Lithophysae (singular = lithophysa) are nearly spherical to lenticular structures with concentric or onion-like shells and cavities. These spherulite-like structures occur mainly in rhyolite lava flows and compacted or welded tuffs.
Spherulites are spheroidal or globular structures with dense acicular or elongated/needle-like crystal masses radiating from a central point. They occur primarily in glassy volcanic rocks.
However, unlike lithophysae, spherulites don’t have cavities or hollows, i.e., they are always filled.
Most lithophysae have diameters in centimeter scale. However, large ones over 12 feet in diameter occur in Silver Cliff in Colorado in the USA.
The name lithophysa means bubble stone. It describes these bodies or structures with bubbles in their interior.
Lithophysal texture or fabric describes rocks with these hollow, bubbly cavities with concentric shells.
How do they form?
The origin of lithophysae is uncertain. However, those in rhyolite lava flows probably form from the formation process involving rhythmic exsolution and expansion of volatiles as magma crystallizers.
These essentially form vesicles, i.e., gas bubbles trapped by solidifying magma.
On the other hand, those in compacted welded tuff likely form when exsolving gases collectively collect into fragments of pumice.
Related structures
Lithophysa is closely related to geodes, thundereggs and vugs. However, they have some differences.
Let us look at each of these three structures.
1. Geodes
Geodes are hollow or partially filled, nearly spherical rocky bodies filled with crystals or mineral masses, especially jasper, quartz, agate or opal.
They are lined lithophysae with some druzy, i.e., lined with minerals that project into the cavity.
Geodes can be vesicles or form when fossils or crystals dissolve, leaving voids. They are common in volcanic rocks, limestone beds, and, less often, shale and other rocks.
2. Thundereggs
Thundereggs are nearly spherical structures or rock bodies filled with vesicles, which are common in rhyolite ash layers, lava flow, or tuffs.
They are often filled with agate, jasper, and opal and can have other minerals that are not partially filled, like geodes and form from vesicles.
3. Vugs
Vugs are small to medium, irregular-shaped cavities, often lined with mineral crystals. They can form from vesicles, including in cores of pegmatites, or from rock dissolution. Also, they can form from tectonic activities.
In which rocks do lithophysae occur?
Lithophysae occur in fine-grained or aphanitic volcanic rocks like rhyolite lava flows and compacted tuff. Also, they can occur in glassy silica-rich rocks like obsidian.
In rhyolite lava flows, lithophysae are hollow, nearly spherical masses measuring a few centimeters with concentric layers of aphanitic materials.
Their concentric shells are often made of aphanitic to microcrystalline quartz and alkali feldspar.
On the other hand, those in welded or compacted tuffs are disc-like shaped and often lack the concentric structures shells.
Furthermore, some lithophysae may contain minerals quartz, topaz, hematite and clinoferrosilite formed from vapor-phase precipitates.
Lastly, these structures can occur with spherulites. A good example is Lipari in Italy and Yellowstone National Park in the USA, occurring in obsidian rocks.
Why are they important?
Collectors highly price Lithophysae. Also, some may contain valuable vapor-phase precipitates like topaz.