The microscopic morphology relates to the texture or the shape and arrangement of crystals inside a vein. Here I distinguish four primary categories:
1. Blocky
2. Elongate blocky
3. Fibrous
4. Stretched

This list is not exhaustive: especially shallow hydrothermal systems can produce a variety of textures and the reader is referred to Dong et al. (1995) for a review.

2.2.1. Blocky texture

A blocky texture is a texture in which grains are roughly equidimensional and randomly oriented. The texture in most granites could for instance be termed 'blocky'. Blocky textures can be primary, if, during vein growth, nucleation of new grains continues. Blocky textures can, however, also be secondary and due to recrystallisation of a primary texture.

2.2.2. Elongate blocky texture

Crystals in an elongate blocky texture (Fisher & Brantley 1992) are typically moderately elongate (length/width ratio generally in the order of 10) and the long axes of crystals are aligned (Fig. 11). This texture forms when nucleation of new grains does not occur during vein growth, and all growth is by crystallographically continuous overgrowths on existing grains and growth occurs at the tips of existing crystals. The 'seed grains' can be pre-existing grains in the wall rock of a vein, or grains formed during an initial nucleation stage. Elongate blocky textures show evidence for crystallographically controlled growth competition between grains (Mügge 1928). Crystals growing into a fluid typically show faceted morphologies as some crystal faces grow faster than others. Some grains, which are crystallographically oriented favourably with respect to the general growth direction, will outgrow unfavourably oriented grains. The faster growing 'winner' grains not only grow faster, but also wider, at the expense of the 'looser' grains. This leads to a gradual increase in grain width in the growth direction and the development of a crystallographically preferred orientation for the 'winner' grains (Mügge 1928, Cox & Etheridge 1983) (see Appendix B).
 

Figure 11. Photomicrograph of elongate blocky texture in a quartz vein. Vein crystals grew out from quartz grains in the sandstone wall rock (below), towards the centre of the vein (top). Growth competition reduced the number of grains away from the vein margin. Approximately horizontal dust and fluid inclusion trails suggest step wise crack-seal growth. Folded Palaeozoic turbidites, East Gippsland, Victoria (Australia). Width of view 8 mm, crossed polars.

2.2.3. Fibrous texture

In a fibrous texture, the rod-shaped grains can achieve a much higher length/width ratio than in elongate blocky textures (Fig. 12). As in an elongate blocky texture, the grains’ long axes are aligned. The distinguishing feature is that fibrous veins hardly show any growth competition. All grains have approximately the same shape. As with elongate blocky texture, a fibrous texture can only develop if no nucleation takes place after growth started.
 

Figure 12. Photomicrograph of an antitaxial fibrous calcite vein. Fibre growth was outward from the median line, which is marked by a string of inclusions of the calcareous shale host rock. Outward growth can be determined by the slight increase in average fibre width away from the median line. Small blade-like quartz crystals precipitated at the vein margin, possibly onto existing small quartz grains in the shale. Tapley Hill Formation, Opaminda Creek, Arkaroola, South Australia. Width of view 4 mm, crossed polars.

It should be noted here, that I support the distinction between elongate blocky veins and fibrous veins as used by Fisher & Brantley (1992). This distinction is currently not usually made by other workers, who tend to call both categories 'fibrous'. However, until the seventies, it was recognised that the two are different (Durney & Ramsay 1973). The popularity of the 'crack-seal' mechanism, first proposed in the paradigmatic paper by Ramsay (1980) is perhaps the cause for the grouping together of the two categories. Although a crack-seal origin for all "fibrous veins" is favoured by some (e.g. Cox & Etheridge 1983, Cox 1987, Urai et al. 1991), different vein forming mechanisms may operate and therefore a distinction in fibrous and elongate blocky textures should be made.

2.2.4. Stretched crystals

In the previous textures, additional vein material formed by precipitation on the surface of existing grains. The primary distinction between the previous textures and stretched crystals is that in stretched crystals, additional growth took place inside the grains (on the surfaces of the half grains), with the space for new-growth provided by (micro-) fractures that cut through the grains (Fig. 13). Fluid inclusions, dust rims or cathodo-luminescence images may reveal this. Stretched crystals often have jagged boundaries ("radiator" structure) and sometimes the two halves of the original grain can still be recognised at both ends of a stretched crystal.
 

a b

Figure 13. Photomicrograph of stretched quartz crystals in a sandstone hosted vein in (a) plane polarised light and (b) cross polarised light. The vein material was formed by adding material to existing grains in the sandstone. White arrows indicate parts of a stretched vein crystal that formed two halves of one grain before the vein formed. Vein wall parallel dust trails and wall rock inclusions (black arrows) indicate that this vein formed by repeated crack-sealing, which can also produce characteristic serrated grain boundaries ("radiator" structure, red arrows). Folded Palaeozoic turbidites, East Gippsland, Victoria (Australia). Width of view 2.3 mm.

2.2.5. Combinations of textures

Not all veins display only one texture. It is not uncommon for veins to be partly fibrous and partly (elongate) blocky as in figure 14. 'Polytextured' could be a possible term for such veins, but it is more important that the different textures for such a vein are described than to define a new and less meaningful single term for the many different possible combinations of textures. Two types of polytextured veins can be distinguished: (1) sequential growth of different textures as in fig. 14a&b, where first one texture forms and then another one, and (2) simultaneous growth of different textures at different sites within one vein (Fig. 14c). Veins of the second type occur in at Opaminda Creek, Arkaroola, when veins cut shales and carbonaceous silt stone layers. Fibrous textures form in the shale, but stretched crystals develop where the vein transects silt stone. Different mechanical properties probably play a role here, with fractures only forming in the silt stone layers and vein sections in the shale growing without any fracturing (see Ch. 3.2.2).
 

a b c

Figure 14. Photomicrograph of a polytextured, fibrous and blocky calcite vein in (a) plane polarised light and (b) cross polarised light. The initial veins is marked by a thin line of wall rock inclusions (S) and a line of quartz crystals (Q). Subsequently, two stages of vein growth occurred: (1) antitaxial fibrous growth towards the left and (2) open cavity growth on the right. The cavity infill took place by overgrowth of the first vein material (at line Q), but also by nucleation and growth of new crystals, resulting in a dominantly blocky texture. Infill of the cavity was not complete as cavities with faceted grain surfaces remained (C). This indicates that the blocky growth was probably the last growth event, post-dating leftward antitaxial fibrous growth from line S (right half of vein not shown). Width of view 10 mm. (c) Antitaxial fibrous calcite vein in shale (right) grading into stretched calcite in silt layer (left). Crossed polars; width of view 12.5 mm. Tapley Hill Formation, Opaminda Creek, Arkaroola, South Australia..

2.2.6. Partially filled veins

Veins may contain voids or cavities. Such cavities may be a result of incomplete filling of the vein. One form of incomplete filling is where a continuous crust of crystals lines the wall rock, with the vein crystals often having euhedral crystal faces facing the remaining cavity. Another form is where individual crystals span the entire vein width, but have open space in between (Henderson et al. 1990). These voids can later be filled by side-ways overgrowth of the first crystals (Fig. 15). If the vein is completely filled, the original existence of such voids is sometimes only visible with cathodo-luminescence.
 

Figure 15. Photomicrograph of quartz vein with a void between elongate crystals. Crystals grew from left to right in many increment as indicated by repeated vertical dust trails. The void between crystals A and B is being partially filled by side-ways growth (arrows) of grains A and B. Folded Palaeozoic turbidites, East Gippsland, Victoria (Australia). Width of view 3.2 mm, plane polarised light.