Crystal Habit

Crystal habit refers to the practical, observational vocabulary that has developed alongside formal crystallography for describing what a mineral actually looks like in hand specimen. ^[1] Where crystallographic notation assigns precise geometric relationships to faces and planes, habit terms communicate shape, proportion, and growth pattern in language that can be read directly from a specimen without calculation. The two vocabularies complement each other: crystallography explains why a particular habit is possible; habit terms let that habit be recorded and communicated quickly in the field. The same underlying structure can produce very different outward forms depending on how fast different faces grew, whether growth space was available, and whether the mineral precipitated from solution or crystallised in a crowded melt.

Face Development

Whether a crystal displays well-formed faces at all depends on the physical conditions it encountered during growth. The three most basic habit terms describe this. ^[1] A euhedral crystal grew where it had room - in an open cavity, a fluid-filled fracture, or a late-stage pegmatite void - and its faces are geometrically complete expressions of the mineral’s symmetry. An anhedral grain grew hemmed in by its neighbours: every face that might have formed was blocked by an adjacent crystal, leaving the grain bounded entirely by irregular surfaces dictated by whatever space was available. Subhedral sits between the two: some faces are recognisable and properly planar, others are incomplete or irregular. In practice, most grains in coarse-grained igneous rocks are subhedral to anhedral simply because the crystals grew simultaneously and competed for space, while euhedral crystals are more characteristic of late-stage or secondary growth in open spaces.

Shape of Individual Grains

Beyond face development, habit vocabulary captures the three-dimensional proportions of a grain - whether it is roughly equal in all directions, stretched into a rod, or flattened into a sheet. ^[1] The terms describe two opposing spectrums that diverge from the equant middle. Equant grains are roughly equal in all three dimensions - a cube or dodecahedron is equant. Moving toward greater elongation, the terms run from columnar (a modest rod, length clearly exceeding width) through acicular (slender needle-like crystals) to fibrous, filiform, or capillary (hair-thin strands). Moving in the opposite direction toward increasing flatness, the sequence runs from stubby columnar through platy and scaly or micaceous (thin, sheet-like grains). Tabular crystals are book-shaped - definitly flattened, but not to the thinness of a platy grain. Bladed grains are both elongate and thin, like a knife blade: longer than tabular but narrower and flatter. These proportions directly reflect which crystallographic directions grew fastest during crystallisation. A mineral whose bonds are strongest along one axis tends to grow fastest along that axis, producing elongate or acicular crystals. One with especially strong bonding in a plane grows fastest across that plane and is typically platy. Understanding habit therefore gives immediate qualitative insight into bond anisotropy within the structure.

Patterns of Grain Intergrowth

When many crystals of the same mineral grow together to form a mass, the collective texture of that mass depends on the shapes of the individual grains. ^[1] Equant grains pack together in a granular texture - the grains interlock in a roughly isotropic mass with no preferred direction. Elongate grains can organise themselves in two contrasting ways: if they all align along the same axis the aggregate is parallel, and if they fan outward from a common point or plane the aggregate is radiating. Platy or micaceous minerals that align in parallel planes form a foliated aggregate - the stacked mica sheets in a schist are a direct expression of foliated habit. The same platy minerals radiating from a centre produce a plumose aggregate instead. These intergrowth patterns matter well beyond mere description: foliated aggregates are a primary texture of metamorphic rocks, and their presence or absence is one of the first observations a field geologist makes to distinguish deformed metamorphic material from undeformed igneous or sedimentary equivalents.

Habits on Pre-existing Surfaces

When minerals precipitate onto an already-existing rock surface - the wall of a vein, the lining of a cavity, or the face of a fracture - the resulting habits form a distinctive group. ^[1]

Drusy refers to a surface encrustation of small euhedral crystals, all anchored to the substrate and projecting outward. ^[1] The individual crystals are well-formed but carpet the surface rather than growing freely into space. Quartz-lined geodes are the most familiar example of drusy habit.

Colloform habit produces smooth, rounded surfaces rather than faceted crystal faces - the aggregate consists of radiating clusters of elongate grains, and the outer surface curves rather than planes. ^[1] Colloform morphology is characteristic of minerals that precipitated from a gel or supersaturated solution where the chemistry favoured rapid, disorganised growth rather than slow, face-controlled crystallisation. It is especially common in supergene (near-surface, low-temperature) ore deposits. Within the colloform category, four terms describe progressively different geometries.

Botryoidal, mammillary, and reniform are arranged in order of decreasing surface lumpiness, and the boundaries between them are not sharp - the terms grade into one another. ^[1] Botryoidal surfaces have the most pronounced, grape-like protrusions. Mammillary surfaces are smoother and broader. Reniform specimens have the gentlest curvature, suggesting the smooth curve of a kidney rather than the lumpy cluster of a grape bunch. Because the transitions are gradational, the most useful field practice is to choose the term that best fits the dominant visual character of the specimen.

Additional Aggregate Terms

A further group of terms describes mineral masses whose character comes from their internal structure or the spatial arrangement of constituent grains rather than from surface curvature.

These terms span a wide range of appearances - from the fine, concentric layering of banded chalcedony to the open, skeletal grid of reticulated gypsum. The list is not exhaustive; any term that accurately and efficiently communicates form is legitimate in mineralogical usage. ^[1] What the vocabulary as a whole provides is a shared shorthand: a few well-chosen words can convey what would otherwise require a paragraph of description, and that precision is what makes the habit vocabulary useful in both field notes and published mineral descriptions.

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