MINERAL AGGREGATES
Large single crystals with fully developed faces are relatively rare. Usually numerous crystals are formed at the same time, and thus interfere with each other’s development. Masses of closely packed minerals which consist mainly of one mineral species are known as mineral aggregates. According to their structure and appearance such aggregates may be described as: radially fibrous (natrolite, Plate 19, wavellite, Plate 49), concentrically banded (malachite, Plate 9), granular (lapis lazuli, Plate 67), botryoidal (blende, Plate 38, azurite, Plate 55, chrysocolla, Plate 52, adamite, Plate 62), nodular (smithsonite, Plate 61), encrusting (antimony ochre, Plate 63), stalagmitic (limonite, Plate 48), fibrous (malachite, Plate 54), sheaf-like (calcite, Plate 14), scaly (chlorite, Plate 76, 80), matted fibres (amianthus, i.e. actinolite asbestos, Plate 79), and stellate, i.e. fibres radiating from a centre to produce star-like forms. There is an almost limitless variety of forms amongst mineral aggregates. Many joint surfaces are lined with innu¬merable small closely packed minerals. The form of native metals, such as copper, silver or gold, may take the shape of thin wires, foils or sheets, or be tree-like or moss-like (Plates 10, 34, 36).