Megacrystals (Calcite) – DP/EC 130
Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx (genitive calcis) with the suffix -ite used to name minerals.
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite". Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite at 380–470 °C, and vaterite is even less stable.
Properties
Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra (perfectly shaped crystals - well defined crystal faces), tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes. Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. (Source)
Uses
The properties of calcite make it one of the most widely used minerals. It is used as a construction material, abrasive, agricultural soil treatment, construction aggregate, pigment, pharmaceutical and more. It has more uses than almost any other mineral.
Calcite as Limestone and Marble
Limestone is a sedimentary rock that is composed primarily of calcite. It forms from both the chemical precipitation of calcium carbonate and the transformation of shell, coral, fecal and algal debris into calcite during diagenesis. Limestone also forms as a deposit in caves from the precipitation of calcium carbonate.
Marble is a metamorphic rock that forms when limestone is subjected to heat and pressure. A close examination of a broken piece of marble will usually reveal obvious cleavage faces of calcite. The size of the calcite crystals is determined by the level of metamorphism. Marble that has been subjected to higher levels of metamorphism will generally have larger calcite crystals. (Source)
The EarthCache
You are at an old calcite quarry. This calcite results from the growth of large megacrystals assocuiated with a falut system that flanks this quarry.
To validate your FOUND in this cache you have to answer the following questions:
- 1- At the cache coordinates, measure the length of the biggest calcite rhombohedra.
- 2- What is the habit (form) of these rhombohedra? (Do they grow straight, radially or in every direction?)
- 3- Do the crystals grow together without spaces between them or are there spaces between them?
Remember: No photos of the calcite so as to give away the answers!
Note: This is an old quarry! Danger of falling! Do not take unnecessary chances! Do not visit in darkness! Feel free to collect samples from the floor.
Calcite é um mineral com composição química CaCO3, com clivagem perfeita e romboédrica.
Fórmula: CaCO3
Dureza na escala de Mohs: 3.
Peso específico: 2,6 - 2,8.
Cor: incolor, branca
Cor do traço: branco.
Brilho: vítreo.
Cristais: trigonal.
Clivagem: perfeita, em três direções.
Origem: hidrotermal, sedimentar.
Etimologia: do latim calx, cal queimada.
Reage fortemente com ácido clorídrico, mesmo diluído e a frio
Calcite é um mineral de carbonato e o polimorfo mais estável de carbonato de cálcio (CaCO3). A escala de Mohs de dureza mineral, com base na comparação da dureza do risco, define o valor 3 como "calcite". Outros polimorfos do carbonato de cálcio são os aragonite minerais e vaterite. Aragonite converte para calcite a 380-470 ° C, e a vaterite é ainda menos estável.
A EarthCache
Estás numa antiga pedreira de calcite onde os fluidos que circularam entre duas falhas que ladeiam o local deram origem a megacristais de calcite.
Para validar o teu registo nesta cache, tens de responder às seguintes perguntas:
1- Nas coordenadas desta cache qual é o comprimento médio dos cristais de calcite romboédricos?
2- Qual a forma de crescimento destes cristais nas coordenadas da cache? (Linear, radial ou sem orientação definida?)
3- Mais uma vez, nas coordenadas da cache os cristais crescem sem especços entre eles ou há espaços entre os mesmos?
Lembra-te: Nada de fotos da calcite de forma dar as respostas!
NOTA: Estás numa pedreira! Perigo de queda! Não arrisques! Não aproximar sem luz!
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