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

Skeletal texture

Skeletal crystals are incomplete, polyhedral single crystals with internal cavities that are commonly crystallographically controlled. In skeletal crystals in igneous rocks, the cavities are filled with glass or crystalline groundmass material.

Skeletal and dendritic crystals are forced to develop by constitutional (compositional) supersaturation. In a polycomponent system (such as a silicate melt), if low-temperature components not wanted by the growing crystal cannot diffuse away into the liquid fast enough, they become concentrated in a narrow zone adjacent to the interface, especially if their diffusion rate in the liquid are very low at the temperature concerned (Fig.1). In addition, the liquid right at the interface has the highest temperature of any of the surrounding melt, owing to release of latent heat of crystallization by the growing crystal (Fig.2), although this diffuses away much faster than impurity components, and so has a much smaller effect.

Fig.1: Sketch of spike projecting from crystal corner, through layer of liquid rich in impurity components into pure liquid, initiating dendritic growth habit. Modified from Vernon, R. H. (2018).

Fig.2: Temperature field around a growing crystal. Modified from McLachlan & Carlson (1952).

The accumulation of impurity components around the crystal means that the supersaturation of the melt increases with increasing distance from the interface until it reaches a steady value. Further crystal growth is only possible in liquid farther away from the interface, where liquid of normal composition is available. Planar interfaces are unstable under these conditions of strong constitutional supersaturation and change into projections or spikes (Fig.1). The reason is that, if part of the interface advances ahead of the rest of the crystal, it encounters liquid that is poorer in impurity components and richer in nutrient components, and so can continue to grow. Moreover, because of its large interface area, the spike can lose impurity components more easily. However, owing to the tendency for heat and impurity components to accumulate around the spikes themselves, secondary spikes (Fig.3) may be forced to develop on the primary spikes for crystallization to continue. This leads to a dendritic habit (Fig.4). Skeletal, dendritic and spiky crystals of plagioclase, olivine, pyroxene and magnetite occur in quenched volcanic rocks, including submarine basalts and Archaean magnesian basalts or komatiites.

Fig.3: Sketch showing development of secondary spike on primary spike in response to accumulation of impurity components around the primary spike. Modified from Vernon, R. H. (2018).

Fig.4: Development of dendritic spike (stages of growth, 1 to 5) in response to accumulation of impurity components around the crystal. . Modified from Kretz, R. (2003).

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.

Etruscan Furnace Slag

Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 2x (Field of view = 7mm)
Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)
Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. XPL image, 2x (Field of view = 7mm)
Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 2x (Field of view = 7mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. XPL image, 2x (Field of view = 7mm)	Skeletal pyroxene crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)
Skeletal pyroxene and olivine (colorless) crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene and olivine (colorless) crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene and olivine crystals in a Etruscan Furnace Slag, Campiglia Marittima (Livorno), Italy. XPL image, 10x (Field of view = 2mm)

Skeletal Plagioclase

Skeletal plagioclase crystals in a pillow lava. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals in a pillow lava. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals in a pillow lava. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystals in a pillow lava. XPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals in a pillow lava. XPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals in a pillow lava. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystal with internal void in a pillow lava. PPL image, 40x (Field of view = 0,2mm)	Skeletal plagioclase crystal with complex form in a pillow lava. PPL image, 40x (Field of view = 0,2mm)	Skeletal plagioclase crystal with complex form in a pillow lava. PPL image, 40x (Field of view = 0,2mm)
Skeletal plagioclase crystals (feathery) in a pillow lava. PPL image, 20x (Field of view = 1mm)	Plagioclasi scheletrici. Immagine a N//, 10x (lato lungo = 2mm)	Skeletal plagioclase crystal with long spikes. XPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystals. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals. PPL image, 2x (Field of view = 7mm)
Skeletal plagioclase crystals. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals. XPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)
Skeletal plagioclase crystals. PPL image, 2x (Field of view = 7mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)	Skeletal plagioclase crystals. PPL image, 10x (Field of view = 2mm)

Skeletal Leucite ("cartwheel")

Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Alban Hills (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal leucite crystals in a Tephrite from Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)

Skeletal Olivine

Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Pillow lava. XPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
Skeletal olivine in a Pillow lava. XPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)	Skeletal olivine. PPL image, 10x (Field of view = 2mm)	Skeletal olivine. XPL image, 10x (Field of view = 2mm)
Skeletal olivine. PPL image, 10x (Field of view = 2mm)

Skeletal Pyroxene

Skeletal elongated pyroxene crystals. XPL image, 10x (Field of view = 2mm)	Skeletal elongated pyroxene crystals. XPL image, 10x (Field of view = 2mm)	Skeletal elongated pyroxene crystals. PPL image, 10x (Field of view = 2mm)
Skeletal elongated pyroxene crystals. XPL image, 10x (Field of view = 2mm)	Skeletal elongated pyroxene crystals. XPL image, 10x (Field of view = 2mm)	Skeletal elongated pyroxene crystals. PPL image, 10x (Field of view = 2mm)
Skeletal elongated pyroxene crystals. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystal with internal void. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystal (basal section) with internal void. XPL image, 10x (Field of view = 2mm)
Skeletal pyroxene crystals (basal section) with internal void. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystal (basal section) with internal void. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxene crystal (basal section) with internal void. XPL image, 10x (Field of view = 2mm)
Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)
Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)
Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)
Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)	Skeletal Augite crystals in a alkaline basalt. PPL image, 10x (Field of view = 2mm)

Skeletal Phlogopite

Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 2x (Field of view = 7mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 2x (Field of view = 7mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)
Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)
Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Phlogopite crystal in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)

Skeletal Sanidine

Fibrous Skeletal Sanidine crystals in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Fibrous Skeletal Sanidine crystals in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)
Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)
Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 2x (Field of view = 7mm)	Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)	Skeletal Sanidine crystals (with spike) in a lamproite from Orciatico (Tuscany, Italy). PPL image, 10x (Field of view = 2mm)

Skeletal Magnetite

Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)