 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
| Agate | |
|---|---|
| General | |
| Category | Mineral |
| Chemical formula | Silica, SiO2 |
| Identification | |
| Colour | White to grey, light blue, orange to red, black. |
| Crystal habit | Cryptocrystalline silica |
| Crystal system | Hexagonal |
| Cleavage | None |
| Fracture | Conchoidal with very sharp edges. |
| Mohs Scale hardness | 7 |
| Luster | Waxy |
| Refractive index | Translucent to transparent |
| Pleochroism | None |
| Streak | None |
| Specific gravity | 2.6 |
| Fusibility | ? |
| Solubility | ? |
| Major varieties | |
| None | |
Agate (ag' it) is a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly ChalcedonyChalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy luster. It may be semitransparent or translucent and is usually white to gray, grayish-blue or some shade of brown, sometimes nearly black. Other shades have been gi.
According to TheophrastusTheophrastus the successor of Aristotle in the Peripatetic school, a native of Eresus in Lesbos, was born c. His original name was Tyrtamus but he later became known by the nickname "Theophrastus," given to him, it is said, by Aristotle to indicate the gr the agate (achates) was named from the river Achates, now the Drillo , in SicilySicily Sicilia in Italian) is an autonomous region of Italy and the largest island in the Mediterranean Sea, with an area of 25,700 sq. 1 million inhabitants. Towns and Cities Sicily's principal cities include the regional capital Palermo, together with t, where the stone was first found.
Most agates occur as nodules in eruptive rocks, or ancient lavaLava is molten rock that a volcano expels during an eruption. Due to its high temperature, lava can be quite fluid when first exuded from a volcanic vent, but eventually solidifies into rock. However, the lava may flow many miles before solidification.s, where they represent cavities originally produced by the disengagement of vapour in the molten mass, and since filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section, whence such stones are known as banded agate, riband agate and striped agate.
In the formation of an ordinary agate, it is probable that waters containing silica in solution – derived, perhaps, from the decomposition of some of the silicates in the lava itself – percolated through the rock, and deposited a siliceous coating on the interior of the vapour-vesicles. Variations in the character of the solution, or in the conditions of deposit, may have caused corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartzFor other uses of this word, see Quartz (disambiguation). Milk quartz rock Quartz is the most abundant mineral on Earth (about 12% vol. It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO), with a hardness of 7, Several vapour-vesicles may unite while the rock is viscous, and thus form a large cavity which may become the home of an agate of exceptional size; thus a BrazilianThis article is about Brazil, the country. For other article subjects named Brazil see Brazil (disambiguation). The Federative Republic of Brazil Republica Federativa do Brasil in Portuguese) is the largest and most populous country in South America. geode, lined with amethyst, of the weight of 35 tons, was exhibited at the Dusseldorf Exhibition of 1902.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, like celadonite , delessite or " green earth ," which are rich in iron, derived probably from the decomposition of the augite in the mother-rock. This green silicate may give rise by alteration to a brown oxide of iron ( limonite), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate, freed from its matrix, is often pitted and rough, apparently in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the "priming," and upon this basis zeolitic minerals may be deposited.
Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethyst, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity or geode.
A Mexican agate, showing only a single eye, has received the name of " cyclops." Included matter of a green colour, like fragments of "green earth," embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to moss agate .
Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum, due to the extreme delicacy of the successive bands, whence they are termed rainbow agates .
On the disintegration of the matrix in which the agates are embedded, they are set free, and, being by their siliceous nature extremely resistant to the action of air and water, remain as nodules in the soil and gravel, or become rolled as pebbles in the streams.
| Politics | Business | Finance |
 |
 |
 |
|