The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

Next sample:

Tinguaite. Altered phonolite (QAPF field 11), Fen complex. Trachytic groundmass of sanidine laths, aegerine needles, and small aggregates of anhedral granular mineral, likely muscovite/paragonite secondary to groundmass nepheline.

Small phenocrysts of biotite, large phenocrysts of sanidine, often partially altered into natrolite and large blocky phenocrysts of what was likely nepheline, now pseudomorphically altered into disordered microscopic anhedral aggregations of presumably the same mineral (biotite or paragonite) as the groundmass as well as the altered nepheline found in samples Fen 248, 249, and 250; unfortunately, definitive identification is not possible.

AI coughed up the following- keep in mind that the pre-existing country rock, Telemark gniess, has significantly less Aluminum and significantly more Sodium compared to the rest of the rocks on earth, so the gneiss is classified as “peralkaline”. Within the Fen complex, due to the ijolite/carbonatite intrusions, the Fen rocks are usually classified as “silica-undersaturated peralkaline”.

“The alteration of sanidine into natrolite is a hydrothermal process, typically occurring within silica-undersaturated volcanic rocks like phonolites or nepheline syenites. While sanidine itself is a high-temperature potassium-sodium feldspar, its alteration to natrolite—a hydrated sodium-aluminum silicate zeolite—represents a low-temperature, secondary, or "deuteric" alteration event.”

 

[Andy Resnick]

Altered Phonolite (QAPF field 11), Fen complex. Groundmass primarily consists of trachtytic sanidine laths and aggregates of anhedral grains of a mineral, likely cancrinite replacing groundmass nepheline. In addition, minor amounts of aegirine needles, and opaques are present.

Blocky rectangular and hexagonal nepheline phenocrysts have pseudomorphically altered into cancrinite, and sanidine phenocrysts have a dusty/perthic appearance. In some places, the sanidine has partially altered into small elongated anhedral grains of (likely) orthoclase and in other, sanidine phenocrysts have partially altered to microcline.

(Along the right side is the edge of the sample.)

It’s unclear if enough aegirine is present to classify this rock as a Tinguaite.

Image triplet: In the center of the field of view, a pair of diamond-shaped titanite crystals, partially oxidized to ilmenite, leucoxene, and calcite. At bottom, carbonate (probably calcite) and titanite (with some ilmenite opaques) have pseudomorphically replaced a mineral with cleavage pattern similar to biotite, but I can’t identify the original mineral.

“Cancrinite commonly forms as a secondary alteration product of nepheline in alkaline-rich igneous rocks, such as nepheline syenites, typically through low-temperature hydrothermal processes or subsolidus reactions. This alteration occurs via the interaction of nepheline with CO2- and alkali-rich fluids. The transformation, which occurs in the presence of water, involves the addition of Ca, Na and carbonate to the nepheline structure. The alteration typically occurs in the subsolidus range, with hydrothermal fluids (often around 400 - 500°C or lower) supplying the necessary components."