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Forsterite

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St John's Island (Zagbargad; Zabargad; Zebirget; Seberged; Topazios), Red Sea, Egypt
© 2006 M. Kampf

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Formula:
Mg
 
2
SiO
 
4
System:OrthorhombicColour:Green, pale yellow or ...
Lustre:VitreousHardness:7
Member of:Olivine Group
Name:In honor of Adolarius Jacob Forster (1739-1806), English mineral collector and dealer.
Polymorph of:Ringwoodite, Wadsleyite


Olivine Group. Fayalite-Forsterite Series, and the Forsterite-Tephroite Series.
The magnesium analogue of Fayalite, Tephroite, and Calcio-Olivine.

Visit gemdat.org for gemological information about Forsterite. Currently in public beta-test.

Classification of Forsterite

IMA status:Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
Strunz 8th edition ID:8/A.04-10
Nickel-Strunz 10th (pending) edition ID:9.AC.05

9 : SILICATES (Germanates)
A : Nesosilicates
C : Nesosilicates without additional anions; cations in octahedral [6] coordination
Dana 8th edition ID:51.3.1.2

51 : NESOSILICATES Insular SiO4 Groups Only
3 : Insular SiO4 Groups Only with all cations in octahedral [6] coordination
Hey's CIM Ref.:14.4.1

14 : Silicates not Containing Aluminum
4 : Silicates of Mg
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Type Occurrence of Forsterite

Type Locality:Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy
Year of Discovery:1824

Occurrences of Forsterite

Geological Setting:In mafic and ultramafic igneous rocks, metamorphosed impure dolomites

Physical Properties of Forsterite

Lustre:Vitreous
Diaphaneity (Transparency):Transparent, Translucent
Colour:Green, pale yellow or white.
Streak:White
Hardness (Mohs):7
Tenacity:Brittle
Cleavage:Perfect
Perfect on {010}
imperfect on {100}
Fracture:Conchoidal
Density (measured):3.275 g/cm3
Density (calculated):3.271 g/cm3

Crystallography of Forsterite

Crystal System:Orthorhombic
Class (H-M):mmm (2/m 2/m 2/m) - Dipyramidal
Cell Parameters:a = 4.754Å, b = 10.1971Å, c = 5.9806Å
Ratio:a:b:c = 0.466 : 1 : 0.587
Unit Cell Volume:V 289.92 ų (Calculated from Unit Cell)
Z:4
Morphology:Euhedral to subhedral crystals
Twinning:{100} {011} {012}
Comment:Data for synthetic material.
Structure
  Reference
Birle J D Gibbs G V Moore P B Smith J V (1968) Crystal structures of natural olivines. American Mineralogist 53:807-824.

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More Crystal Structures
Click here to view more crystal structures at the American Mineralogist Crystal Structure Database
X-Ray Powder Diffraction:
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Radiation - Copper Kα
Data Set:
Horizontal Axis: ° to ° Vertical Axis: % Source Data: Filtered Data:
Data courtesy of RRUFF project at University of Arizona, used with permission.

Optical Data of Forsterite

Type:Biaxial (+)
RI values: nα = 1.636 - 1.730 nβ = 1.650 - 1.739 nγ = 1.669 - 1.772
2V:Measured: 74° to 90°, Calculated: 56° to 84°
Maximum Birefringence:δ = 0.033 - 0.042

Chart shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:High
Dispersion:r > v or r < v

Chemical Properties of Forsterite

Formula:
Mg
 
2
SiO
 
4
Essential elements:Mg, O, Si
All elements listed in formula:Mg, O, Si
Common Impurities:Fe

Relationship of Forsterite to other Species

Series:Forms a series with Fayalite (see here)
Forms a series with Fayalite (see here)
Forms a series with Tephroite (see here)
Member of:Olivine Group
Other Members of Group:

- +
Calcio-Olivine
Ca
 
2
SiO
 
4
Fayalite
Fe
2+
2
SiO
 
4
Glaucochroite
CaMn
2+
 
SiO
 
4
Kirschsteinite
CaFe
2+
 
SiO
 
4
Laihunite
Fe
2+
 
Fe
3+
2
(SiO
 
4
)
 
2
Liebenbergite
(Ni,Mg)
 
2
SiO
 
4
Monticellite
CaMgSiO
 
4
Olivine
(Mg,Fe
2+
 
)
 
2
SiO
 
4
Tephroite
Mn
2+
2
SiO
 
4
Related Minerals - Nickel-Strunz Grouping):

- +
9.AC.05Fayalite
Fe
2+
2
SiO
 
4
9.AC.05Glaucochroite
CaMn
2+
 
SiO
 
4
9.AC.05Kirschsteinite
CaFe
2+
 
SiO
 
4
9.AC.05Laihunite
Fe
2+
 
Fe
3+
2
(SiO
 
4
)
 
2
9.AC.05Liebenbergite
(Ni,Mg)
 
2
SiO
 
4
9.AC.05Tephroite
Mn
2+
2
SiO
 
4
9.AC.10Monticellite
CaMgSiO
 
4
9.AC.15Brunogeierite
Fe
2+
2
Ge
4+
 
O
 
4
9.AC.15Ringwoodite
(Mg,Fe
2+
 
)
 
2
SiO
 
4
9.AC.20Chesnokovite
Na
 
2
[SiO
 
2
(OH)
 
2
] · 8H
 
2
O
Related Minerals - Hey's Index Grouping:

- +
14.4.2Enstatite
MgSiO
 
3
14.4.3Clinoenstatite
MgSiO
 
3
14.4.4Chrysotile
Mg
 
3
(Si
 
2
O
 
5
)(OH)
 
4
14.4.5Clinochrysotile
14.4.6Orthochrysotile
14.4.7Parachrysotile
14.4.8Lizardite
Mg
 
3
(Si
 
2
O
 
5
)(OH)
 
4
14.4.9Talc
Mg
 
3
(Si
 
4
O
 
10
)(OH)
 
2
14.4.10Spadaite
MgSiO
 
2
(OH)
 
2
 · H
 
2
O (?)
14.4.11Sepiolite
Mg
 
4
(Si
 
6
O
 
15
)(OH)
 
2
 · 6H
 
2
O
14.4.12Loughlinite
Na
 
2
Mg
 
3
Si
 
6
O
 
16
 · 8H
 
2
O
14.4.13Eifelite
KNa
 
3
Mg
 
4
Si
 
12
O
 
30
14.4.14Stevensite
(Ca,Na)
 
x
Mg
 
3-x
(Si
 
4
O
 
10
)(OH)
 
2
14.4.15Antigorite
Mg
 
3
(Si
 
2
O
 
5
)(OH)
 
4
14.4.16 Magnesio-anthophyllite

Other Names for Forsterite

Synonyms:
BoltonitePeridoto BiancoWhite Olivine
Other Languages:
German:Boltonit
Forsterit
Russian:Форстерит
Simplified Chinese:镁橄榄石
Spanish:Boltonita
Forsterita
Varieties:
Peridot

Other Information

Health Warning:No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Forsterite

Reference List:

- +
Dana, E.S. (1892) System of Mineralogy, 6th. Edition, New York: 450-451, 451-456 (as chrysolite).

Kushiro, I. (1969), The system forsterite-diopside-silica with and without water at high pressures: American Journal of Science: 267: 269-294.

Hazen, R.M. (1976) Effects of temperature and pressure on the crystal structure of forsterite. American Mineralogist: 61: 1280-1293.

Deer, W.A., Howie, R.A., and Zussman, J. (1982) Rock-Forming Minerals. Volume 1A. Orthosilicates. Longmans, 2nd. Edition, 919 pp.: 3-336.

Francis, C.A. (1985) New data on the forsterite-tephroite series. American Mineralogist: 70: 568-575.

Zha, C.-S., Duffy, T.S., Downs, R.T., Mao, H.-K., and Hemley, R.J. (1996) Sound velocities and elasticity of single-crystal forsterite to 16 GPa. Journal of Geophysical Research: 101(B8): 17535-17545.

Brodholt, J.P. (1997), Ab initio calculations on point defects in forsterite (Mg2SiO4) and implications for diffusion and creep: American Mineralogist: 82: 1049-1053.

Brodholt, J.P. and Refson, K. (2000) An ab initio study of hydrogen in forsterite and a possible mechanism for hydrolytic weakening. Journal of Geophysical Research: 105: 18977-18982.

Internet Links for Forsterite

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  • Raman and XRD data at RRUFF project
  • American Mineralogist Crystal Structure Database
  • Forsterite details from Handbook of Mineralogy (PDF)
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    • Localities for Forsterite

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