AragoniteAragonite and the more common mineral Calcite are polymorphous to each other. Although Aragonite and Calcite contain the same chemical composition, they differ in crystal structure. Aragonite forms orthorhombic crystals, whereas Calcite forms trigonal crystals. Sometimes the crystals of Aragonite and Calcite are too small to be detected, and it is only possible to distinguish these two minerals by complex scientific optical tests. Since the true identity ofmicrocrystalline forms of Aragonite or Calcite may not be known, they may be mislabeled as the wrong mineral.
Almost all Aragonite crystals are twinned growths of three individual crystals that formpseudohexagonal trillings . Therefore, although Aragonite crystallizes in the orthorhombicsystem, virtually all its crystals are hexagonal shaped. The trillings can be noted from the basalstriations on each of the three individual crystals running in different directions, causing a hexagonal crystal to have three striation directions on each base. Other minerals frequently pseudomorph Aragonite. Calcite commonly pseudomorphs after Aragonite, and although it contains the same chemical composition and crystal forms as Aragonite, it is really Calcite. There are many Aragonite crystals sold to collectors that are in fact really Calcite pseudomorphs after Aragonite. A rare but popular pseudomorph of Copperafter Aragonite comes from Corocoro, Bolivia. Aragonite frequently contains sand inclusions, which give a specimen a brown color. It forms in many environments, but a particularly interesting one is its formation from deposition of hot, mineral-rich springs. The water releases the calcium it contains upon reaching the air, and forms mounds and thick crusts around the springs. When these deposited mounds and crusts are banded, they are sometimes carved and termed "Onyx Marble", "Suisan Marble", "California Onyx", or "Mexican Onyx". Aragonite is the main component of many organic substances, such as pearl and coral. The iridescent surface of pearl and mother-of-pearl is a layer of Aragonite secreted by mollusks and related invertebrates. http://www.minerals.net/mineral/aragonite.aspx AzuriteAzurite owes its name to its beautiful azure-blue color, which makes it a very popular and well-known mineral. It usually occurs with green Malachite, which may form green stains or specks on Azurite crystals or aggregates. The two minerals sometimes occur admixed or bandedtogether, forming what is called "Azure-malachite" in the gem and mineral trades. A rarer Azurite mixture, known as "Bluebird", is Azurite mixed with dark red Cuprite. Azurite, "Azure-malachite", and "Bluebird" all have gem uses.
In some localities, the Azurite undergoes a chemical change and loses some hydroxyl, alteringthe Azurite to Malachite, but retaining the crystal shape of the original Azurite. Sometimes, only part of the Azurite is altered to Malachite, while the other part remains as Azurite. Such specimens are green on one end and blue on the other. Linarite specimens are sometimes mistakenly sold as Azurite, since they both occur in the same locations and may strikingly resemble each other. However, simple tests on physical properties can accurately distinguish the two. http://www.minerals.net/mineral/azurite.aspx CalciteCalcite is the one of the most common minerals on earth. It occurs in a seemingly unlimited variety of shapes and colors. It constitutes a major portion of many of the earth's rocks. Calcite belongs to the calcite group of minerals, a group of related carbonates that are isomorphouswith one another. They are similar in many physical properties, and may partially or fully replaceone another, forming a solid solution series. All members of the calcite group crystallize in thetrigonal system, have perfect rhombohedral cleavage, and exhibit strong double refraction in transparent rhombohedrons.
Calcite and Aragonite are polymorphous to each other. Although Calcite and Aragonite contain the same chemical composition, they differ in crystal structure. Calcite forms trigonal crystals, whereas Aragonite forms orthorhombic crystals. Sometimes the crystals of Calcite and Aragonite are too small to be detected, and it is only possible to distinguish these two minerals by complex scientific optical tests. Since the true identity of microcrystalline forms of Calcite or Aragonite may not known, they may be mislabeled as the wrong mineral. A microcrystalline type of Calcite in globular form is common in certain regions. This Calcite forms from precipitating calcium-rich water inside caverns or on limestone cliffs. It exists in the form of stalagmites, stalactites, flowstone, and strange globular growths. These growths constantly accumulate, forming layers. They are frequently impure, trapping in organic matter such as leaves, twigs, and moss as they accumulate. Because of their impure status, they are classified by some as rocks. These calcareous growths have designated names based on their shape, habit, or formation. Most of these growths are Calcite, but some are crystallized asAragonite. The environment of formation, however, can be a key guide to whether the mineral crystallized as Calcite or Aragonite. Aragonite will generally develop only at hot springs, whereas most other calcareous growths will be Calcite. Calcite may form as an undesirable coating on top of another mineral. The calcite can be easily burned off by soaking it in acid, which will cause it to effervesce and eventually dissolve, leaving the mineral below exposed. http://www.minerals.net/mineral/calcite.aspx DolomiteDolomite is a very common mineral and is best known for its saddle-shaped curved crystal aggregates. A unique, isolated Dolomite occurrence in Eugui, Spain has provided colorless transparent crystals that resemble the Iceland Spar variety of Calcite, and have no resemblance to Dolomite specimens from any other localities. The occurence of Kolwezi, in the Congo (Zaire) has produced some fascinating, cobalt-rich specimens that are a beautiful hot pink color and are extremely popular.
Dolomite occurs in a different crystal class than the Calcite Group. This can be noted by the fact that Dolomite generally forms more elongated crystals than the Calcite Group. In addition, Dolomite never occurs in scalenohedral crystakss, whereas minerals of the Calcite Group do. Dolomite is both a mineral and a rock. The mineral is the pure form, and Dolomite rock is composed mostly of Dolomite but also with impurities such as Calcite, Quartz, and Feldspar. http://www.minerals.net/mineral/dolomite.aspx MalachiteMalachite is a very popular mineral with its intense green color and beautiful banded masses. The banded specimens are formed by massive, botryoidal, reniform, and especially stalactiticMalachite that are dense intergrowths of tiny, fibrous needles. Dense banded specimens are often sliced and polished to bring out their beautiful coloring. The bands may consist ofconcentric rings with interesting patterns; such specimens are highly sought after. These concentric banded specimens are most commonly from African sources. Polished, banded Malachite has been carved into ornaments and worn as jewelry for thousands of years, and in some ancient civilizations it was thought to be a protection from evil if worn as jewelry.
Malachite is generally found together with blue Azurite, and sometimes the two may occur admixed or banded together, forming what is commonly known in the gem and mineral trade as "Azure-Malachite". Malachite may also replace Azurite crystals, retaining the original Azurite shape but chemically altering it. http://www.minerals.net/mineral/malachite.aspx |
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