Fayalite

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Fayalite
Fayalite crystal group - Ochtendung, Eifel, Germany.jpg
Fayalite crystal group from Ochtendung in the Eifel
General and classification
chemical formula Fe 2 2+ [SiO 4 ]
Mineral class
(and possibly department) 		Silicates and germanates - island silicates (nesosilicates)
System no. to Strunz
and to Dana 		9.AC.05 ( 8th edition : VIII / A.04)
03/01/01/01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Pbnm (No. 62, position 3)Template: room group / 62.3
Lattice parameters a  = 4.82  Å ; b  = 10.48 Å; c  = 6.09 Å
Formula units Z  = 4
Physical Properties
Mohs hardness 6.5 to 7
Density (g / cm 3 ) measured: 4.392; calculated: [4.40]
Cleavage good after {010} after {100}
Break ; Tenacity shell-like to uneven
colour greenish yellow, light yellow to amber yellow, yellow brown, red brown to black
Line color White
transparency transparent to opaque
shine Glass gloss, resin gloss on fractured surfaces
Crystal optics
Refractive indices n α  = 1.731 to 1.824
n β  = 1.760 to 1.864
n γ  = 1.773 to 1.875
Birefringence δ = 0.042 to 0.051
Optical character biaxial negative
Axis angle 2V = 74 ° to 47 ° (measured); 54 ° to 66 ° (calculated)
Other properties
Chemical behavior soluble in HCl

Fayalite is a rather seldom occurring mineral from the mineral class of " silicates and germanates " with the chemical composition Fe 2 2+ [SiO 4 ] and is therefore chemically an iron silicate . With forsterite and tephroit it forms a seamless mixed series , the links of which are called olivine .

Fayalite crystallizes in the orthorhombic crystal system and develops mostly tabular to prismatic crystals , but also granular to massive aggregates of pale yellow, greenish-yellow, yellow-brown and red-brown to black color.

Etymology and history

Fayalite was first found in 1840 on the Ilha do Faial (old spelling Fayal ), an Azores island belonging to Portugal, and described by Christian Gottlob Gmelin , who named the mineral after its type locality .

classification

In the old (8th edition) and new systematics of minerals according to Strunz (9th edition) , the fayalite belongs to the division of " island silicates (nesosilicates)" and there to the olive group , formed from the minerals fayalite, forsterite, laihunite , love mountainite and tephroit .

The revised 9th edition of Strunz's mineral systematics divides this department more precisely according to the presence or absence of further anions and the coordination of the cations involved . The fayalite is accordingly in the sub-section of the “island silicates without further anions; with cations in octahedral [6] coordination ”and is still a member of the olive group there, which has, however, been expanded to include the minerals glaucochroit and cherry stone .

The systematics of minerals according to Dana , which is common in the English-speaking world , assigns the fayalite similarly to the new Strunz'sche mineral systematics in the department of " island silicates: SiO 4 groups with all cations only in octahedral [6] coordination ". The Oliving group, which can also be found there, consists of the members Fayalit, Forsterit, Laihunit, Liebenbergit and Tephroit, as in the old Strunzian system, but expanded to include olivine, for which the IMA / CNMNC is still lacking recognition.

Chemism

In its pure form, fayalite consists of 70.51% FeO and 29.49% SiO 2 , but this is only achieved with synthetic fayalite. Natural fayalite can contain up to 10% forsterite. Furthermore, foreign admixtures of a few percent MnO, ZnO, Al 2 O 3 as well as chromium, titanium, cobalt, calcium and others were observed.

Fayalite (Fe 2 2+ [SiO 4 ]) is the iron analogue of the magnesium silicate forsterite (Mg 2 2+ [SiO 4 ]) as well as the manganese silicate Tephroit (Mn 2 2+ [SiO 4 ]) and forms with these a complete, isomorphic mixed series down to low temperatures.

The intermediate links of the mixed series Fayalit-Forsterit were given their own names - similar to those of the plagioclase - with a fixed composition:

Fe 2 2+ [SiO 4 ] Surname Mg 2 2+ [SiO 4 ]
90-100% Fayalite 0-10%
70-90% Ferrohortonolite 10-30%
50-70% Hortonolite 30-50%
30-50% Hyalosiderite 50-70%
10-30% Chrysolite 70-90%
0-10% Forsterite 90-100%

Crystal structure

Fayalite crystallizes orthorhombically in the space group Pbnm (space group no. 62, position 3) with the lattice parameters a  = 4.82  Å ; b  = 10.48 Å and c  = 6.09 Å as well as four formula units per unit cell . Template: room group / 62.3

properties

In nature, fayalite is only rarely found in its pure form, but almost always with fluctuating contents of forsterite and / or tephroit. Color factor are the (brown to black) in fayalite predominant iron - ions or (gray, red) in Tephroit predominant manganese ions. Depending on the percentage of forsterite that is colorless in its pure state, the colors of the fayalite are weakened accordingly.

The mineral is soluble in hydrochloric acid , forming SiO 2 gel. In front of the soldering tube , fayalite melts to a black, magnetic glass.

Modifications and varieties

Molar volume as a function of pressure at room temperature

At high pressure to convert fayalite by phase transformation in the high pressure - modification Ahrensit order. This represents the iron-containing analogue of ringwoodite , i.e. unlike forsterite there is no intermediate form analogous to wadsleyite . Under the conditions that prevail in the upper mantle, the transition from fayalite to ahrensite takes place at around 6 to 7 GPa, i.e. at significantly lower pressure than the phase transformations of forsterite. In high pressure experiments, however, the phase transition can take place with a delay, so that fayalite can remain metastable up to almost 35 GPa at room temperature (see figure). However, it then becomes amorphous rather than changing into a crystalline structure such as ahrensite.

So far, two manganese-containing fayalite varieties are known, which are referred to as hortonolite and knebelite (named after Walther von Knebel ).

Education and Locations

Fayalite between sanidine crystals

Fayalite forms in ultramafic volcanics and plutonites . There it occurs in paragenesis with a whole range of minerals such as almandine , various minerals of the amphibole group , apatite , arfvedsonite , augite , grunerite , hedenbergite , ilmenite , magnetite , microcline , plagioclase , sanidine and spinel . Unlike olivine rich in magnesium, fayalite can also occur together with quartz or tridymite . The paragenesis fayalite + quartz is only replaced by ferrosilite at high pressures above 10 kBar .

As a rather rare mineral formation, fayalite can sometimes be abundant at various sites, but overall it is not very common. So far (as of 2014) around 300 sites for fayalite are known. In addition to its type locality Ilha do Faial, Fayalit was found in Portugal on Pico and on Água de Pau on São Miguel.

Other sites are Egypt , Algeria , Antarctica , Ethiopia , Australia , Brazil , Chile , China , Germany , Eritrea , Finland , France , French Polynesia , Greenland , India , Indonesia , Ireland , Israel , Italy , Japan , Cameroon , Canada , Madagascar , Morocco , Mexico , Mongolia , Myanmar , Namibia , New Zealand , Netherlands , Nigeria , Norway , Oman , Austria , Romania , Russia , Saudi Arabia , Sweden , Switzerland , Slovakia , Spain , St. Lucia , South Africa , Tajikistan , Czech Republic , Ukraine , Hungary , the United Kingdom (Great Britain) and the United States (USA).

Fayalite was also found in rock samples that were brought back from the moon by the Apollo 11 and Apollo 14 missions .

In addition, fayalite is produced synthetically from the crystallization of iron-rich slag during the smelting of iron ores .

use

As a gem

The minerals of the Oliving group are mainly processed into gemstones if they are of good quality . Clear varieties usually have a facet cut in different shapes, while cloudy varieties tend to have a cabochon cut. They are commercially available under the name " Peridot " or "Chrysolite".

Due to the color, there is a risk of confusion with beryl , chrysoberyl , demantoid , diopside , prasiolite , prehnite , sinhalite , emerald , tourmaline and vesuvianite .

In order to enhance stones with a weak color, a green foil is occasionally placed under them in ring or pendant settings. Also imitations of colored glass or synthetic corundum or spinel issued by rogue vendors as peridot. In contrast to these, the fayalite or its mixed crystals can be recognized by the strong birefringence, which can be recognized when looking through thicker, faceted stones from the doubling of the lower facet edges even without a magnifying glass.

In high temperature experiments

Fayalit has the property of reacting reversibly with oxygen at high temperatures:

This reaction can be used to set a defined partial pressure or a defined fugacity of oxygen in high-temperature experiments. The system is also known as "FMQ buffer" (fayalite magnetite / quartz buffer).

See also

literature

Web links

Commons : Fayalite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  538 .
  2. ↑ Webmineral - Fayalite (English)
  3. ↑ a b c Fayalite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 75  kB ]).
  4. ↑ a b c d e Mindat - Fayalite (English)
  5. ↑ a b c Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  459 .
  6. ^ Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  655 .
  7. ^ Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  657 .
  8. ↑ DC Presnall: Phase diagrams of Earth-forming minerals . In: TJ Ahrens (Ed.): Mineral Physics & Crystallography - A Handbook of Physical Constants (=  AGU Reference Shelf ). No. 2 . American Geophysical Union, Washington, DC 1995, ISBN 0-87590-852-7 , pp. 248-268 .
  9. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 , pp. 115, 138 .
  10. ↑ Steven R. Bohlen, Eric J. Essene, AL Boettcher: REINVESTIGATION AND APPLICATION OF OLIVINE-QUARTZ-ORTHOPYROXENE BAROMETRY . In: Earth and Planetary Science Letters . tape 47 , 1980, pp. 1–10 ( umich.edu [PDF; 774 kB ; accessed on April 26, 2019]).
  11. ↑ Mindat - Number of localities for Fayalite
  12. ^ List of localities for fayalite at the Mineralienatlas and at Mindat
  13. ^ A b c Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 174 .
  14. ↑ Martin Okrusch, Siegfried Matthes: Mineralogie. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 372 .