24-rock-n-gem-magazine-big

Published Spring 2003

Contents: Silver minerals, fossil store horrors, refractometers, competition.

Buy Issue 24, Choose Destination

 Silver Minerals

Silver, Ag, is known as a coinage metal and has an average crustal abundance of 0.08 ppm. Over 10,000 tonnes are produced annually with Mexico, Russia, USA, Peru and Australia being the foremost current world suppliers. Silver is used commercially in photography, silverware and jewellery, silvering mirrors, high capacity batteries, electronics and chemicals. Silver is found mainly as the native element or in countless sulphides, sulphosalts and in halides. Silver occurs in an array of interesting habits and forms very desirable specimens for collectors. Crude masses, wires, sheets, leaves and plates are all very typical for this mineral. Distinct rough cubic crystals are rarer and can include octahedra and dodecahedra. Twinned parallel arborescent to dentritic habits are also very enticing. Its colour is silver-white but this soon tarnishes grey to black with a metallic lustre. It has a variable specific gravity of around 10.5 and has good malleability. It has a shining white streak with a hackly fracture and a low hardness. The above characteristics and simple chemical tests are useful in identifying silver. It occurs as a primary hydrothermal mineral and as a secondary species in the oxidised portions of mineral deposits. Silver is chiefly associated with acanthite, chlorargyrite, various silver sulphosalts and many other minerals. Silver deposits are known throughout the world in many countries. USA, Canada, Norway (Kongsberg being noted for the best crystallised and wire specimens!), Germany, Czech Republic, Australia, Mexico, Peru, Bolivia and Kazakhstan have all produced fantastic specimens. Silver specimens are usually available at Rock "n" Gem Shows. The greatest number of silver-bearing minerals is found among the sulphides and sulphosalts and they provide an interesting group.

Acanthite, Ag2S, is a monoclinic mineral that forms prismatic crystals. Usually found as a paramorph after the high temperature polymorph in pseudo-cubic or pseudo-octahedral habits to several ems in size. Acanthite is iron-black in colour with a black streak and a high specific gravity of 7.2. It has an uneven fracture and a hardness of 2.0-2.5. The habit, lack of cleavage and chemical tests helps towards its identification. It is a widespread mineral of moderately low-temperature sulphide deposits and in secondary enrichment zones, along with silver, pyrargyrite, proustite, polybasite, stephanite, galena, chalcopyrite, sphalerite, calcite and quartz. Good specimen localities are Freiberg, Schneeberg, Annaberg, Marienberg and St. Andreasberg in Germany; countless sites in Mexico; USA; Cobalt area, Ontario, Canada; Chanarcillo, Atacama, Chile and Jachymov, Czech Republic.

Hessite Ag2Te, can be found as appealing pseudo-cubic crystals to about 1.7 cm. Massive, compact or fine-grained aggregates are commonly encountered. It is lead-grey to steel-grey 111 colour tarnishing to black, and possessing a smooth and even fracture. Hessite frequently occurs in low to moderate-temperature hydrothermal veins and in some massive pyrite deposits. It is associated with calaverite, sylvanite, altaite, petzite, gold, galena, tetrahedrite, chalcopyrite and other minerals. Crystal form and chemical tests aid in the identification of hessite. The best samples are obtained from the Bote mine, Zlatna, Romania; Kalgoolie,Western Australia; Altia Mts, Siberia in the former USSR; California, Arizona, Colorado, USA; and in various localities in Canada.

Stromeyerite, AgCuS, generally occurs as massive or compact dark steel-grey aggregates. Orthorhombic prismatic crystals are discovered infrequently. Its physical properties include a steel-grey streak, metallic lustre, nd a hardness of 2.5 to 3. and a subconchoidal fracture. Stromeyerite is mostly encountered as a secondary accessory mineral in hydrothermal deposits. It also is known as a primary species. Freibergite, bornite, chalcocite, chalcopyrite and galena often accompany this mineral. Quite a few localities for stromeyerite are known. These include: Dzhezkazgan, Kazakhstan; Silver King and Magma mines, Arizona, numerous mines in Colorado, USA; Rudelstadt and Kupferberg, Silisia, Poland; Mt Lyell, Tasmania, Australia; and from Cobalt, Ontario and Silver King mine, British Columbia, Canada.

Proustite.AgrAs'Sr, is one of the most desirable minerals to have in any collection due to its beautiful crystals. Great specimens are both difficult and extremely expensive to buy. It forms various hexagonal prismatic crystals to 8 cm in length and can be twinned. Massive to compact aggregates are also found. Proustite is translucent and scarlet-vermilion in colour that darkens with exposure to light. The mineral is brittle with a conchoidal to uneven fracture and a distinct cleavage. It has a low hardness and a specific gravity of 5.5. It has a vermillion streak and an adamantine lustre. It is usually lighter than the next mineral pyrargyrite and chemical tests are useful guides to its identity along with its representative associations. It is found as a late-forming hydrothermal mineral and in oxidised zones, frequently together with silver, arsenic, stephanite, acanthite, tetrahedrite, arsenopyrite, chlorargyrite and barite. Proustite is not uncommon, but only occasionally do localities provide ore or wonderful specimens.
Exceptional crystal groups came from Chanarcillo, Chile; also at Jachymov and Pribram, Czech Republic; Freiberg, Niederschlema and Schneeberg, Germany; from diverse locations in Mexico; Poorman mine, Idaho, USA, and at other sites worldwide. Proustite has occasionally been cut into striking gems, but it is really too soft to wear and also suffers from light damage.

Pyrargyrite, Ag3SbS3, isanother gorgeous silver mineral It crystallises in the hexagonal system with prismatic, hernimorphic crystals that are sometimes twinned. Other habits are massive and granular forms. Pyrargyrite has an amazing deep red colour and is translucent with an adamantine lustre. It also darkens on exposure to light. Other physical properties are a purplish red streak, distinct cleavage, conchoidal to uneven fracture and a hardness of 2.5. Distinguishing features are its crystal habit, the antimony test and its darker hue than proustites. Pyrargyrite and proustite should both be kept out of the light. The mineral is characteristic of late-stage low-temperature primary species in quite a few hydrothermal deposits, and is also found as a secondary mineral. Silver, acanthite, tetrahedrite, other silver sulphosalts, calcite, quartz and dolomite regularly occur with pyrargyrite. Premier specimens are acquired from Colquechaca, Bolivia; Chanarcillo, Chile; Fresnillo, Zacatecas and Guanajuato, Mexico; San Genaro mine, Huancavelica, Peru; St. Andreasberg, Germany and San Carlos mine, Heindelaencina, Spain. Cut stones up to 50 carats have been faceted, but are inclined to be too dark.
The halide class includes: Chlorargyrite, AgCl, bromargyrite, AgBr and iodargyrite, Agi. They are all secondary minerals typically occurring in oxidised portions of silver deposits, commonly in arid climates.
Chlorargyrite is found as attractive modified cubic crystals or more usually as parallel groups, massive, stalactitic, fibrous or as aggregates forming crusts and films. It is colourless, yellowish to grey white when fresh, but turns violet-brown to purple when exposed to light. The mineral possesses a resinous, adamantine to waxy lustre and is translucent to transparent. It is sectile, ductile and very plastic in character. It has a white streak, low hardness and a high specific gravity. Physical characteristics, chemical tests and its mode of occurrence are distinguishing properties. The mineral is associated with silver, cerussite, iodargyrite, atacamite, malachite, jarosite, goethite, and pyromorphite. Fine samples are obtained from many nations. Germany, Czech Republic, England, Russia, USA, Mexico, Chile, Bolivia and Australia are noted sources for chlorargyrite.

Bromargyrite can occur as cubic or cubeoctahedral combinations up to 7 mm across. Ordinarily massive, parallel or subparallel aggregates are found. The mineral is yellowish, greenish brown to bright green in colour and is resinous, adamantine or waxy in lustre. It gives a white to yellowish white streak and has a hardness of 2.5. It is sectile, ductile and very plastic. Bromargyrite possesses a high specific gravity of 6.4 and has an uneven to subconchoidal fracture. The above features and its chemistry aid identification. Normally it occurs together with silver, iodargyrite, smithsonite, wulfenite and various Fe-Mn oxides. Although an uncommon species, it is found at a variety of localities. Good sources include Huelgoat, France; Dernbach and Bad Ems, Germany; in Arizona, New Mexico, Idaho and Colorado, USA; copious sites in Mexico; Broken Hill, Australia and at Chanarcilio, Chile.

Iodargyrite is a mineral with an appealing range of crystals. Prismatic to tabular hexagonal crystals occur with occasional complex pyramidal forms. Lamellar, scaly, globular, rosettes and massive habits also occur. It has a perfect cleavage with a conchoidal fracture and is both sectile and flexible. It has a very low hardness (1.5) and its specific gravity is 5.6. It is usually colourless but becomes yellow on exposure to light. Greenish yellow, brown or grey are other hues commonly encountered. Iodargyrite has a variable lustre ranging from resinous, adamantine to pearly on cleavage. Cleavage and other properties are useful tests for the mineral. Silver, acanthite, chlorargyrite, bromargyrite, cerussite, goethite, vanadinite and descloizite are all closely associated with iodargyrite. It is the rarest of the three silver halides discussed. Delightful examples have been discovered III Mexico, USA, Chile, Australia, Kazakhstan, Germany, Spain and France.

Aurorite, (Mn,Ag,Ca)Mn3073H20, is the only currently recognised silver-bearing mineral which forms an oxide. It sporadically forms in vein lets filling microfractures in calcite. It occurs as small irregular masses and as platy or
scaly grains. The mineral is black and opaque with a low hardness. The specific gravity is estimated to. be about 3.8. Usually associated minerals are todorokite, cryptomelane, pyrolusite, chlorargyrite, silver and quartz. Chemical tests and x-ray diffraction are used in its identification. Aurorite is present at a number oflocations, but is very scarce. Aurora mine and Yucca Mts, Nevada, Ash Peak mine, Sheep Tanks district, Arizona; in the Kawazu mine, Shizuoke Pefecture,]apan are known sites for the material.
The minerals of barium will be discussed in the next issue.
John Betterton

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