For such an apparently simple element, the mineralogy of gold is quite complex. To begin with, gold can occur in a wide variety of forms. In massive quartz reefs, gold occurs as disseminated, irregular grains, scales, plates and veinlets with microscopic dimensions, and as larger compact, reticulated, spongy or hackly masses or slugs.
Gold occasionally takes forms that lend themselves to descriptive terms such as wire gold, nail gold, mustard gold and paint gold. While all gold has a crystalline structure, distinct crystals showing well-formed faces are relatively rare. They require special conditions to form, in particular space in which to grow. Hence crystals of gold are found in cavities in quartz reefs or in softer minerals such as iron oxides where they have been able to push aside the enclosing material as they grew. Gold crystallises in the cubic system, and perhaps the most common variety is the eight-sided octahedron.
Possibly the best surviving Australian specimen is a group of sharp, branching, octahedral crystals from Matlock, in the Woods Point goldfield of Victoria. A superb 715 gram (23 troy ounce) mass of crystals known as the Latrobe Nugget was found in the Heathcote district in Victoria and obtained by the British Museum in 1858.
Other important metal-bearing minerals can also be found in the quartz reefs with the gold. The presence or absence of these minerals can be used to help classify the type of gold field. The most common and widespread are pyrite and arsenopyrite, two minerals containing iron and sulphur. This assemblage is distinctive in many of the Victorian goldfields. Sulphides of lead, zinc, silver, bismuth and antimony also occur and may be locally abundant in some gold fields. Massive deposits of these metal sulphides may contain only small proportions of gold, but their overall size makes them significant producers. In the famous Golden Mile at Kalgoorlie, Western Australia, unusual minerals containing tellurium combined with gold have been extremely important.
The relative softness of gold means it can be scratched by harder grains during erosion and transport. However, gold’s malleability often leads to particles being bent or twisted, rather than reduced in size. Gold grains that haven’t travelled far from the quartz reefs often preserve many of their original features, such as their basic shape or the imprints of quartz crystals. Generally speaking, finer gold particles known as gold dust have been carried further from their source reefs, possibly by fast-flowing streams. A common observation by early Victorian diggers working alluvial deposits along streambeds was for the gold particles to become smaller and more worn further downstream. This was especially evident in the Woolshed Valley in the Beechworth district in northeastern Victoria, where Reedy Creek flowed vigorously through a steep-sided gorge cut in granite.
Credits: By Dr Bill Birch, Senior Curator, Geosciences, Museum Victoria
Gold occasionally takes forms that lend themselves to descriptive terms such as wire gold, nail gold, mustard gold and paint gold. While all gold has a crystalline structure, distinct crystals showing well-formed faces are relatively rare. They require special conditions to form, in particular space in which to grow. Hence crystals of gold are found in cavities in quartz reefs or in softer minerals such as iron oxides where they have been able to push aside the enclosing material as they grew. Gold crystallises in the cubic system, and perhaps the most common variety is the eight-sided octahedron.
Possibly the best surviving Australian specimen is a group of sharp, branching, octahedral crystals from Matlock, in the Woods Point goldfield of Victoria. A superb 715 gram (23 troy ounce) mass of crystals known as the Latrobe Nugget was found in the Heathcote district in Victoria and obtained by the British Museum in 1858.
Other important metal-bearing minerals can also be found in the quartz reefs with the gold. The presence or absence of these minerals can be used to help classify the type of gold field. The most common and widespread are pyrite and arsenopyrite, two minerals containing iron and sulphur. This assemblage is distinctive in many of the Victorian goldfields. Sulphides of lead, zinc, silver, bismuth and antimony also occur and may be locally abundant in some gold fields. Massive deposits of these metal sulphides may contain only small proportions of gold, but their overall size makes them significant producers. In the famous Golden Mile at Kalgoorlie, Western Australia, unusual minerals containing tellurium combined with gold have been extremely important.
The relative softness of gold means it can be scratched by harder grains during erosion and transport. However, gold’s malleability often leads to particles being bent or twisted, rather than reduced in size. Gold grains that haven’t travelled far from the quartz reefs often preserve many of their original features, such as their basic shape or the imprints of quartz crystals. Generally speaking, finer gold particles known as gold dust have been carried further from their source reefs, possibly by fast-flowing streams. A common observation by early Victorian diggers working alluvial deposits along streambeds was for the gold particles to become smaller and more worn further downstream. This was especially evident in the Woolshed Valley in the Beechworth district in northeastern Victoria, where Reedy Creek flowed vigorously through a steep-sided gorge cut in granite.
Credits: By Dr Bill Birch, Senior Curator, Geosciences, Museum Victoria
