Minerals are naturally occurring materials, formed through geological processes, and each has a specific chemical composition with atoms arrayed in an orderly crystal structure.
Thanks to the varying conditions of their formation and geological history, minerals occur in an endless variety of forms - and minute traces of other elements can completely change their colours.
Silica is the key constituent of the majority of the minerals presented here: if carbon can be regarded as the element of life , then silica deserves recognition as the element of beauty .
The Rock Cycle
Earth is a dynamic planet in a state of constant change. Rocks and minerals are perpetually created, destroyed, transported and re-formed into new generations of rocks and minerals. Geologists use the expression the rock cycle to encompass this grand, planetary-scale tour.
Earth itself has a simple basic structure: a solid nickel and iron inner-core; a molten outer-core comprising the same elements; a solid but pliable mantle rich in magnesium and silicon - and a relatively thin, but rigid crust with oceanic and continental components.
Earth's crust can be divided into about a dozen major plates: the continental plates (like Africa, Europe, Australia and the Americas) are about 35km thick, and separated from one another by oceanic plates (like the Pacific, Atlantic, Indian and Antarctic) with a crust about 6km thick. In the process of plate tectonics, rafts of cool, rigid crust are driven in conveyor-belt fashion across the globe.
Energy as heat from the core and mantle is convected to Earth's surface, creating bodies of molten rock (magma). Because it is hot and buoyant, this rises to the surface creating linear chains of volcanic activity that traverse the major oceans. At these oceanic-ridges, magma erupts into the crust, then cools to form solid rock. Slowly, over millions of years, the process known as subduction can then causes entire oceanic plates to be dragged back down into the earth, leading to the collision of continental plates - and the massive pressures exerted forcing up mountain ranges, like the Himalaya.
The Impact Of The Earth's Surface
The rock cycle creates the three types of rock - igneous (formed of fire), sedimentary (settled down) and metamorphic (from the Greek to change form) - but then the Earth's oceans and atmosphere act to destroy rocks.
Igneous rocks and minerals, for example, are created at high temperatures and high pressures deep in the crust, so are often inherently unstable in the cold, wet conditions of Earth s surface. They weather chemically, breaking down to their constituent minerals, or are altered to more stable minerals such as clays.
Physical processes are also at work. Ocean waves pound coastal cliffs, glaciers reduce granites to rock flour , and rivers carve vast canyons. Glacial debris from the Himalaya is transported by vast river systems and is eventually deposited in deltas and in ocean basins. Particles of sand are driven thousands of miles across deserts by the wind.
As the energy available for transport wanes, particles are deposited forming unconsolidated sand, mud and clay sediments. Over time, these build up, layer by layer, bed by bed, to form thick sediment sequences. The sediments become progressively buried and transformed by cementation and compaction into solid sedimentary rocks like sandstones and mudstones.
Sedimentary rocks themselves can be eroded time after time, forming and reforming sediments and sedimentary strata. Volcanism, metamorphism and deep burial in the crust create hot fluids which can dissolve rocks and minerals, and which can also create mineral veins and pods within older rocks. Where precipitation occurs within open fractures, large, perfectly-formed crystals develop into minerals such as quartz.
Each image is a graphical representation of a point within this grand planetary scale cycle frozen in time. Each contains glimpses of this story played out at the scale of molecules. Some, such as the Paesina Stones, look uncannily like real landscapes, and in part this is because of a genuine correspondence between small- and large-scale processes. With many others, such similarities of appearance are deceptive. Horizontally-banded agates, for example, might appear to be the product of a simple process of sedimentation, whereas in fact the process is more complex - and far from fully understood.
Agates are formed of chalcedony, a microcrystalline form of quartz, and the most spectacular examples come from open pockets, created by trapped gas bubbles in basalt lavas into which ground water percolates. The structure of agate is comprised of concentric shells, differentiated by colour, which are derived from differences in the orientation of the microscopic fibrous crystals that make up the rock, plus variations in trace elements within the crystals.
Crystal structure and mineral habits
We are all familiar with the six-sided quartz crystal with a pyramid-like point or metallic cubes of pyrite - so-called Fool's Gold . Crystals conform to a limited number of crystal systems: cubes, hexagons and sheets, for example, because their constituent elements, oxygen and silica in quartz, or copper and carbonate in malachite, can only bond in a few predefined geometric configurations.
Other minerals have extremely homogeneous structures, such as agates, which are structurally identical in all directions, while Mica forms tabular, plate-like sheets.
Branching patterns are among the most familiar in nature - from plants to river-deltas. Known generically as dendrites (from the Greek word dendros, meaning leaf), the classic embodiment of dendritic mineral formation occurs where manganese in solution diffuses into a crack within a rock, and reacts with oxygen to create manganese oxide.
A multitude of phenomena produce colours in minerals, the most common source being small amounts of metallic elements, such as iron, manganese and copper distributed in the crystal lattice.
These absorb certain wavelengths of light: iron and manganese in quartz produce the purple of amethyst, and copper in malachite gives green. Microscopic structural features of crystals also provide colour, as light is scattered or bent through them. Tiny fibrous crystals within chalcedony - the microcrystalline form of quartz found in agates and jasper - give a milky blue colouration as light breaks into rays, the red component is generally absorbed and the blue reflected.
Tiny finely distributed pockets of gas, liquid or solid can scatter light, and cleavage, the regular arrangement of planes of weakness in a crystal structure, gives colour in moonstone and labradorite.
In opal, the mineral structure is created by microscopic silica spheres packed in a regular arrangement, which produce a brilliant play of colour as light is reflected and diffracted by the silica particles.
Rock degradation and weathering also provide intricate and subtle colour variation, for example iron minerals weather to give reds, and in Indian Paint Stone cracks within the rock have concentrated the flow of water and this has created the sites of oxidation.
For a more comprehensive explanation of how the minerals have been formed, see Formations, Images From Rocks, Richard Weston's book about our Earth Images. You can buy a copy here at a discounted price.