Its coarse grained phaneritic texture indicates it cooled slowly, which implies it cooled underground — that is to say, it is intrusive. Granite is a common rock of the modern continental crust. There are granite-like rocks in Archean cratons, too. Technically, these are not precisely granite, but instead a suite of three similar plutonic rocks: tonalite, trondjhemite, and granodiorite. Here is an example of one such unit: an outcrop of the Kaap Valley Tonalite, a 3.
Explore it and and count how many different textural or compositional aspects to the pluton you can find:. Those minerals are nonferromagnesian and silica-rich, and the result is a felsic magma. Then as now, those magmas cooled slowly underground, developing a coarse-grained texture. Interestingly, the oldest Archean sedimentary rocks are not silicliclastic. There are no sandstones. There are no conglomerates. There are no shales. This implies, quite strongly, that there was no land exposed above sea level — no rock exposures available for weathering and erosion.
But underwater, there were still chemical sedimentary rocks forming due to mineral precipitation from the ocean. Among the oldest Archean sedimentary rocks is chert. It is thick and covers a wide area, and records an extended period of chemical precipitation of silica from seawater, sometime around 3. Examine this boulder of the Buck Reef Chert, and consider its alternating black and white layers, subsequent faulting, veining, and pressure solution, and a modern surface veneer of lichens.
They are chemical sedimentary rocks composed of alternating layers of iron-rich oxides usually hematite or magnetite and silica precipitates chert or jasper. Specifically, BIFs imply a world without widespread free oxygen. But oxygen is a highly reactive element, and it likes to form bonds with other atoms. One possibility, the straightforward one, is that these are primary sedimentary layers: bedding, in other words. In that case, a rhythmic alteration in sediment source is required.
However, another possibility is that the original sedimentation was more or less homogeneous, and only later during diagenesis did the silica-rich and iron-rich components separate out from each other into layers. Later in the Archean, enough land had risen above sea level that significant quantities of siliciclastic sediments were produced: mud, sand, and pebbles.
Turbidity currents flowed into deep-water marine basins of the Archean, adjacent to significant sources of clastic sediment such as proto-continental terranes.
Then, as now, these turbidity currents transported a slurry of sediments of different sizes. As the current slowed, its capacity dropped, and sedimentary grains settled out, in order of their weight. The biggest ones tended to be the heaviest ones, and settled out first, followed by progressively more and more lightweight grains which tended to be smaller. The result was a graded bed: coarse at the bottom, fine at the top. Abundant chlorite and actinolite in greenschist-facies basalts imparts a green color that gives rise to the name greenstone belt.
Most units are steeply dipping, tightly folded, and cut in places by large shear zones, which develop as the belts are accreted to the continent following their generation in oceanic crustal or arc Skip to main content Skip to table of contents. This service is more advanced with JavaScript available. But nephrite, which traditionally comes from western China, was in use long before jadeite became popular.
Older Chinese nephrite pieces sometimes command huge figures at auction. Investors typically seek out something with broader appeal. Often that means jadeite jewellery. Art deco pieces are particularly prized. Picking a winner can be tricky, though, because every piece of jade is different, and figuring out what each one is worth is an art rather than a science.
Colour is often the first thing buyers look for. Jade is almost synonymous with green, and bright emerald shades of the colour are still the most prized. Buyers also look for size, shape, transparency, texture and craftsmanship. A jade bangle, for instance, is often a good investment because it can only be made with high-quality material, according to Wong.
Here, our master carver has chosen to accentuate certain aspects of the design with the natural details of the raw material. The carving process can be thought of as a relationship between the raw material and the carving design, each informing one another.
The yellow ochre tinting down the left-hand of this kete design is evidence of compounds in the grain of the mineral. The carver has chosen to highlight this in the design, and likely chose the stone because of this special detail.
As a starting point, jade is a given or cultural, name and it originates from the ornamental stone jade, which is used in artwork and in jewellery making. Etymology is the study of the origin of words and the way in which their meanings have changed throughout history, and it can be an enlightening way to trace history. There are two types of Jade; jadeite and nephrite. Jadeite is a silicate of sodium and aluminium and is found in an area of Northern China. Nephrite is a silicate of calcium and magnesium and is found in New Zealand.
Nephrite jade is what the Maori term pounamu refers to. Damor in Before this, all rock specimens that looked like jade were named jade, irrespective of their variety. It is now agreed that the two main types of jade, nephrite and jadeite are chemically quite distinct.
There was also a third, called chlormilite, which is now regarded by most authorities as an iron-rich variety of jadeite. Named by Damour from the word jade, Jadeite has a wide colour range and is harder than nephrite jade. The origin of the term nephrite is much older than the word jade. People who suffered from this disease either ate the stone ground to a paste or wore it as a charm.
0コメント