Spherulites are small, spheroidal, or globular structures common in glassy volcanic rocks.
These bodies have dense masses of tiny, fibrous, prism-like, or acicular crystals of one or more minerals radiating from a common central point, often visible using a hand lens.
Acicular describes a crystal habit characterized by slender, needle-like crystals, often fragile.
Most of the acicular crystals in spherulitic bodies are alkali feldspars and quartz but can have other minerals. They form primarily from the devitrification of volcanic glass.
Lastly, the texture or fabric resulting from glassy rocks having many spherulites is known as spherulitic texture.
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Description and appearance
Spherulites are nearly rounded masses with an aggregate of tiny, feathery crystals or crystallites radiating from one or more common centers.
Usually, they radiate in three dimensions, i.e., they are not flat and appear as dull areas surrounded by a glassy rock base.
1. Size: Microspherulites to microspherules
Their size ranges from microscopic to about 20 cm or more. Microscopic spherulites or microspherulites are those that resolve only under a microscope. The resultant texture is microspherulitic.
On the other hand, larger, visible ones up to 20 cm are macroscopic, and the texture they form is spherulitic.
Those larger than 20 cm, with some as large as two or three meters in diameter, are megaspherulites. For instance, Silver Cliff, Colorado, has megaspherulites with a diameter ranging from 0.3 to 4.3 meters.
Other large ones are Cerro Piedras Bola in Mexico and Steens Mountain in Oregon, near Klondyke in Arizona in the US.
2. Radiating centers
Most spherulite crystals appear to radiate or nucleate from pre-existing phenocrysts, oolites, or pisolites, with some growing from a void. Some of these centers have visible crystals.
Phenocrysts are larger, well-formed crystals in a finer-grained matrix.
On the other hand, oolites are small, spherical grains with concentric layers in sedimentary rocks, while pisolites are concretionary grains made of calcium carbonate. They resemble oolites but can be larger.
3. Acicular crystals
The radiation acicular crystals have glass in between the fibers. These fibers, microscopic to macroscopic, can branch along their length or remain single.
They are mostly made of alkali feldspar, quartz, or silica polymorph. However, they can form from other minerals too.
Lastly, the optical orientation of the fibers may vary.
Varieties
Spherulites nucleating or radiating needle-like crystals in a hollow center are called hollow spherulites.
On the other hand, lithophysae have partially hollow concentric shells on their inside that represent a glass with a vesicle or gas bubble. They are often large, common rhyolite, obsidians, and similar volcanic glasses.
The other variety is varioles. These are mm to cm pale-colored spherulites that radiate fibers of mostly feldspar, usually oligoclase, a sodium-rich plagioclase. They occur mostly in extrusive volcanic rocks with a mafic composition.
Lastly, axiolites are elongated spherulites whose aggregate of minute acicular crystals radiate from a linear nucleus, not a point. They are often at right angles to their nucleus.
Spherulite examples
Some examples of spherulitic textures include:
1. Bird’s eye quartz
Bird’s eye quartz refers to jasper with small spherulitic particles of colorless quartz that look like a bird’s eye. They come in various colors: yellowish, tan, burnt sienna, brown or pinkish.
2. Flowering obsidian
A white to grayish black obsidian matrix with white or lighter spherulitic minerals. They are polished cut en cabochon or used to make beads.
3. Snowflake obsidian
This is an obsidian with gray, white, or reddish spherulites ranging from microscopic to a meter or more in diameter.
4. Peanut obsidian
Spherulitic obsidian with radiates feldspars crystals. It occurs in Sonora, Mexico, and resembles marekanite.
How do spherulites form?
Spherulitic textures form either from volcanic glass devitrification or magma’s rapid undercooling.
Glass is unstable. Therefore, it will undergo devitrification to a stable state. Diversification is how a glass transforms into a crystalline solid with minerals like cristobalite and feldspars.
One of the products and evidence of the devitrification of glass is the development of spherulites. It produces a radial aggregate of crystals, especially cristobalite or tridymite, and feldspars.
On the other hand, supercooling will result in faster crystal growth than nucleation by orders of magnitude. This can allow the growth of elongated crystals characteristic of spherulites.
Where do they occur?
Spherulitic textures occur mostly in felsic volcanic glasses, especially rhyolite, pitchstone, agate, pumice lapilli, chalcedony, and obsidian.
However, a variety called variole occurs in fine-grained or aphanitic mafic rocks. These rocks include basalts, tachylite, and komatiite. They occur in submarine pillow lava, volcanic dikes, or subaerial lava flows.
Usually, spherulites occur as individual structures scattered throughout the rock or in clusters. They are more concentrated in areas with favorable chemistry that forms them.