Best Field Indicators are crystal habit, cleavage and, perhaps most importantly, density.
| By Name A list of minerals in alphabetical order By Class Elements, Oxides, Carbonates, etc. Interesting Groupings Gemstones, Birthstones, etc. Full Text Search Mineral identification by keyword searching Physical Properties Keys to identifying minerals Garnet Jewelry  |  |
Garnets as a group are relatively common in highly metamorphosed rocks and in some igneous formations. They form under the high temperatures and/or pressures that those types of rocks must endure. Garnets can be used by geologists as a gauge of how much temperature and pressure the rock has endured. As a gemstone, garnets have had a mixed reputation. Garnets do possess high indices of refraction, are hard enough, have pretty colors, are wonderfully transparent, lack cleavage and are durable; thus making good candidates for gemstones. However, many people consider garnets to be inferior to other colored gems. This may be due to garnet's relative abundance and widespread use, and therefore a (typically) low price. Garnets are greatly variable in colors and varieties, though, and many of these are both rare and beautiful, producing genuinely precious gemstones. Some garnets are truly unique in the mineral kingdom and have much to offer as both gemstones and mineral specimens. Of course, garnets are the January Birthstone. The general formula for most of the garnets is A3B2(SiO4)3. The A represents divalent metals such as calcium, iron, magnesium and/or manganese. The B represents trivalent metals such as aluminum, chromium, iron and/or manganese and in the rarer garnets; vanadium, titanium, zirconium and/or silicon. The general formula for a couple of rarer garnets (hibschite and katoite) is A3B2(SiO4)3-X(OH)4X. The main differences in physical properties among the members of the garnet group are slight variations in color, density and index of refraction. Garnets are isostructural, meaning that they share the same crystal structure. This leads to similar crystal shapes and properties. Garnets belong to the isometric crystal class, which produces very symmetrical, cube-based crystals. The most common crystal shape for garnets however is the rhombic dodecahedron, a twelve sided crystal with diamond-shaped (rhombic) faces. This basic shape is the trademark of garnets, for no other crystal shape is so closely associated with a single mineral group like the rhombic dodecahedron is with garnets. Most garnets are red in color, leading to the erroneous belief that all garnets are red. In fact a few varieties, such as grossular, can have a wide range of colors, and uvarovite is always a bright green. As a mineral specimen, garnets usually have well shaped and complex crystals and their color and luster can make for a very beautiful addition to a collection. At times, garnets are accessory minerals to other valuable and pretty gem minerals such as topaz, beryl, tourmaline, vesuvianite and diopside making these specimens extra special. The Garnet Group is actually a larger group than most people know. In the table below are the more common members with their chemistries and their significant divergent properties: density (SG), index of refraction (IR), and the typical color as well as their general environment of formation. Under environment of formation: metamorphic is indicated with an M and igneous with an I. All garnets may contribute to detrital sedimentary rocks but they are not of original sedimentary origin.
Other rarer members of the garnet group inlcude calderite, goldmanite, henritermierite, hibschite, katoite, kimzeyite, knorringite, majorite, morimotoite and schorlomite. The mineral yafsoanite, a tellurate, the halide mineral cryolithionite, arsenate minerals berzeliite and manganberzeliite and the vanadate mineral palenzoaite share the same structure with the Garnet Group. |
THE MINERAL GASPEITE
- Chemistry: (Ni, Mg, Fe)CO3, Nickel Magnesium Iron Carbonate
- Class: Carbonate
- Group: Calcite
- Uses: As an ornamental stone and as mineral specimens.
- Specimens
Gaspeite is considered a rare mineral, being found in only a few localities, but it is becoming a popular mineral in the semi-precious stone markets. It is being used as a cabochon in sterling silver jewelry with such notable minerals and material as turquoise, malachite, azurite, onyx and coral. Gaspeite's light green, almost apple green color is quite unique and not hard to mistake. It real accents the banded greens of malachite and the blue green of turquoise. Carved material is generally homogenous, but it may contain portions of its brownish host rock which may give it a distinctive character.
Gaspeite is found as a secondary mineral around nickel sulfide deposits. It was thought of as just a gangue mineral by miners when it was encountered and usually placed in the mines dumps or tailings. A gangue mineral is a mineral that probably contains the ore metal, but its chemistry is such that processing it is either impossible or unprofitable. Such was the case with gaspeite despite being rich in nickel. Of course lapidary craftsmen have a different view of gaspeite's profitability! Gaspeite is named for its type locality of Gaspe' Peninsula, Lemieux Township, Gaspe'-ouest County, Quebec, Canada, but the best material of late is coming from North of Perth, Australia.
PHYSICAL CHARACTERISTICS:
- Color is a pale green to apple green.
- Luster is vitreous to dull.
- Transparency: Crystals are usually translucent, massive material is opaque.
- Crystal System is trigonal; bar 3 2/m.
- Crystal Habits include rhombohedrons and scalenohedrons, but crystals are very rare. More commonly found massive.
- Cleavage is perfect in 3 directions forming rhombohedrons.
- Fracture is uneven.
- Hardness is 4.5 - 5.
- Specific Gravity is 3.7 (somewhat heavy for a translucent mineral).
- Streak is yellowish green.
- Other Characteristics: Effervesces slightly in hydrochloric acid.
- Associated Minerals include millerite, pentlandite, skutterudite, annabergite and other nickel minerals.
- Notable Occurrences include the type locality of Gaspe' Peninsula, Lemieux Township, Gaspe'-ouest County, Quebec and Sudbury, Ontario, Canada and a new source of carvable material from Kambalda and Widgie Mooltha which is North of Perth, Australia.
- Best Field Indicators are color, locality, reaction to acids, cleavage and density.
THE MINERAL GAYLUSSITE
- Chemistry: Na2Ca(CO3)2 - 5H2O , Hydrated Sodium Calcium Carbonate.
- Class: Carbonates
- Uses: Only as mineral specimens.
- Specimens
Gaylussite is named for French chemist and physicist J. L. Gay-lussac. It is sometimes a well crystallized mineral displaying good, intricately faceted monoclinic forms. Gaylussite is one of several carbonate minerals that form in non-marine evaporite deposits. Other such carbonates include trona, pirssonite, northupite, nahcolite and thermonatrite. They are all difficult to tell apart from each other except when good crystal form is present or when optical or X-ray techniques can be used. Gaylussite is subject to dehydration which can cause cloudiness and deterioration and should therefore be stored in a sealed container.
PHYSICAL CHARACTERISTICS:
- Color is colorless, white or yellowish.
- Luster is vitreous.
- Transparency: Crystals are transparent to translucent.
- Crystal System is monoclinic; 2/m.
- Crystal Habits include intricately faceted prismatic to tabular crystals, but also massive and encrusting.
- Cleavage is perfect in two directions.
- Fracture is conchoidal.
- Hardness is 2 - 3.
- Specific Gravity is 1.9 - 2.0 (well below average)
- Streak is white.
- Associated Minerals include pirssonite, northupite, trona, analcime and halite.
- Notable Occurrences include Searles Lake, San Berardino County; Deep Spring and Owens Lake, Inyo County; Borax Lake, Lake County and China Lake, Kern County, California, USA; Gobi Desert, Mongolia and Lagunillas, Merida, Venezuela.
- Best Field Indicators: Crystal habit, environment of formation, cleavage, density and locality.
THE MINERAL GERSDORFFITE
- Chemistry: NiAsS, Nickel Arsenic Sulfide
- Class: Sulfides
- Group: Cobaltite
- Uses: As a very minor ore of nickel and as mineral specimens.
- Specimens
Gersdorffite, which is named for Johann von Gersdorff, the owner of the mine at which it was found, is a very uncommon nickel mineral that belongs to the Cobaltite Group of minerals. Most members of this group are rare to extremely rare except for cobaltite. Like other members of this group, gersdorffite can form nice crystals that are either cubic, octahedral, tetrahedral or even pyritohedral, the classic crystal form of pyrite.
Gersdorffite's symmetry is isometric and it has the same unique symmetry as pyrite, skutterudite and cobaltite. They all belong to the Diploidal symmetry class; bar 3 2/m, a class that, although isometric, lacks four fold symmetry. The gersdorffite structure is similar to pyrite's except that the S2 groups of pyrite are replaced by As-S groups in gersdorffite. The arsenic and sulfur are bonded together with a covalent bond. If the arsenic and sulfur have an ordered arrangement (such that all of the arsenic atoms are to the left or all are to the right of the As-S bonds), then the symmetry is reduced to the lower symmetry of the Tetartoidal Class; 2 3. This is the lowest symmetry class of the isometric system and contains only three fold and two fold axes. Most gersdorffite has an unordered arsenic-sulfur structure and therefore belongs to the higher Diploidal symmetry class.
Gersdorffite is very similar to a nickel antimony sulfide called ullmannite. Ullmannite's formula is NiSbS and shows the substitution of antimony for the arsenic in gersdorffite's formula. The two elements are themselves very similar and therefore do not cause appreciable differences in their respectively enriched minerals. The two minerals are almost indistinguishable by ordinary means although gersdorffite is much more common.
Gersdorffite forms a solid solution series with cobaltite, meaning the nickel and cobalt in the two minerals are substitutable. Both minerals are found in magma derived sulfide deposits. Gersdorffite is also found in hydrothermal veins that crystallized at moderate temperatures.
PHYSICAL CHARACTERISTICS:
- Color is white to gray.
- Luster is metallic.
- Transparency: Crystals are opaque.
- Crystal System: Isometric; bar 3 2/m.
- Crystal Habits include cubic, octahedral, tetrahedral and pyritohedral crystals (sometimes tabular) as well as massive and granular forms.
- Cleavage is good in three directions forming cubes.
- Fracture: Uneven.
- Hardness is 5 - 5.5
- Specific Gravity is 6.6 - 6.7 (heavier than average for metallic minerals)
- Streak is gray-black.
- Other Characteristics: Crystals are sometimes striated.
- Associated Minerals include nickeline, wollastonite, sphalerite, calcite, pyrrhotite, lollingite, annabergite, skutterudite, cobaltite, arsenopyrite, erythrite, nickelskutterudite, pentlandite, siderite and other nickel and iron minerals.
- Notable Occurrences include the type locality of Schladming, Salzburg, Austria as well as Sudbury, Ontario and Mont Saint-Hilaire, Quebec, Canada; Lobenstein, Germany; Yudjinyi Mine, Southern Ural Mountains, Russia; Noire, France; Kragero, Norway; Tasmania, Australia; Scordale Lead Mines, Warcop, United Kingdom; Franklin, New Jersey; Henshaw Quarry, San Bernardino County and the Jamestown Mine, Tuolumne County, California; Montana and in the Black Hawk District, Grant County, New Mexico, USA
- Best Field Indicators are crystal habit, cleavage, hardness and density.
THE MINERAL GIBBSITE
- Chemical Formula: Al(OH)3, Aluminum Hydroxide
- Class: Oxides and Hydroxides
- Uses: A minor source of metallic aluminum.
- Specimens
Gibbsite is an important ore of aluminum and is one of three minerals that make up the rock Bauxite. Bauxite is often thought of as a mineral but is really a rock composed of aluminum oxide and hydroxide minerals such as gibbsite, boehmite, AlO(OH) and diaspore, HAlO2, as well as clays, silt and iron oxides and hydroxides. Bauxite is a laterite, a rock formed from intense weathering environments such as found in richly forested, humid, tropical climates.
Gibbsite's structure is interesting and analogous to the basic structure of micas. The basic structure forms stacked sheets of linked octahedrons of aluminum hydroxide. The octahedrons are composed of aluminum ions with a +3 charge bonded to six octahedrally coordinated hydroxides with a -1 charge. Each of the hydroxides is bonded to only two aluminums because one third of the octahedrons are vacant a central aluminum. The result is a neutral sheet since +3/6 = +1/2 (+3 charge on the aluminums divided by six hydroxide bonds times the number of aluminums) and -1/2 = -1/2 (-1 charge on the hydroxides divided between only two aluminums); thus the charges cancel. The lack of a charge on the gibbsite sheets means that there is no charge to retain ions between the sheets and act as a "glue" to keep the sheets together. The sheets are only held together by weak residual bonds and this results in a very soft easily cleaved mineral.
Gibbsite's structure is closely related to the structure of brucite, Mg(OH)2. However the lower charge in brucite's magnesium (+2) as opposed to gibbsite's aluminum (+3) does not require that one third of the octahedrons be vacant of a central ion in order to maintain a neutral sheet. The different symmetry of gibbsite and brucite is due to the different way that the layers are stacked.
It is the gibbsite layer that in a way forms the "floor plan" for the mineral corundum, Al2O3. The basic structure of corundum is identical to gibbsite except the hydroxides are replaced by oxygen. Since oxygen has a charge of -2 the layers are not neutral and require that they must be bonded to other aluminums above and below the initial layer producing the framework structure that is the structure of corundum.
Gibbsite is interesting for another reason because it is often found as a part of the structure of other minerals. The neutral aluminum hydroxide sheets are found sandwiched between silicate sheets in important clay groups: the Illite, Kaolinite and Montmorillonite/smectite groups. The individual aluminum hydroxide layers are identical to the individual layers of gibbsite and are referred to as the "gibbsite layers".
PHYSICAL CHARACTERISTICS:
- Color is white or colorless with shades of gray, blue and green.
- Luster is vitreous to dull; cleavage surfaces have a pearly luster.
- Transparency Crystals are translucent and rarely transparent.
- Crystal System is monoclinic; 2/m.
- Crystal Habit is usually massive but rare crystals are found in flattened tabular crystals. In many bauxite specimens gibbsite is found in a pisolitic habit. Also found as botryoidal encrustations, concretionary, stalactitic and foliated masses.
- Cleavage is perfect in one direction, basal.
- Fracture is uneven.
- Hardness is 2.5 - 3.5
- Specific Gravity is 2.4 (slightly below average)
- Streak is white.
- Other Characteristics: When breathed on, gibbsite gives off a noticeable clay smell.
- Associated Minerals are boehmite, diaspore, azurite, hydrozincite and aurichalcite.
- Notable Occurrences include Vogelsberg, Germany; Gant, Hungary; Les Baux, France; Lavrion, Greece; Guyana and Arkansas, USA.
- Best Field Indicators are crystal habits, hardness, smell and associations.
THE MINERAL GLAUBERITE
- Chemistry: Na2Ca(SO4)2, Sodium Calcium Sulfate
- Class: Sulfates
- Uses: A minor source of "glauber's salt", Na2SO4, and as mineral specimens.
- Specimens
Glauberite is a mineral known better for when it is not there, than from when it is there. What does that mean? Well glauberite is often pseudomorphed and "cast" by other minerals.
A pseudomorph occurs when a mineral is replaced by another mineral at such a slow pace, essentially atom by atom, that no deteriorating effects occur to the shape of the crystal. The pseudomorph (meaning false shape) may look like glauberite's crystals but it is actually another mineral or even opal in one occurrence. Several minerals have been found to replace glauberite such as calcite, quartz, gypsum and as mentioned, opal.
Glauberite also becomes cast by several minerals. It is soluble in water and probably this is the reason for this tendency. After glauberite forms other minerals grow on top of the crystals. Later, the glauberite dissolves away, leaving a cast of itself as the only evidence of its past presence. Its crystal habit is unique enough that both pseudomorphs and casts can usually be identified as being produced from a glauberite crystal. Some quartz and prehnite casts from Paterson, New Jersey were once thought to belong to a strange mystery mineral that no longer exists at the site. But most of these casts are now confidently assigned as glauberite casts.
Glauberite is named for the salt that helps compose its chemistry called "Glauber's salt" after a German chemist. The Na2SO4, is used in dying, solar energy cells and in medicine. Glauberite could be thought of, in very general terms, as a combination of anhydrite, CaSO4, and glauber's salt.
Crystals of glauberite are distinctive and difficult to describe. They are steeply inclined wedge to tabular shaped dipyramidal crystals. Their monoclinic character is easily seen as most symmetry is lacking. However they are usually well-formed and interesting to examine.
Glauberite forms in saline environments with other evaporite minerals such as halite, gypsum, calcite and thenardite. Modern as well as prehistoric evaporite deposits can have glauberite crystals. Some glauberite has been identified in sublimate deposits on lavas off the coast of Sicily. Hydrothermal precipitation of glauberite is also seen.
PHYSICAL CHARACTERISTICS:
- Color is ordinarily white, yellow, gray or colorless.
- Luster is vitreous, greasy to dull.
- Transparency crystals are transparent to translucent.
- Crystal System is monoclinic; 2/m.
- Crystal Habits include the steeply inclined wedge to tabular shaped dipyramidal crystals.
- Cleavage is perfect in one direction (basal).
- Fracture is conchoidal.
- Hardness is 2.5 - 3
- Specific Gravity is approximately 2.7 - 2.8 (average for translucent minerals)
- Streak is white.
- Other Characteristics: Bitter salty taste, striations on most crystal faces and glauberite will dissolve in water but not as readily as halite.
- Associated Minerals are calcite, halite, thenardite, gypsum and certain clays.
- Notable Occurrences include Saline Valley, Inyo Co. and Searles Lake, San Bernardino Co., California, Great Salt Lake, Utah, Texas, New Mexico and Verde Valley, Arizona, USA; Strassfurt, Germany; Russia; Kenya; Chile; Toledo, Spain; Salzburg, Austria; France; Canada; Sicily and India. Pseudomorphs and casts come from many of the previously mentioned localities and from Paterson and Great Notch, New Jersey, USA.
- Best Field Indicators are crystal habit, softness, cleavage, taste, solubility and striations.
THE MINERAL GLAUCOPHANE
- Chemistry: Na2 (Mg, Fe)3Al2Si8O22(OH)2, Sodium Magnesium Iron Aluminum Silicate Hydroxide.
- Class: Silicates
- Subclass: Inosilicates
- Group: Amphibole
- Uses: Only as mineral specimens.
- Specimens
Glaucophane is named from its typical blue color (in Greek glaucophane means "blue appearing"). The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckite are the only common amphibole minerals that are typically blue.
Glaucophane forms a solid solution series with ferroglaucophane; Na2 (Fe, Mg)3Al2Si8O22(OH)2. Glaucophane is the magnesium rich member and ferroglaucophane is the iron rich member. Ferroglaucophane is similar to glaucophane but is denser, generally darker in color with a diminished pearly luster.
Glaucophane is formed typically in a highly metamorphic zone known by the geologic term blueschist facies. This facies forms from material caught under subduction zones in mountain belt regions. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle. Its glaucophane's color that gives this facies its name. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.
PHYSICAL CHARACTERISTICS:
- Color is blue to dull gray.
- Luster is vitreous to pearly.
- Transparency: Crystals are generally translucent.
- 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 56 and 124 degrees.
- Fracture is conchoidal to splintery.
- Hardness is 5 - 6.
- Specific Gravity is approximately 3 - 3.2 (slightly above average for non-metallic minerals).
- Streak is pale gray to blue.
- Other Characteristics: Pleochroic.
- Associated Minerals include chlorite, epidote, lawsonite, omphacite, garnets, albite, barroisite, cummingtonite, aragonite, sphene, rutile, quartz, jadeite, actinolite and pumpellyite.
- Notable Occurrences include the type locality of Syra Island, Cyclades Islands, Greece as well as numerous localities in the Coastal Range of California and the Kodiak Islands, Alaska, USA; St. Marcel, Val d'Aosta and Piollore (Biella), Italy; Anglesey, Wales, UK; Australia and Japan.
THE MINERAL GOETHITE
- Chemical Formula: FeO(OH), Hydrated Iron Oxide
- Class: Oxides and Hydroxides
- Group: Goethite
- Uses: important ore of iron, pigment
- Specimens
Goethite has had a reputation for making rather uninteresting, dull and "dirty" mineral specimens. This is definitely not true of many of the fine specimens available to collectors today. Some good pieces show splendid radiating clusters of deep black crystals. Massive stalactitic forms show rainbows of irridescent colors on surreal landscapes. Many times Goethite forms the backdrop or matrix for other minerals such as red Vanadinite, making a visually appealing mineral specimen.
PHYSICAL CHARACTERISTICS:
- Color is yellow, brown, brownish red to black. Sometimes tarnished with irridescent colors.
- Luster is earthy to dull for massive forms and metallic for crystals.
- Transparency crystals are opaque.
- Crystal System is orthorhombic; 2/m 2/m 2/m
- Crystal Habits include prismatic and platy crystals, massive and aggregate forms include internally radiating or fibrous; reniform, botryoidal and stalactitic.
- Cleavage is perfect in one direction.
- Fracture is uneven to splintery.
- Hardness is 5-5.5
- Specific Gravity is variable at 3.3 to 4.3 (light for metallic minerals)
- Streak is brown, brownish yellow to orange yellow.
- Associated Minerals include a large list of minerals (particularly secondary deposit minerals).
- Notable Occurences include many iron mines around the world; especially nice specimens come from England, Mexico, and the Lake Superior region in Michigan, USA.
- Best Field Indicators are crystal habit, streak and tarnish.
THE MINERALS GRAPHITE
- Chemistry: C, Elemental Carbon
- Class: Native Elements
- Subclass: Non-metallics
- Group: Carbon
- Uses: for the lead in pencils, as a toughener of steel and as a lubricant.
- Specimens
Graphite is a polymorph of the element carbon. diamond is another polymorph. The two share the same chemistry, carbon, but have very different structures and very different properties.
- Diamond is hardest mineral known to man, Graphite is one of the softest.
- Diamond is an excellent electrical insulator, Graphite is a good conductor of electricity.
- Diamond is the ultimate abrasive, Graphite is a very good lubricant.
- Diamond is usually transparent, Graphite is opaque.
- Diamond crystallizes in the Isometric system and graphite crystallizes in the hexagonal system.
Somewhat of a surprise is that at surface temperatures and pressures, Graphite is the stable form of carbon. In fact, all diamonds at or near the surface of the Earth are currently undergoing a transformation into Graphite. This reaction, fortunately, is extremely slow.
All of the differences between graphite and diamond are the result of the difference in their respective structures. Graphite has a sheet like structure where the atoms all lie in a plane and are only weakly bonded to the graphite sheets above and below. Diamond has a framework structure where the carbon atoms are bonded to other carbon atoms in three dimensions as opposed to two in graphite. The carbon-carbon bonds in both minerals are actually quite strong, but it is the application of those bonds that make the difference.
Graphite can only be confused with the mineral molybdenite which is metallic bluish silver in color. However, molybdenite is much denser and has a silver blue streak.
Most graphite is produced through the metamorphism of organic material in rocks. Even coal is occassionally metamorphosed into graphite. Some graphite is found in igneous rocks and also as nodules inside of iron meteorites.
PHYSICAL CHARACTERISTICS:
- Color is black silver.
- Luster is metallic to dull.
- Transparency crystals are opaque
- Crystal System is hexagonal; 6/m 2/m 2/m
- Crystal Habits include massive lamellar veins and earthy masses. also as scaly granules in metamorphic rocks.
- Hardness is 1 - 2
- Specific Gravity is 2.2 (well below average)
- Cleavage is perfect in one direction.
- Fracture is flaky.
- Streak is black gray to brownish gray.
- Associated Minerals include quartz, calcite, micas, iron meteorites and tourmalines.
- Other Characteristics: thin flakes are flexible but inelastic, mineral can leave black marks on hands and paper, weakly conducts electricity.
- Notable Occurrences include New York and Texas, USA; Russia; Mexico; Greenland and India.
- Best Field Indicator is softness, luster, density and streak.
THE MINERAL GRATONITE
- Chemistry: Pb9As4S15, Lead Arsenic Sulfide
- Class: Sulfides
- Subclass: Sulfosalts
- Uses: Mineral specimens and as a very minor ore of lead.
- Specimens
Gratonite is a rare lead sulfide mineral. It forms nice crystals that can be arranged into rosette-like clusters with projecting rhombohedral terminations. Clusters can be attractive and good specimens command large prices, mostly because of the rarity of the mineral. Gratonite is a sulfosalt, a segment of sulfides where the arsenic acts more like a metal than a non-metal and occupies a position where it is bonded to sulfurs.
PHYSICAL CHARACTERISTICS:
- Color is a dark lead gray.
- Luster is metallic.
- Transparency: Crystals are opaque.
- Crystal System: Trigonal; 3
- Crystal Habits include rhombohedral as well as hexagonal prismatic crystals capped by three rhombohedral faces. Clusters can be found in rosette-like aggregates, also massive.
- Cleavage: None.
- Fracture: Subconchoidal.
- Hardness is 2.5
- Specific Gravity is 6.2 (heavier than average for metallic minerals)
- Streak is gray.
- Notable Occurrences are limited to the Excelsior Mine, Cerro de Pasco, Peru.
- Best Field Indicators are crystal habit, locality, color and density.
THE MINERAL GREENOCKITE
- Chemistry: CdS, Cadmium Sulfide
- Class: Sulfides
- Uses: A minor ore of cadmium and as mineral specimens.
- Specimens
Greenockite is a rare mineral and the only real ore of cadmium. It is a much sought after mineral by collectors both for its rarity and for its nice color and crystal habit. Although it is the most common cadmium mineral, most of the world's supply of cadmium is supplied through the processing of zinc and lead ores where cadmium is a common trace element.
Greenockite has the same structure as the zinc iron sulfide wurtzite. Greenockite's structure is composed of SCd4 tetrahedrons that are stacked in a layered structure with every other layer exactly the same in an AB AB AB ... hexagonal sequence.
The tetrahedrons in greenockite all are oriented in one direction and produce the hexagonal (six fold rotational) symmetry. The symmetry is broken in a perpendicular direction to the major axis and results in a hemimorphic crystal structure. In other words, there is no symmetry element, like a mirror or two fold rotational axis, perpendicular to the major axis and thus crystal faces on top of the crystal are not repeated on the bottom of the crystal. Hemimorphic crystals have different looking tops from their bottoms, as if they never completed the opposite, symmetrical, side; therefore the term hemimorphic or half shape. Other minerals that have a hemimorphic character are the tourmalines, hemimorphite (obviously), wurtzite and zincite among others.
Crystals of greenockite are typically six sided pyramids where the hemimorphic character is easily seen as the top of the crystal is the point and the bottom of the crystal is the pyramid's base. Often the crystals are platy with a hexagonal outline and the hemimorphic character is then not easily distinguished.
PHYSICAL CHARACTERISTICS:
- Color is honey yellow, orange, red or light to dark brown.
- Luster is adamantine to resinous.
- Transparency crystals are transparent to translucent.
- Crystal System is hexagonal; 6 m m
- Crystal Habits include small tapering hemimorphic six sided pyramids and platy hexagonal crystals. Also seen as crusts or dustings over crystals of calcite, smithsonite and sphalerite.
- Cleavage is poor in one direction (basal) and good in three other directions (prismatic).
- Fracture is conchoidal.
- Hardness is 3 - 3.5
- Specific Gravity is approximately 4.5 - 5 (much heavier than average).
- Streak is red, orange or light brown.
- Other Characteristics: Striations on pyramidal faces parallel to base, an index of refraction of 2.50 - 2.52 and is soluble in hydrochloric acid giving off hydrogen sulfide gas
- Associated Minerals include fluorite, chalcopyrite, quartz, smithsonite, calcite, pyrite, sphalerite, marcasite and prehnite.
- Notable Occurrences include Greenock (hence the name), Scotland; Llallagua, Bolivia; Paterson, New Jersey; Joplin, Missouri; Arkansas, Illinois and Kentucky, USA.
- Best Field Indicators are crystal habit, color, streak and cleavage.
THE MINERAL GROSSULAR
- Chemistry: Ca3Al2(SiO4)3, Calcium Aluminum Silicate
- Class: Silicates
- Subclass: Nesosilicates
- Group: Garnets
- Uses: abrasive, mineral specimens and as a Gemstone
- See our natural Grossular Garnet Mineral Specimens
Also see variety specimens:
Grossular (also known as grossularite), like other garnets, forms rounded crystals with 12 rhombic or 24 trapezoidal faces or combinations of these and some other forms. This crystal habit is classic for the garnet minerals. Grossular is the calcium aluminum garnet and forms in contact or regional metamorphic enviroments as does andradite the calcium iron garnet. It is believed that these garnets form from the metamorphism of impure siliceous limestones. Grossular has many color possibilities and is probably the most colorful of the garnets. The orange variety is the most common and specimens of orange grossular crusts are prized by many collectors. Green variety is called tsavorite and is occassionally cut as a gem. Hessonite is a red variety of grossular and is also sometimes used as a gem.
PHYSICAL CHARACTERISTICS:
- Color is colorless, yellow, orange, green, red, grey and black.
- Luster is vitreous.
- Transparency crystals are transparent to translucent.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits include the typical rhombic dodecahedron. also seen is the 24 sided trapezohedron. Combinations of these forms are common and sometimes the rare faces of the hexoctahedron, a 48 sided crystal habit that rarely is seen by itself, can also combine with these other forms making very attractive, complex and multifaceted crystals. Massive and granular occurrences are also seen. Commonly forms crusts that show many rhombic faces
- Cleavage is absent.
- Fracture is conchoidal.
- Hardness is 6.5 - 7
- Specific Gravity is approximately 3.5+ (above average for translucent minerals).
- Streak is white.
- Associated Minerals are micas, chlorite, diopside, calcite, idocrase and serpentine.
- Other Characteristics: index of refraction is 1.75 and many faces show a sort of srtiation caused by multiple crystal forms.
- Notable Occurrences are Asbestos, Canada; Mexico; Kenya; Italy and Sri Lanka.
- Best Field Indicators are crystal habit, color, enviroment and hardness.
THE MINERAL GYPSUM
- Chemistry: CaSO4-2(H2O), Hydrated Calcium Sulfate
- Class: Sulfates
- Uses: plaster, wall board, some cements, fertilizer, paint filler, ornamental stone, etc..
- Specimens
Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water.
Gypsum has several variety names that are widely used in the mineral trade.
- "Selenite" is the colorless and transparent variety that shows a pearl like luster and has been described as having a moon like glow. The word selenite comes from the greek for Moon and means moon rock.
- Another variety is a compact fiberous aggregate called "satin spar" . This variety has a very satin like look that gives a play of light up and down the fiberous crystals.
- A fine grained massive material is called "alabaster" and is an ornamental stone used in fine carvings for centuries, even eons.
PHYSICAL CHARACTERISTICS:
- Color is usually white, colorless or gray, but can also be shades of red, brown and yellow.
- Luster is vitreous to pearly especially on cleavage surfaces.
- Transparency crystals are transparent to translucent.
- Crystal System is monoclinic; 2/m
- Crystal Habits include the tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces dominate with jutting prism faces on the edges of the tabular crystals. Long thin crystals show bends and some specimens bend into spirals called "Ram's Horn Selenite" Two types of twinning are common and one produces a "spear head twin" or "swallowtail twin" while the other type produces a "fishtail twin". Also massive, crusty, granular, earthy and fiberous.
- Cleavage is good in one direction and distinct in two others..
- Fracture is uneven but rarely seen.
- Hardness is 2 and can be scratched by a fingernail.
- Specific Gravity is approximately 2.3+ (light)
- Streak is white.
- Associated Minerals are halite, calcite, sulfur, pyrite, borax and many others.
- Other Characteristics: thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Also some samples are fluorescent. Gypsum has a very low thermal conductivity (hence it's use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz.
- Notable Occurances include Naica, Mexico; Sicily; Utah and Colorado, USA; and many other locallities throughout the world.
- Best Field Indicators are crystal habit, flexible crystals, cleavage and hardness.
THE MINERAL GYROLITE
- Chemistry: Ca2 Si3 O7(OH)2 -H2 O, Hydrated calcium silicate hydroxide.
- Class: Silicates
- Subclass: Phyllosilicates
- Uses: mineral specimen
- Specimens
Gyrolite often forms nodular aggregates. These aggregates can appear glassy, dull or even fiberous. Unlike other similar looking minerals (such as prehnite or smithsonite), gyrolite usually forms individual nodules as opposed to botryoidal or crustal growths. The aggregate nodules can often accompany many fine and rare minerals such as apophyllite, okenite and many of the zeolites. Much gyrolite forms inside of volcanic bubbles called vesicles and can only add another element to the surreal "landscape" inside.
PHYSICAL CHARACTERISTICS:
- Color is white or colorless. Also green or brown.
- Luster is vitreous to dull.
- Transparency crystals are transparent to translucent.
- Crystal System is triclinic; bar 1
- Crystal Habits include the nodules described above. Nodules or concretions are lamellar to fiberous.
- Cleavage is perfect in one direction.
- Fracture is uneven.
- Hardness is 3 - 4.
- Specific Gravity is approximately 2.3+ (below average)
- Streak is white.
- Other Characteristics: lacks the luster of smithsonite and prehnite is usually greener in color.
- Associated Minerals are okenite, apophyllite, quartz, laumontite and other zeolites.
- Notable Occurrences include Poona, India; California and Northern Ireland.
- Best Field Indicators are crystal habit, luster and associations.
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