Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Lapillistone

Pyroclasts are defined as fragments generated by disruption as a direct result of volcanic action. The fragments may be individual crystals, or crystal, glass or rock fragments. Their shapes acquired during disruption or during subsequent transport to the primary deposit must not have been altered by later redepositional processes. If the fragments have been altered they are called "reworked pyroclasts", or "epiclasts" if their pyroclastic origin is uncertain.

The various types of pyroclasts are mainly distinguished by their size:

Bombs: pyroclasts the mean diameter of which exceeds 64 mm and whose shape or surface (e.g. bread-crust surface) indicates that they were in a wholly or partly molten condition during their formation and subsequent transport.
Blocks pyroclasts the mean diameter of which exceeds 64 mm and whose angular to subangular shape indicates that they were solid during their formation.
Lapilli pyroclasts of any shape with a mean diameter of 64 mm to 2 mm.
Ash grains pyroclasts with a mean diameter of less than 2 mm They may be further divided into coarse ash grains(2 mm to 1/16 mm) and fine ash (or dust) grains (less than 1/16 mm).

Pyroclastic deposits are defined as an assemblage of pyroclasts which may be unconsolidated or consolidated. They must contain more than 75% by volume of pyroclasts, the remaining materials generally being of epiclastic, organic, chemical sedimentary or authigenic origin. When they are predominantly consolidated they may be called pyroclastic rocksand when predominantly unconsolidated they may be called tephra.

The majority of pyroclastic rocks are polymodal and may be classified according to the proportions of their pyroclasts Fig.1 as follows:

Agglomerate: a pyroclastic rock in which bombs > 75%.
Pyroclastic breccia: a pyroclastic rock in which blocks > 75%.
Tuff breccia: a pyroclastic rock in which bombs and/or blocks range in amount from 25% to 75%.
Lapilli tuff: a pyroclastic rock in which bombs and/or blocks < 25%, and both lapilli and ash < 75%.
Lapillistone: a pyroclastic rock in which lapilli > 75%.
Tuff or ash tuff: a pyroclastic rock in which ash > 75%.

Tuff may be further divided into coarse (ash) tuff (2 mm to 1/16 mm) and fine (ash) tuff (less than 1/16 mm). The fine ash tuff may also be called dust tuff. Tuffs and ashes may be further qualified by their fragmental composition, i.e a lithic tuff would contain a predominance of rock fragments, a vitric tuff a predominance of pumice and glass fragments, and a crystal tuff a predominance of crystal fragments.

Classification of polymodal pyroclastic rocks based on the proportions of blocks/bombs, lapilli and ash. Modified from Le Maitre, 2005

Lapilli

Lapilli are spheroid, teardrop, dumbbell or button-shaped droplets of molten or semi-molten lava ejected from a volcanic eruption that fall to earth while still at least partially molten. Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions. Here, thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff.

The heat of the newly-deposited volcanic pile tends to cause the semi-molten material to flatten out as they become welded. Welded tuff textures are distinctive (termed eutaxitic), with flattened lapilli, fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. Rounded tephra balls are called "accretionary lapilli" if they consist of volcanic ash particles. Accretionary lapilli are formed in an eruption column or cloud by moisture or electrostatic forces, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.

The processes of collision, binding, and breakup of particles in moisture-bearing eruption plumes lead to the formation of aggregates of ash that in turn play an important role in the deposition of ash from phreatoplinian plumes. Ash aggregates fall with greater velocities than their component particles and so the process of aggregation results in the premature flushing of large quantities of fine ash and the simultaneous deposition of a wide range of grain sizes. This creates poorly sorted and fine-grained fall deposits even close to the vent.

The formation of aggregates is governed by factors such as the concentration, shapes, and size distributions of ash, the abundance and physical state of water in the plume, and the temperature and relative humidity of the plume and surrounding atmosphere. Phreatoplinian ash aggregates form a spectrum, which can be related, in the first instance, to the liquid content (water plus dissolved gas species) of the depositing pyroclast assemblage. The spectrum ranges from weak clusters of grains at the dry end to ash-free rainfall at the wet end. In between these two endmembers are accretionary lapilli of various types, sizes, and structures and mud rain. The dry end of the ash aggregation spectrum is represented by weak clusters of grains, which have been observed to fall from dry plinian plumes.

With condensation of water in an eruption plume, ash will accrete into denser clumps as accretionary lapilli. Recent experimental work shows that the likely origin of accretionary lapilli is through collision of liquid-coated ash particles in the plume. Ash particles in a water-rich plume act as condensation nucleii for water vapor and clasts and may be continually coated in acidic liquid or ice. When clasts collide, due to different fall velocities and convection in the plume, the potential for binding is enhanced because of the effect of strong, short-range surface tension forces. Electrostatic attraction is also important for attraction of particles and the fine-grained outer layers on many accretionary lapilli probably form this way.

Two morphologically different types are distin-guished:

(1) Rim-type lapilli are composed of a coarse-grained core surrounded by a fine-grained rim. Rims are internally graded or made up of several layers of alternating fine and very-fine grained ash.

(2) Core-type lapilli lack fine-grained rims. Field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits.

lapilli2019(1)

Lapillistone with accretionary lapilli. Black Mountain, California, USA. From Wooster Geologists

Accretionary lapilli. From Stephen Hui Museum

Accretionary lapilli from Tenerife. From Dr Richard J Brown

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.

Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)	Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)	Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)
Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)	Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)
Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)	Rounded carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)
Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)	Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. XPL image, 2x (Field of view = 7mm)	Tear-drop carbonatitic lapilli (with calcite and magnetite phenocrysts) set in a secondary calcite groundmass. Lapillistone from Henkenberg, kaiserstuhl, Germany. PPL image, 2x (Field of view = 7mm)