Veins
Terminology referring to mineral orientation inside the vein
Antiaxial vein – minerals growth from the inside out.
Syntaxial vein – minerals crystallize from the wall toward the core. The core of the vein has opened several time.
In both cases (antiaxial and syntaxial) the long axes of minerals is oriented parallel to the least principal stress (i.e., are often normal to the wall of the vein). Also, the orientation of the minerals can vary from a layer to the next, pointing to variations in stress field orientation.
Comb texture – minerals crystallized from the wall toward the core of a vein, and usually crystallized in open space. Minerals long axis is normal to the wall of the vein (and grain size is usually uniform).
Symmetric or asymmetric vein – without or with mineral or textural variations between one and the other half (wall to wall) of the vein.
Vein types
Stockwork – irregular network of veins.
Fibrous vein – produced by many opening events. An opening event (which is due to fluid pressure exceeding lithostatic pressure) induce a pressure drop, which destabilize the fluid and induce mineral deposition. Then pressure increase and can locally dissolve the mineral (quartz) deposited during the pressure drop.
Crack-seal vein – produced by many opening and depositional events (crack-seal process). The layers of quartz deposited during one opening event tend to be thicker compared to the layers of fibrous vein.
Ribbon texture – also similar to crack-seal veins, with thick layers. Trails of wall rock inclusions can be observed in the quartz vein. These trails point to successive opening occurring along the wall of the vein, with parts of the wall rock entrained inside the vein as inclusions at each opening.
Laminated vein – also the result of a crack-seal process, with several types of minerals being deposited (as opposed to the generally quartz mono-mineralic of many veins).
Texture that can be observed inside a vein
Acicular/bladed calcite (or quartz), platy quartz – blade-shaped quartz or calcite formed by an ebullition event. For examples of lattice bladed, ghost bladed, parallel bladed, see here.
Amorphous, or radial growth of small crystals with long axes normal to the free surface – textures that can be observed in colloform zoning.
Crustiform texture – coating (deposition of thin layers of minerals) over larger minerals (e.g., bladed quartz) in open space.
Crustification banding/zoning – coating, generally attributed to several opening and filling events in a vein, with the deposition of different minerals as the composition of the hydrothermal fluid evolves.
Cockade texture – coating of a brecciated or other fragmental rock, where successive ‘crust’ of hydrothermal minerals deposit around rock fragments.
Colloform (scalloped) banding/zoning – concentric layers with a curvature that is convex toward the layer deposited last (or toward a free surface). Generally developed in subsurface conditions (with cold water involved, but there are exceptions, e.g., agate). The colloform texture suggests deposition of particles transported as colloids (gel) by the fluid. The colloform texture can point to fast crystallization in a destabilized system (due to fluid mixing, ebullition, etc.).
Reniform, botryoidal, mammillary, moss, stalactitic (e.g., agate, chalcedony, malachite) – texture of the free surface in a vein with colloform zoning.