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."

 

[Andy Resnick]

Next up:

Altered Tinguaite/Phonolite (QAPF field 11), Fen complex. This rock has an appearance very similar to Fen 252. Trachytic groundmass of sanidine laths, aegerine needles, and small aggregates of anhedral granular mineral, likely paragonite replacing groundmass nepheline.

Small phenocrysts of biotite with opaques (either magnetite or ilmenite), large phenocrysts of sanidine, some with Baveno twinning, 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 (paragonite) as the altered nepheline found in samples Fen 248, 249, and 250; unfortunately, definitive identification (by me) is not (currently) possible.

“The alteration of nepheline into paragonite represents a metamorphic or hydrothermal process, often occurring in Na-rich alkaline environments. Paragonite, a sodium-rich mica, forms from nepheline through hydration and silica uptake, often during the alteration stage of nepheline syenites or related pegmatites”

 

[Andy Resnick]

Next up:

Altered Tinguaite/Phonolite (QAPF field 11), Fen complex. Similar appearance as Fen 253. Blocky phenocrysts of cancrinite, likely as pseudomorphic altered nepheline. Phenocrysts of sanidine heavily altered (dusty appearance, perthic texture). Groundmass is disordered, consisting of agglomerated anhedral cancritine grains, both laths and anhedral grains of feldspar, and irregularly shaped grains of green aegirine. What would typically be aegirine needles instead have a highly degraded/corroded appearance, unclear if this rock would be classified as a Tinguaite.

Above image pair: upper right corner is dusty (PP) sanidine; lower right corner is cancritine (pseudomorphic altered nepheline); groundmass is (approximately) along the the left half of the frame, and in between cancritine and groundmass, a thin layer of anhedral grains of calcite. Within the groundmass, grain of blue mineral is either fluorite, lazurite, or haüyne.

Large isotropic grain, likely sodalite:

Several “phenocrysts” consisting of a distinct outer shell (dark hematite and feldspar laths, mostly dusty in PP) surrounding an interior region, consisting of a mixture of calcite and needle-like aragonite (a polymorph of calcite):

Red-brown “smoke”, likely hematite:

Thin veinlet of unknown mineral- possibly hematite, here traversing cancrinite; a zone of calcite surrounds the veinlet.

Isotropic blue mineral is either fluorite, lazurite, or haüyne.

Also present in the sample when viewed through epi-darkfield: purple-ish ilmenite; some crystals of titanate have pseudomorphically altered to ilmenite. Silvery rhomboids and truncated hexagons of magnetite.