Phonolite
Category: Volcanic
Type
Commons Phonolite is a relatively rare volcanic rock with a chemical composition intermediate between mafic and felsic rocks. Its intrusive equivalent is nepheline syenite. It consists predominantly of alkali feldspar, with lesser amounts of feldspathoids (foids) and mafic minerals. If the only feldspathoid present is nepheline and/or haüyne, the rock is termed phonolite. However, if leucite predominates over nepheline, the rock is classified as leucite phonolite, or leucite-nepheline phonolite if nepheline exceeds leucite (Le Maitre et al., 2002). Phonolites containing sodic pyroxene and/or amphibole are classified as peralkaline phonolites.
Name origin The name originates from the rock itself, which produces a metallic sound — its "rings"—when struck. It is derived from the Greek words phone – sound and lithos – stone. Cordier was the first who published this name of the rock in 1816.
Locality Pico del Teide ("Hell's Mountain"), Tenerife, Canary Islands, Spain (sample courtesy of J. Pavlík)
GPS: 28° 16' 17,2" N, 16° 38' 18,9" W
Major minerals Alkali feldspar – anorthoclase (Ab69.5–70.4An6.0–9.2Or20.3–25.3), clinopyroxene – salite – phenocryst (Fs11–18En37–41Wo45–47) (Ablay et al., 1998) and (Fs15.2–15.9En38.6–39.6Wo45.0–45.5) (electron microprobe analysis – unpublished data). Currently, salite is classified as an Fe-rich variety of diopside. Some salites from Pico de Teide are Na-rich (up to 1 wt% Na2O) and are classified as Na-salite. The glass in the intergranular spaces is of phonolitic composition, with an SiO2 content of 58.1–58.2 wt% and an alkali content (Na2O + K2O) of 15.7–15.8 wt% (electron microprobe analysis – unpublished data, Huraiová 2022).
Accessory minerals Fe-Ti oxides (magnetite and Ti-magnetite), ilmenite, biotite and apatite.
Classification Based on modal composition in the QAPF diagram for volcanic rocks (Streckeisen, 1978), the phonolite field is defined by a modal foid content of 10–60 % and a plagioclase ratio P/(P + A) between 0 and 10. If the modal composition cannot be determined, the rock is classified by the TAS (Total alkali-silica diagram) diagram for volcanic rocks based on SiO2, Na2O, and K2O content (Le Bas et al., 1986), where it falls within the phonolite field.
Colour Black.
Structure Massive, porous, locally fluidal.
Granularity A fine-grained rock with a glassy luster. Feldspar and pyroxene phenocrysts are visible to the naked eye.
Texture Hemicrystalline, pilotaxitic, porphyritic.
Alterations The rock is unaltered, it is completely fresh.
Petrographic characteristics The rock is black and slightly porous, with a striking glassy luster, indicating a very fresh specimen. Macroscopically, light-colored feldspar phenocrysts up to 2 mm in size stand out. Closer microscopic examination reveals clinopyroxene phenocrysts as well—ranging from Fe-rich diopside and salite to augite. The sample is from the summit area of the Pico de Teide stratovolcano, located in the central part of Tenerife. With an elevation of 3,718 m, it is the highest point in Spain and the highest peak among all islands in the Atlantic Ocean. Tenerife, the largest island in the Canary archipelago, is an oceanic island situated on an oceanic plate. In terms of height (from the Atlantic seabed to the summit—approximately 7,000 m) and the volume of volcanic material, it ranks as the third-largest volcano in the world. The summits of the Pico de Teide and nearby Pico Viejo stratovolcanoes are part of Teide National Park, which was inscribed on the UNESCO World Heritage List in 2007. Due to its proximity to populated areas, the volcano is included in the list of the World’s 16 most dangerous volcanoes—the Decade Volcanoes. The most recent volcanic activity occurred in 1909, when a 5 km-long lava flow moved down the northwestern slope of Pico Viejo. The stratovolcano is structurally unstable and poses a significant geohazard. Therefore, geophysical measurements are continuously being conducted here to locate magmatic reservoirs beneath the volcano (PiñaVaras et al., 2018).
Usage Used for the production of paving stones and phonolite glass. It is also used as an additive in concrete or for the production of crushed stone.
Literature Ablay, G.J., Carroll, M.R., Palmer, M.R., Martí, J., Sparks, S.J., 1998 : Basanite-phonolite Lineags of the Teide-Pico Viejo Volcanic Complex, Tenerife, Canary Islands. Journal of Petrology, 39, 5, 905-936. Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. & Zanettin, B., 1986: A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram. Journal of Petrology, 27, 3, 745 – 750. Cordier, P.L.A., 1816: Sur les substances minerales dites en masses qui entrent dans la composition des roches volcaniques de tous les ages. Journal de Physique, de Chimie et d’Histoire Naturelle. Paris. Vol.83, p.352–386. [Alloite, Asclerine, Cinerite, Peperite, Phonolite, Pumite, Spodite, Trassoite, Tufaite] Le Maitre, R.W., Streckeisen, A., Zanettin, B., Le Bas, M.J., Bonin, B., Bateman, P., Bellien,i G., Dudek, A., Efremova, S., Kelle,r J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H., Woolley, A.R., 2002: Igneous Rocks: A Classification and Glossary of Terms. Recommendationas of the International Union of Geological Sciences. Subcommision on the Systematics of Igneous Rocks. Cambridge University Press, Cambridge, UK, 236 pp. PiñaVaras, P., Ledo, J., Queralt, P., Marcuello, A., Perez, N., 2018: On the detectability of Teide volcano magma chambers (Tenerife, Canary Islands) with magnetotelluric data. Earth, Planets and Space, 70, 14, 11 pp. https://doi.org/10.1186/s40623-018-0783-y Streckeisen, A., 1978: IUGS Subcommision on the Systematics of Igneous Rocks. Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites and melilite rocks. Recommendatiins and suggestions. Neues Jahrbuch für Mineralogie. Stuttgard. Abhandlungen. 143, 1-14.
Photomicrographs
Idiomorphic porphyritic phenocrysts of clinopyroxene (Cpx) - Fe-rich diopside-salite, alkali feldspar (Kfs), and magnetite (Mag) in a fine-grained groundmass composed of plagioclase (Pl) and glass (Glass). Apatite (Ap) also occurs sporadically in the rock (bottom-right image). Magnetite appears black in both plane-polarized light and under crossed polars because it reflects most of the light striking its surface. Magnetite it is classified as an opaque mineral. Under crossed polars, clinopyroxene displays vivid interference colors, unlike alkali feldspars and plagioclase, which exhibit interference colors in shades of grey-white and grey. Volcanic glass appears light brown in plane-polarized light, while tiny lath-like plagioclase (Pl) phenocrysts are visible within it in the BSE image. The Fe-rich diopside-salite phenocrysts exhibit oscillatory zoning; this is clearly visible not only in transmitted light with a single polarizer (showing a different shade of green at the rim) but especially in BSE (back-scattered electron) images obtained via electron microprobe (bottom-right image), where the clinopyroxene core appears as a lighter shade of grey due to its higher Fe content. Under microscopic observation, the rock appears porous—a feature clearly visible in plane-polarized light (as small, rounded voids formed by the escape of volcanic gases) or in BSE images, where the pores appear black. The abundance of volcanic glass indicates that the phonolite solidified rapidly at the surface. A silicate melt inclusion within an alkali feldspar (Kfs) crystal provides evidence of the rock's magmatic origin; this melt inclusion was trapped during the growth of the alkali feldspar. The width of the upper photomicrographs is 2.2 mm. The two lower images have their own separate scale bars.
Normative composition
Phonolite has a high content of normative albite – ab, orthoclase – or, and nepheline – ne. In addition to common normative minerals, the phonolite from the Teide volcano in the Canary Islands also contains normative acmite – ac, which incorporates the remaining molar proportion of Na after the formation of albite. The rock's silica undersaturation is manifested by the presence of normative nepheline – ne. Phonolite is not a normative corundum – c rock.
Normative minerals
SiO2
TiO2
ZrO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
S
CO2
Total
Molar proportion of normative mineral
Molecular mass of normative mineral
Weight % of normative mineral
Oxide
(wt. %)
58.92
0.76
19.12
3.82
0.00
0.17
0.72
1.32
8.86
5.27
0.13
99.09
Molecular
weight
60.08
79.88
101.96
159.69
71.85
70.94
40.31
56.08
61.98
94.20
141.95
Molecular
proportion
0.9807
0.0095
0.1875
0.0239
0.0024
0.0024
0.0179
0.0235
0.1429
0.0559
0.0009
ap
0.0031
0.0009
0.0009
328.68
0.30
il
0.0024
0.0024
0.0024
151.75
0.36
ru
0.0071
0.0071
79.90
0.57
or
0.3357
0.0559
0.0559
0.0559
556.67
31.14
ac
0.0455
0.0114
0.0114
0.0114
462.02
5.25
hm
0.0126
0.0126
159.69
2.00
di
0.0357
0.0000
0.0179
0.0179
0.0179
216.56
3.87
wo
0.0026
0.0026
0.0026
116.17
0.30
ab
0.4471
0.0745
0.0745
0.0745
524.46
39.08
ne
0.1141
0.0571
0.0571
0.0571
284.11
16.21
D: 0.2283
D1: 0.2283
D2: 0.2283
Mg/(Mg+Fe2+): 99.09
Total of normative wt. % 99.09
Comment A portion of the provisional albite – ab´ was converted into nepheline – ne, while the remainder forms the actual normative albite – ab. Normative ilmenite – il was formed from the molar proportion of MnO, which is combined with the molar proportion of FeO because all iron in the chemical analysis was determined as Fe2O3. Normative diopside – di was formed from the molar proportion of MgO, which unusually was lower than the molar proportion of CaO. Normative wollastonite – wo was formed from the remaining CaO. The residual molar proportion of TiO2 remaining after the formation of normative ilmenite – il is assigned to rutile – ru. The residual molar proportion of Fe2O3 remaining after the formation of ilmenite – il is assigned to hematite – hm.
Chemical composition
Based on its SiO2 content, phonolite has an intermediate composition ranging from 52 to 63 wt.%. It is an alkaline rock with an alkali content (Na2O + K2O) at least 12 wt.%, in which Na2O predominates over K2O. Phonolite from Teide (chemical analysis – sample T1-27-2 from tf1 unit; Ablay et al., 1998) exhibits a molecular excess of Na2O and K2O relative to Al2O3, evidenced by the presence of normative acmite, it is thus a peralkaline rock with a peralkaline index (P.I. = NK/A) of 1.05. The peralkaline index for sample TEI-1 (Huraiová 2022, unpublished analysis) is also 1.05. The peralkaline index represents the molar ratio (Na2O + K2O)/Al2O3, which is always greater than 1 for peralkaline rocks. Additionally, the rock is quartz-undersaturated, as it contains normative nepheline. The phonolite at Pico Teide is thought to have formed through the fractional crystallization of a basanitic parental magma, from which phenocrysts of alkali feldspar (anorthoclase) and Fe- and Na-rich diopside successively crystallized.
Pico del Teide, Tenerife, Canary Islands, Spain - sample T1-27-2, tf1 unit
SiO2
58.72TiO2
0.76Al2O3
19.12Fe2O3
3.82FeO
0.00MnO
0.17MgO
0.72CaO
1.32Na2O
8.86K2O
5.27P2O5
0.13LOI
0.60Total
99.69Mg(Mg/Fe2+)
1.000A/CNK
0.84A/NK
0.94
Ablay et al. 1998.