vastiform

Vastiform is a descriptive term in mineralogy that denotes a crystal habit or aggregate that is broad, sheetlike, and tabular. Vastiform crystals tend to grow as wide, flat plates or sheets that span substantial dimensions within a vein, cavity, or fracture, rather than forming slender, fibrous, or acicular aggregates.

In terms of formation, vastiform forms arise in environments where space for growth is ample, such as hydrothermal veins or open fissures. The habit yields specimens with large, flat faces that can be parallel or subparallel to the host vein, producing a veiny, laminated, or draped appearance along the cavity walls.

Usage and scope: Vastiform describes external morphology rather than mineral identity; it can apply to a range of minerals, including carbonates, silicates, sulfates, and related groups, when the growth produces broad, sheetlike masses. The term is commonly encountered in field descriptions, collections, and literature on crystal habit.

See also crystal habit, mineralogy, tabular crystals.