Fundamental Minerals

Sialic Minerals

• Feldspar
Feldspar
Sanidine
Plagioclase

• Feldspathoid
Feldspathoids
Haüyne
Kalsilite
Leucite
Nepheline


• Quartz

Femic Minerals

• Amphiboles
• Aenigmatite
• Biotite
• Sagenitic biotite
• Melilite
• Olivine
• Pyroxene
Structure
Clinopyroxene
Orthopyroxene

Accessory Minerals

• Andradite
• Apatite
• Cordierite
• Chromite
• Magnetite
• Ilmenite
• Titanite

Very Rare Minerals

• Gregorite
• Nyerereite
• Priderite

Secondary Minerals

• Calcite
• Chlorite
• Hematite
• Epidote
• Orpiment
• Phillipsite
• Pinite

Texture

• Amygdales
• Embayed
• Eutaxitic
• Felsic
• Glomerophyric
• Hyalopilitic
• Hypocrystalline
• Holohyaline
• Intergranular
• Intersertal
• Magmatic flow
• Opacitic rim
• Ophitic
• Poikilitic
• Pilotassitic
• Porphyritic
• Sieve
• Skeletal
• Spinifex
• Trachytic
• Vitrophyric
• Zoning
• Xenocryst
• Xenolith

Alteration Products

• Bowlingite
• Iddingsite
• Pseudomorph
• Saussurite
• Sericite
• Uralite

Glass and related Texture

• Volcanic glass
• Fiamme
• Perlite
• Palagonite
• Spherulites
• Shards

Volcanic Rocks

• Aillikite
• Alnöite
• Andesite
• Ankaramite
• Basalt
• Benmoreite
• Bergalite
• Comendite
• Coppaelite
• Dacite
• Dacite (devitrified)
• Garnet dacite
• Foidite
• Foidite (nepheline)
• Haüynophyre
• Hyaloclastite
• Horneblende Andesite
• Ignimbrite
• Ignimbrite (pantelleritic)
• Kaiserstuhl
Kaiserstuhl
Bergalite
Carbonatitic lapillistone
Diatreme breccia
Limburgite
Olivine nephelinite
Tephrite
Sövite
Wollastonite phonolite

• Kersantite
• Kimberlites
Kimberlites
Varty lake (Canada)
Sturgeon lake (Canada)
Monastery (S.Africa)
Udachnaya (Russia)
• Komatiites
Komatiites
Alexo (Canada)
Belingwe (Zimbawe)

• Lamprophyres
Lamprophyres
Aillikite
Alnöite
Bergalite
Kersantite
Monchiquite
Sannaite
Spessartite
Vosegite

• Lapillistone
• Latite
• Leucitite
• Limburgite
• Madupite (lamproite)
• Mafurite
• Monchiquite
• Natrocarbonatite
• Obsidian
• Oldoinyo Lengai
Ol Doinyo Lengai
Natrocarbonatite lava

• Peperine
• Pyroclastic flow
• Pitchstone
• Pillow lava
• Pumice
• Rhyolite
• Sannaite
• Scoria (basaltic)
• Scoria (phonolitic)
• Spessartite
• Tephrite
• Tephritic leucitite
• Tephritic phonolite
• Trachybasalt
• Trachyte
• Trachyte (foid-bearing)
• Venanzite
• Vitrophyre
• Vosegite

Spinifex texture

Nesbitt (1971) formally introduced the term spinifex when he described and classified several different types of skeletal crystals in komatiites from Australia and Canada. The term derives from an Australian spiky grass (Triodia spinifex).

Petrologists who initially studied spinifex texture noticed that the skeletal olivine phenocrysts in komatiites resemble quench crystals (formed at very rapid cooling rates) in experimental melt. Solidification under the conditions of supercooling (low nucleation rates and rapid crystal growth rates) produces a few large skeletal or dendritic crystals. However, this model cannot explain the occurrence of spinifex texture within thick komatiite flows well below the upper chilled crust. Large skeletal crystals may have crystallized at depths, many metres below the surfaces of the flows where cooling rates must have been low.

Nesbitt was first to recognize what has come to be known as the spinifex paradox: Spinifex texture is commonly found in the interior of komatiite flows, well below the upper chilled crust. In the thickest units, large dendritic crystals appear to have crystallized at depths 10 or more meters below the surface of the flow. Under such circumstances, the loss of heat from the interior of the flow is controlled by conduction through the upper solidified crust. In a typical 2-m-thick komatiite flow, the cooling rate during crystallization of the lower part of the spinifex layer is only 1 to 3°C per hour. In thicker flows the rate is far lower. In contrast, the morphologies of the olivine or pyroxene crystals in spinifex-textured lavas resemble those produced experimentally at cooling rates never less than about 30°C/hr.

As the spinifex texture is confined to MgO-rich basaltic to ultramafic rocks, a partial explanation is that spinifex texture is related to the temperature difference between the liquidus and solidus (Fig.2), which is very large for komatiites and Mg-rich rocks (400-500°C; Faure et al. 2006) compared with typical basaltic rocks (<100°C). However, in detail, the origin of the spinifex texture remains problematic.

Fig.2: Diagram of percentage liquid vs temperature illustrating the large difference in the liquidus-solidus gap between komatiite and basalt. Modified from Arndt, N., & Fowler, A. (2004).

A factor that has been mentioned in many papers on spinifex texture, but has not received sufficient attention, is the role of constrained crystal growth during solidification of the crust of a komatiite flow. Constrained growth refers to the crystallization of parallel grains of olivine or pyroxene in the downward-growing crust of a lava flow. The crystals compete with one another for nutrients. Probably it is this competition that leads to the preferred, near-perpendicular orientation of the olivine crystals in spinifex textures (Fig.3).

Fig.3: Three stages during the solidification of a spinifex-textured komatiite flow. Modified from Arndt, N., & Fowler, A. (2004).

Erupted komatiite contains a small proportion of olivine phenocrysts that grew either during ascent to the surface or during flowage on the surface. During cooling, some of these phenocrysts become trapped in the crust that forms right at the top of the flow, others settle towards to the base of the flow and become part of the lower cumulate layer. Olivine then nucleates in the layer of crystal-free liquid just beneath the crust. The crystals that grow from these nucleii have highly skeletal morphologies, due to the high cooling rate in the crust of the flow, and they are randomly oriented.

As cooling proceeds, these olivine grains continue to grow. Those with a near-vertical orientation are favored because their tips extend downwards into unfractionated nutrient-rich liquid; those with orientations closer to horizontal find only nutrient-poor liquid or collide with other crystals, and they cease to grow. The crystallization of olivine produces a residual liquid with a composition different from that of the parental liquid, depleted in Mg and enriched in Si, Al, Ca and Na. Its density is less than that of the parental liquid. As downward growth proceeds, this liquid is expelled and it accumulates as a layer of low-density at the base of the crystal front (Turner et al., 1986). The growing tips of the spinifex crystals are bathed in a liquid depleted in the components they require to grow.

Large, downward-radiating spinifex textures in komatiitic. Komati River Valley, South Africa. From Large igneous provinces.

Spinifex metakomatiite. Komati River Valley, South Africa. From James St. John.

Spinifex komatiite sills in cumulate komatiite lava flow (Upper Komatiitic Unit). Kidd-Munro Assemblage, Timmins, Ontario, Canada. From James St. John.

Spinifex komatiite. Kidd-Munro Assemblage, Timmins, Ontario, Canada. From James St. John.

Spinifex komatiite. Kidd-Munro Assemblage, Timmins, Ontario, Canada. From James St. John.

Bibliography

• Arndt, N.T. and Fowler, A.D. 2004. Textures in komatiites and variolitic basalts. In: The PreCambrian Earth: Tempos and Events. Pat Eriksson, Wlady Altermann, David Nelson, Wulf Muller, and Octavian Catuneanu (eds.). Developments in Precambrian Geology 12.
• Dann, J.C ., 2001, Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: Inflation of submarine lavas and origin of spinifex zones: Bulletin of Volcanology, v. 63, p. 462-481.

Photo

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized), regularlyarranged, in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxene. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Olivine (spinal column). Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Olivine (spinal column). Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)