Wollastonite phonolite

Fohberg quarry (Fig.1) is the only active quarry in volcanic rock of the Kaiserstuhl. The petrography of the Fohberg phonolite was investigated by Wimmenauer (1962). The rock has a slightly porphyritic texture with phenocrysts of feldspathoids, aegirine-augite, wollastonite and andradite in a greenish-grey fine- to medium-grained matrix of sanidine, aegirine-augite and feldspathoid minerals. Phenocrysts are in general euhedral; however, garnet phenocrysts tend to be subhedral due to possible corrosion.

A prominent feature of Fohberg phonolite samples is the complete decomposition of feldspathoid minerals, and the partial replacement of wollastonite. Secondary minerals, which also form large portions of the matrix, include natrolite as sole Na-zeolite endmember, gonnardite, minor thomsonite (both Ca-Na zeolite species), and calcite. Additionally, pectolite and sepiolite occur in minor quantities as alteration products. On an outcrop scale (Fig.1 b-c-d), and in contrast to the Endhalden and Kirchberg phonolites, the Fohberg phonolite hosts a network of numerous, mainly steep dipping fractures. These are partially or totally filled with a secondary zeolite-dominated assemblage similar to the altered rock matrix. The unusually high CaO of the bulk rock, combined with the high alkalies and agpaitic index, are the major reasons for the crystallization of wollastonite.

a Fohberg phonolite intrusion. b View from the center of the quarry into north direction. The phonolite intrusion shows a discordant contact to overlying ocher Pleistocene loess sediments. People for scale. c Enlargement of rectangle given in b. Essexitic dike, approximately 2 m in width is crosscutting the phonolite intrusion and indicates magmatic activity after phonolite emplacement. d alteration is most obvious along white natrolite (ntr) veins and open mineralized fractures. The grayish rock matrix is also highly affected by zeolitization. Hammer for scale. from Weisenberger, T. B. et al., (2014).

Bibliography

• Ulianov, A., Müntener, O., Ulmer, P., & Pettke, T. (2007). Entrained macrocryst minerals as a key to the source region of olivine nephelinites: Humberg, Kaiserstuhl, Germany. Journal of Petrology, 48(6), 1079-1118.
• Walter, B. F., Marks, M. A. W., & Markl, G. (2016). The Kaiserstuhl natural laboratory-an introduction and first results. Abstract Malawi Expert Council.
• Weisenberger, T. B., Spürgin, S., & Lahaye, Y. (2014). Hydrothermal alteration and zeolitization of the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany. International Journal of Earth Sciences, 103(8), 2273-2300.
• Weisenberger, T., & Spuergin, S. (2009). Zeolites in alkaline rocks of the Kaiserstuhl Volcanic Complex, SW Germanyâ€“new microprobe investigation and the relationship of zeolite mineralogy to the host rock. Geologica Belgica

Photo
Aegirine (green), calcite and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)	Aegirine, calcite and acicular wollastonite (low birefringence) crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)	Aegirine (green) and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)
Aegirine (green) and acicular wollastonite (low birefringence) crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)	Aegirine (green) and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)	Aegirine (green) and acicular wollastonite (low birefringence) crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)
Aegirine (green) and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)	Aegirine (green) and acicular wollastonite (low birefringence) crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)	Aegirine (green) and acicular wollastonite (low birefringence) crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)
Aegirine (green), brown melanite garnet and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)	Aegirine (green), brown melanite garnet and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 10x (Field of view = 2mm)	Aegirine, brown melanite garnet and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 10x (Field of view = 2mm)
Aegirine (green) and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. PPL image, 2x (Field of view = 7mm)	Aegirine and acicular wollastonite crystals, set in a fine-grained matrix of sanidine. Wollastonite phonolite from Fohberg. XPL image, 2x (Field of view = 7mm)