Fundamental Minerals

Sialic Minerals

Feldspars

Feldspathoids

Quartz

Femic Minerals

Spodumene

Accessory Minerals

Rare Accessory

Alteration Products

Microstructures

Rocks

Jacupiranga complex

Garnet Lherzolite

The name is derived from the Lherz Massif, an alpine peridotite complex (also known as orogenic lherzolite complex). A lherzolite is an ultrabasic igneous rock dominated by essential olivine and clinopyroxene and orthopyroxene in equal proportions. Accessory minerals include plagioclase, spinel, garnet, ilmenite, chromite and magnetite. Lherzolites are a peridotite and the main component of the upper mantle. Their aluminous phases change with pressure, with plagioclase present at low pressures, spinel at intermediate pressure and garnet at high pressure. Metasomatism of lherzolite in the mantle can produce accessory micas and amphiboles. Lherzolite is common as a mantle xenolith within mantle-derived magmas and is present in the mantle sequence of ophiolites. Mantle lherzolite usually contains Cr-diopside as the clinopyroxene and enstatite as the orthopyroxene. Lherzolite can also form as cumulates within layered intrusions.

Plagioclase can occur in lherzolites and other peridotites that crystallize at relatively shallow depths (20 – 30 km). At greater depth plagioclase is unstable and is replaced by spinel through the reaction:

CaAl₂Si₂O₈ (plg) + Mg₂SiO₄ (ol) = 2MgSiO₃ (opx) + CaMgSi₂O₆ (cpx) + MgAl₂O₄ (sp)

At approximately 90 km depth, spinel decompose and pyrope garnet becomes the stable aluminous phase.

MgAl₂O₄ (sp) + 1.55 Mg₂Si₂O₆ (opx) + 0.45 CaMgSi₂O₆ (cpx) = 0.85 Mg₃Al₂Si₃O₁₂ (garnet) + 0.15 Ca₃Al₂Si₃O₁₂ (garnet) + Mg₂SiO₄ (ol)

Garnet lherzolite is a major constituent of the Earth's upper mantle (extending to 300 km depth).

Diagram for Ultramafic rocks. Lherzolite field in blue. From IUGS

Garnet lherzolite: xenolith from a kimberlite pipe, Kimberley, central South Africa. From James St. John .

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 20x (Field of view = 1mm)
Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 20x (Field of view = 1mm)	Olivine and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Olivine and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
Garnet crystals in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)	Garnet crystals in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)	Garnet crystals in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
Garnet crystals in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)
Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)	Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
Orthopyroxene (gray), Clinopyroxene and Olivine. XPL image. 10x (Field of view = 2mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
Olivine and spinel crystals. PPL image. 2x (Field of view = 7mm)	Olivine and spinel crystals. PPL image. 2x (Field of view = 7mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 2x (Field of view = 7mm)
Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 2x (Field of view = 7mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)	Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)