Nodules

Nodules are small, rounded masses that can form in a range of natural and engineered contexts. They are typically solid and distinct from diffuse or cystic features, and their composition and origin vary by field.

In geology and paleontology, nodules are discrete, hard masses that grow by mineral precipitation around a central nucleus within sedimentary rocks. They often form when minerals carried by groundwater precipitate and encrust a grain, fossil fragment, or other core. Common examples include pyrite nodules, carbonate nodules, and chert or flint nodules. They can alter rock texture and weather to reveal their rounded shapes.

In botany and agriculture, root nodules occur on the roots of many leguminous plants in a symbiotic relationship with nitrogen-fixing bacteria (most often Rhizobium). The bacteria reside in nodules and convert atmospheric nitrogen into ammonia, making nitrogen available to the plant and contributing to soil fertility. Nodulation is influenced by plant genetics, soil conditions, and environmental factors.

In medicine and radiology, a nodule describes a well-defined, solid lesion that is typically a few millimeters to several centimeters in size. Lung nodules are commonly detected on chest imaging and may require follow-up imaging, growth assessment, or biopsy to distinguish benign from malignant causes. Thyroid nodules are also common and are evaluated for size, composition, function, and cancer risk. Many nodules are asymptomatic and benign, but some require clinical management.

In materials science, nodular or ductile iron contains spherical graphite nodules that improve toughness compared with flake graphite forms, influencing mechanical properties and applications.