Trachyte

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For other uses, see Trachyte (disambiguation).
Trachyte
Alkali feldspar
SecondaryPlagioclase, quartz, or feldspathoid
TextureAphanitic
EquivalentsIntrusive variant is syenite

Trachyte (

alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals,[1] and is formed by the rapid cooling of lava enriched with silica and alkali metals.[2][3][4] It is the volcanic equivalent of syenite.[5]

Trachyte is common wherever alkali magma is erupted, including in late stages of ocean island volcanism

Gale crater on Mars.[11]

Trachyte has been used as decorative building stone[12] and was extensively used as dimension stone in the Roman Empire and the Republic of Venice.[13]

Chemical composition

TAS diagram with trachyte field highlighted

Trachyte has a silica content of 60 to 65% and an alkali oxide content of over 7%. This gives it less SiO2 than rhyolite and more (Na2O plus K2O) than dacite. These chemical differences are consistent with the position of trachyte in the TAS classification, and they account for the feldspar-rich mineralogy of the rock type. Trachydacite occupies the same field in the TAS diagram as trachyte, but is distinguished from trachyte by a normative quartz content over 20%.[2] Trachydacite is not a recognized rock type in the QAPF classification, where rocks rich in alkali feldspar and with quartz over 20% would be classified as rhyolites.[4]

Mineralogy

QAPF diagram with trachyte fields highlighted
Polished opal on trachyte

The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minor

clinopyroxene and olivine are common accessory minerals. The plagioclase is typically sodium-rich oligoclase. The alkali feldspar is typically also sodium-rich sanidine (anorthoclase) and is often cryptoperthitic, with alternating microscopic bands of sodium feldspar (albite) and potassium feldspar (sanidine).[14]

Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass.

ferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand-sized specimens of these rocks. Two types of groundmass are generally recognized: the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral. Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highly feldspathic.[16]

Trachytes very often have minute irregular

liparites and rhyolites) and oligoclase-trachytes, which are now classified as andesites.[17]

Quartz is rare in trachyte, but

silica) is not uncommon.[14] It is rarely in crystals large enough to be visible without the aid of the microscope, but in thin sections it may appear as small hexagonal plates, which overlap and form dense aggregates, like a mosaic or like the tiles on a roof. They often cover the surfaces of the larger feldspars or line the vesicles of the rock, where they may be mingled with amorphous opal or fibrous chalcedony. In the older trachytes, secondary quartz from the recrystallization of tridymite is not rare.[17]

Of the mafic minerals present, augite is the most common. It is usually of pale green color, and its small crystals are often very perfect in form. Brown hornblende and biotite occur also, and are usually surrounded by black corrosion borders composed of magnetite and pyroxene; sometimes the replacement is complete and no hornblende or biotite is left, though the outlines of the cluster of magnetite and augite may clearly indicate from which of these minerals it was derived. Olivine is unusual, though found in some trachytes, for example those of the Arso in Ischia. Basic varieties of plagioclase, such as labradorite, are known also as phenocrysts in some Italian trachytes. Dark brown varieties of augite and rhombic pyroxene (hypersthene or bronzite) have been observed but are not common. Apatite, zircon and magnetite are practically always present as accessory minerals.[18][14]

Occasionally minerals of the feldspathoid group, such as nepheline, sodalite and leucite, are present in trachytes,[16] and rocks of this kind are known as foid-bearing trachytes.[19] The sodium-bearing amphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes;[14] thus aegirine or aegirine augite forms outgrowths on diopside crystals, and riebeckite may be present in spongy growths among the feldspars of the groundmass (as in the trachyte of Berkum on the Rhine). Glassy forms of trachyte (obsidian) occur, as in Iceland, and pumiceous varieties are known (in Tenerife and elsewhere), but these rocks as contrasted with the rhyolites have a remarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous.[16]

Geographic distribution

The Breadknife is a peralkaline trachyte dike in the Warrumbungles of eastern Australia.
Iron Pot is one of several trachyte plugs in the Hedlow Creek region west of Yeppoon in Central Queensland.

Trachyte is the usual silica-rich end member of the

Campi Flegrei volcanic field,[21] where trachytes have been erupted.[22]

Trachytes are well represented among the

lava flows and as dikes or intrusions,[23] but they are much more common on the continent of Europe, as in the Rhine district and the Eifel, also in Auvergne, Bohemia and the Euganean Hills. In the neighborhood of Rome, Naples and the island of Ischia trachytic lavas and tuffs are of common occurrence.[16] Trachytes are also found on the island of Pantelleria. In the United States, trachytes crop out extensively in the Davis Mountains, Chisos Mountains, and Big Bend Ranch State Park in the Big Bend (Texas) region, as well as southern Nevada and South Dakota (Black Hills). There is one known voluminous flow from Puʻu Waʻawaʻa on the north flank of Hualālai in Hawaiʻi.[16] Here the trachyte is glassy and black in color.[15] In Iceland, the Azores, Tenerife and Ascension there are recent trachytic lavas, and rocks of this kind occur also in New South Wales (Cambewarra Range), Queensland (Main Range),[24] East Africa, Madagascar, Yemen and in many other districts.[16]

Among the older volcanic rocks trachytes also are not scarce, though they have often been described under the names orthophyre and orthoclase-porphyry, while trachyte was reserved for Tertiary and recent rocks of similar composition. In England there are Permian trachytes in the Exeter district, and Carboniferous trachytes are found in many parts of the central valley of Scotland. The latter differ in no essential respect from their modern representatives in Italy and the Rhine valley, but their augite and biotite are often replaced by chlorite and other secondary products. Permian trachytes occur also in Thuringia and the Saar district in Germany.[16]

Alkaline rocks such as trachyte are rare in the

continental shields and are evidence of worldwide rifting at that time.[25]

Closely allied to trachyte is the rock type called keratophyre, which is the sodium-rich-plagioclase equivalent of trachyte.[26]

See also

Notes

  1. ISBN 0824808320
    .
  2. ^
    S2CID 28548230
    .
  3. ^ "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999. Retrieved 23 September 2020.
  4. ^ a b "CLASSIFICATION OF IGNEOUS ROCKS". Archived from the original on 30 September 2011.
  5. ISBN 9780521880060
    .
  6. ^ a b c MacDonald 1983, pp. 51-52
  7. ^ a b c d e Philpotts and Ague 2009, pp. 369–370
  8. ISBN 978-3-319-90223-4
    .
  9. ^ a b c Philpotts and Ague 2009, pp. 390-394
  10. ISBN 9780444520463
    .
  11. doi:10.1016/j.lithos.2016.02.023
    .
  12. ISBN 9780128096635
    .
  13. S2CID 133942939
    .
  14. ^
    ISBN 0716724383
    .
  15. ^ a b MacDonald 1983, p. 128
  16. ^ a b c d e f g Flett 1911, p. 117.
  17. ^ a b Flett 1911, p. 116.
  18. ^ Flett 1911, pp. 116–117.
  19. ^ Blatt and Tracy 1996, p.74
  20. S2CID 130297493
    .
  21. S2CID 129909337
    .
  22. S2CID 53633119
    .
  23. PMID 30679443
    .
  24. ^ Stevens, Neville (September 1996). The Main Range (PDF). Brisbane, Queensland: Geological Society of Australia. Archived from the original (PDF) on 19 July 2008.
  25. ^ Philpotts and Ague 2009, pp. 390–391
  26. doi:10.1016/0024-4937(73)90076-5
    .

External links

Wikimedia Commons has media related to Trachyte.

 This article incorporates text from a publication now in the

Flett, John S. (1911). "Trachyte". In Chisholm, Hugh (ed.). Encyclopædia Britannica
. Vol. 27 (11th ed.). Cambridge University Press. pp. 116–117.

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