RockHoundBlog

Amphibole

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

Amphibole (Hornblende)

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

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

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

Chemical formulae

Orthorhombic series

  • Anthophyllite (Mg,Fe)7Si8O22(OH)2

Monoclinic series

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

Descriptions

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

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

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

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

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

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

Amphibole compositions

Amphibole compositions

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

Jade cove field trip- Nephrite Jade, Serpentine, Soap Stone, Actinolite

Filed under: Coming Events,regular postings — Gary April 11, 2007 @ 9:24 am

jade_cove jade_cove_2jade_cove_map

May Field Trip:
Jade Cove May 5th 2007, Sat.10am

General information:
Come for the day or camp overnight
(Robert and Debra Sankovich will be there Thursday 3rd-6th Sunday)
Day use free by side of road, camping $20.00, no Elec hookups

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Jade is the gem name for mineral aggregates composed of either or both of two different minerals, Jadeite and Nephrite.

Filed under: Mineral of the day,regular postings — Gary March 23, 2007 @ 11:15 pm

jade rough_jadequartz_jade_rough

An ornamental stone, jade is applied to two different rocks that are made up of different silicate minerals. Nephrite jade consists of the calcium- and magnesium-rich amphibole mineral actinolite (aggregates of which also make up one form of asbestos). The rock called jadeitite consists almost entirely of jadeite, a sodium- and aluminium-rich pyroxene.

The English word ‘jade’ is derived from the Spanish term piedra de ijada (first recorded in 1565) or ‘loin stone’, from its reputed efficacy in curing ailments of the loins and kidneys. ‘Nephrite’ is derived from lapis nephriticus, the Latin version of the Spanish piedra de ijada.

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Conejo Valley Gem & Mineral Club field trip – Ant Hill-shark teeth fossils

Filed under: Coming Events,regular postings — Gary January 20, 2007 @ 11:28 pm

February Field Trip:
Ant Hill-Feb 24th Saturday 10am-3pm
General information: near Bakersfield California. Come out for a fun
day of digging for shark teeth fossils, the weather should be nice,
cool. The dig site is a walk of 400 yards over semi flat ground, then
up a hill 100 yards. Once there most of the time you’ll be digging in
small area. This is a different site then the Nov 2006 field trip. The
clay is softer and the teeth are harder so you have a better chance of
them not breaking when you remove them from the matrix. This time of
the year there is always the chance of rain, bring the necessary
clothing. If it is raining, we won’t be digging.

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Franklin and Sterling Hill, New Jersey, USA – table of longwave fluorescent minerals

Filed under: regular postings,rockhounding maps — Gary December 18, 2006 @ 6:09 pm

alleghanyiteAlleghanyite is an exceptionally rare member of the humite group, and this specimen is unusually rich with gemmy brown microcrystals to 2 mm flatlaying along an approximately 3-cm vertical axis on this specimen . The matrix is a typical mix of franklinite/calcite/willemite and is highly fluorescent.

The mines of Franklin and the Sterling Hill Mine at Ogdensburg, Sussex County in northwestern New Jersey are world famous and deservingly so. No other site can boast the same assortment of rare and interesting minerals. Over three hundred different minerals were found at these mines and most are listed in The Minerals of Franklin and Sterling Hill Table.

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lapidary- Blue Quartz -Indicolite (blue Tourmaline) in Quartz. Healing?

Filed under: regular postings — Gary October 17, 2006 @ 2:31 am

blue_quartz

Healing Crystals and Gemstones are one of the most beautiful, mystical and profound “energy medicine” tools, which have been used for centuries throughout all cultures, religions and empires. Crystals bring amazing benefits to the healing arena!

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