State of Aggregation. Descriptive Terms

Most mineral specimens, instead of exhibiting well- formed crystals, will tend to be aggregates of many smaller grains, ranging in outline from euhedral through subhedral to anhedraL These crystalline aggregates are traditionally defined by descriptive terms, such as the following (see also Fig. 2.8):

FIG. 2.8. Some common mineral habits and occurrences (From C. Klein, 1994, Minerals and Rocks: Exercises in Crystallography, Mineralogy and Hand Specimen Petrology, rev. ed. New York-. Wiley, p. 305)

Massive, Applied to a mineral specimen totally lacking crystal faces.

Cleavable. Applied to a specimen exhibiting one or several well-developed cleavage directions.

Granular. Made up of mineral grains that are of approximately equal size. The term is mainly applied to minerals whose grains range in size from about 2 to 10 mm. If the individual grains are larger, the aggregate is described as coarse-granular, if smaller, it is fine-granular.

Compact. Applied to a specimen so fine-grained that the state of aggregation is not obvious to the eye.

Lamellar. Made up of layers like the leaves in a book.

Foliated. Made up of thin leaves or plates that can be separated from each other, as in graphite or mica.

Micaceous. Applied to a mineral whose separation into thin plates occurs with great ease, as in mica.

Bladed. With individual crystals (or grains) that are flattened blades or flattened elongate crystals.

Fibrous. Having a tendency to crystallize in needlelike grains or fibers, as in some amphiboles and in asbestos. In asbestos the fibers are separable; that is, they are easy to pull apart.

Acicular. From the Latin root acicula, meaning- needle; describing a mineral with a needlelike habit.

Radiating (or radiated). Describing a mineral in which acicular crystals radiate from a central point.

Dendritic. From the Greek root dendron, meaning tree; applied to a mineral exhibiting a branching pattern.

Banded. Describing a mineral aggregate in which a single species may show thin and roughly parallel banding (as in banded malachite), or in which two or more minerals form a finely banded intergrowth (as in chert and hematite bands in banded iron-formation).

Concentric. With bands or layers arranged in parallel positions about one or more centers (as in malachite).

Mammillary. From the Latin word mamma, meaning breast; with an external form made up of rather large, rounded prominences. Commonly shown by massive hematite or goethite.

Botryoidal. From the Greek root botrys, meaning bunch or cluster of grapes; having the form of a bunch of grapes. The rounded prominences are generally smaller than those described as mammillary. Botryoidal forms are common in smith-sonite, chalcedony, and prehnite.

Globular. Having a surface made of little spheres or globules.

Reniform. From the Latin renis, meaning kidney; with a rounded, kidney-shaped outer surfaice as in some massive hematite specimens (see Fig. 2.9).

FIG. 2.9. Reniform hematite, Cumbria, England. (Harvard Mineralogical Museum)

Colloform. From the Greek root collo, meaning cementing or welding; because there is often no clear distinction between the four previous descriptive terms (mammillary, botryoidal, globular, and reniform), the term colloform includes them all.

Stalactitic. From the Greek stalaktos,- meaning dripping; made up of small stalactites, which are conical or cylindrical in form as is common on the ceilings of caves.

Concretionary. Clustering about a center, as in calcium carbonate concretions in clay. Some concretions are roughly spherical, whereas others assume a great variety of shapes.

Geode. A rock cavity lined with mineral matter but not wholly filled. Geodes may be banded as in agate, through successive depositions of material, and the central part is commonly filled with crystals projecting into an open space.

Oolitic. From the Greek oön, meaning egg; made up of oolites, which are small, round, or ovate (meaning egg-shaped) accretionary bodies, resembling the roe of fish. This texture is common in some iron-rich specimens, made of hematite, known as oolitic iron ore.

Pisolitic. From the Greek pisos, meaning pea; therefore, pea-sized; having a texture similar to that of an oolitic aggregate but somewhat coarser in grain size. Bauxite, the major source of aluminum ore, is commonly pisolitic.