Composição química de rochas

Temos recebido muitas perguntas ultimamente sobre composição química de rochas, principalmente depois da publicação deste post. Comumente perguntam-nos algo do tipo “qual o elemento químico que forma o granito” ou outra rocha qualquer. É necessário que as coisas fiquem bem claras. Uma rocha é, em geral, um agregado de minerais. O granito, por exemplo, é majoritariamente formado dos minerais quartzo, feldspatos e micas. Um mineral, por sua vez, é uma substância, natural ou artificial, de composição química conhecida e característica, com estrutura atômica ordenada, geralmente na forma de cristais. Há minerais formados por apenas um elemento químico, como o ouro, mas a maioria dos minerais é de compostos multielementares. Há grupos de minerais “aparentados”, como os silicatos, formados a partir de inúmeras combinações físicas e químicas a partir de tetraedros de silício. Todos os minerais que compõem o granito por exemplo são silicatos: o quartzo e o feldspato são tectosilicatos e as micas são filossilicatos. A composição mineralógica das rochas ígneas dependerá basicamente da composição do magma a partir do qual se formaram. A composição das rochas metamórficas e sedimentares dependerá da composição das rochas ou sedimentos que lhes deram origem, lembrando que sedimentos são em geral resultado da intemperização (“decomposição”) de outras rochas ou da precipitação de compostos químicos. Dentro de determinado grupo de rochas, no entanto, há predominância ou maior presença de certos elementos químicos. As rochas ígneas ácidas são ricas em silício e relativamente pobres em ferro e magnésio, geralmente apresentando cor clara (leucocráticas) e são também chamadas rochas félsicas (de FELdspato e SÍlica), um exemplo é o próprio granito. Os solos originados destas rochas são geralmente mais amarelados. As rochas ditas básicas são menos ricas em silício e mais ricas em minerais contendo magnésio e ferro, por isso são também chamadas máficas, são rochas mais escuras (melanocráticas) e os solos delas originados costumam ser mais avermelhados. Um exemplo comum de rocha máfica é o basalto. Os minerais que compõem as rochas básicas são predominantemente silicatos: olivina, piroxênio, feldspatóides e feldspatos. As rochas metamórficas costumam ter composição semelhante à da rocha que lhe deu origem: o gnaisse é composto pelos mesmos minerais do granito. Dependendo do ambiente onde se dá o metamorfismo, no entanto, pode haver mudanças na composição em relação à rocha original. A composição mineralógica das rochas sedimentares, como já foi dito, depende da composição dos sedimentos. Rochas formadas a partir da litificação de areias de quartzo, como arenitos, são compostas principalmente por este mineral. Alguns arenitos, chamados de arcosianos, contêm também feldspatos. O calcário é formado da precipitação de carbonato de cálcio. A composição química dos solos, formados a partir do intemperismo químico e físico das rochas, é até certo ponto herdada da rocha parental.  

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Discussão - 20 comentários

  1. Celio Pereira disse:

    Gostaria de saber se com o Calcario Calcitico é possivel tranforma-lo fosfato bicalcico, usando, via acido sulfurico.?
    Gostaria de saber se usando acido fosforico eu posso aumentar a quantidade de fosforo em fosfatos,?
    Gostaria de saber se tem como neutralizar ,ou separar o Fluor de um fosfato,será que por lixiviação com acidos, eu consiga?Como reter o fluor contido no minerio? ele solubiliza<?é metalico

  2. Paulo disse:

    Caros leitores e colaboradores..esta discussão está sadia e enrriquecedora apesar das divergências, então gostaria de colocar mais uma situação. Como que é possível a utilização de calcáreo agrícola, quando este passa por um processo para a liberação de seus agentes e dentro de um perído de seis meses é liberado no solo e outras rochas que derivam do basalto, por exemplo, no processo de moagem em pedreiras da mesma maneira não pode ser aproveitado?

  3. anonimo disse:

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@22

  4. alex disse:

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  5. Julio disse:

    Trabalho em laboratorio de asfalto.
    Gostaria de saber se a temperatura pode mudar a estrutura molecular do agregado

  6. Keka Melo disse:

    quero saber a composiçao quimica do solo de taquaritinga do norte- PE

  7. Rebeca Barbosa disse:

    Olá!
    Poa tarde!
    Gostaria de saber sobre todos os tipos de rochas e de suas composições.
    Mas tambem gostaria de agradecer pela matéria,que me ajudou a passar de ano!!!!
    Até logo!

  8. Rebeca Barbosa disse:

    Olá!
    Poa tarde!
    Gostaria de saber sobre todos os tipos de rochas e de suas composições.
    Mas tambem gostaria de agradecer pela matéria,que me ajudou a passar de ano!!!!
    Até logo!

  9. Boa noite,quero saber se o granito e uma solução, vc pode me justificar por favor.
    obrigado Suzi

  10. gilberto disse:

    Bom dia
    quais os elementos químicos constituintes das rochas vulcânicas?
    grato
    gilberto

  11. manuel disse:

    Caro Ítalo
    Mais um texto seu de que só tardiamente tive conhecimento. O leitor José Salles faz uma interessante pergunta e a resposta que o Ítalo dá tem a minha concordância. O leitor apercebe-se de que a rocha triturada só atenderia à parte mineral,sendo necessário um complemento orgânico.
    De resto,ainda que a ideia não seja nova,ela enquadra-se em práticas muito antigas em agricultura,como o uso de margas,de correctivos calcários moídos,de escórias,ossos triturados,cinzas,gesso,minerais potássicos,eu sei lá.
    No segundo comentário,José Salles alude a autores,em que talvez se incluam personalidades ligadas a Remineralize the Earth,uma organização sem fins lucrativos,com sede em
    Northampton,Massachussetts,USA.,apostada em dar aproveitamento a muito desperdício rochoso que vai por esse
    mundo.
    Desse aproveitamento,não encontrei notícia no American Journal of Soil Science. No Journal of Plant Nutrition and Soil Science,171,de 2006,vem um artigo que parece incluir esse tipo de material,com o título-From marl to rock powder. One history of soil fertility management by rock materials.http://www3.interscience.wiley.com/journal/119877235/abstract. Aponta o resumo para se fazerem experiêcias,de maneira a tirar conclusões.
    Para terminar,fiquei com a certeza de que o leitor José Salles é mais do que um curioso nesta matéria. Quem escreve simulação de condições de intemperismo…,compostagem…,
    uso de sedimentos biodetríticos…,sabe muito.
    E pronto,Ítalo,um abraço,muito boa saúde,e muito bom trabalho.

  12. Patrícia disse:

    Gostei da reportagem….e como estou realizando uma pesquisa sobre oassunto quero fazer uma pequena pergunta: Já que a composição quimica do solo a feta o tipo de carga estatica contida há como mudala ou interpretar sua carga a 11okm por hora???
    Se aguÈm puder me ajudar eu agradeço

  13. Leandro Neves disse:

    ola! gostaria de saber na producao do granito e marmore em peças aquele pó que sai quando é lixado se “quimico” e prejudicial a saúde humana. ficarei muito grato se possivel envia a resposta para o e-mail.
    abraços, Leandro.

  14. Orlando disse:

    Queria informações sobre a composição química de rochas espaciais.

  15. jean disse:

    queria saber mais sobre o mag

  16. Kalinka disse:

    Gostaria de fazer a análise de uma rocha que temos, e saber a composição química da mesma. Quem executa esse trabalho no estado de SP?

  17. Jose Salles disse:

    Antes de tudo, agradeço sua atenção pela resposta, embora, cada vez mais, penso que o mundo acadêmico não deva se distanciar muito do mundo prático. Entretanto, é de grande valia seu esforço na tentativa de me informar sobre seu ponto de vista. Mesmo não sendo o mesmo que o meu e nem sendo especialista no assunto como vc é, discordo da idéia de não acreditar no uso das rochas micronizadas como uma alternativa para a reconstituição do solo. Por outro lado, concordo que em alguns casos, os minerais das rochas trituradas demoram a ser disponibilizado. É por isso que vou além, imaginando que talvez uma simulação das condições do intemperismo pudesse acelerar a disponibilização desses nutrientes minerais, por exemplo através de uma “compostagem” com cepas de microorganismos específicos. Acho que meu comentário é pertinente porque tenho buscado informações sobre o tema em artigos científicos de autores de renome internacional, os quais não acredito que sejam os tais charlatões que você me comentou. Na opinião deles, estaé uma técnica indicada como chave para agricultura moderna sustentável. Entre outras técnicas muito utilizadas, há o uso de sedimentos biodetríticos marinhos que quando micronizados e lançados ao solo são absorvidos quase que cerca de 98% e incorporados como fonte de minerais e calcário. Porém esse recurso biológico, apesar de ser natural não apresenta sustenbilidade de uso a longo prazo porque essa fonte de calcário é derivada de algas calcárias marinhas que crescem cerca de apenas 1mm/ ano. O fato concreto é que estou buscando alternativas ecologicamente corretas para oferecer saúde ao solo e essa técnica já é bem conhecida e difundida em países onde a agricultura é realmente moderna, onde se dá o verdadeiro valor e importância para a terra e que de forma alguma desconsidera o uso de rochas para a fertilização do solo. Minha intensão aqui é trazer à luz a discussão sobre o assunto e buscar opiniões de diversos especialistas. Com certeza a sua é uma forma importante de observar as possibilidades e agradeço muito sua atenção. Um abraço e muito sucesso!

  18. Italo M. R. Guedes disse:

    Caro José Salles,
    Não se pode de forma alguma tirar esta conclusão a partir do que escrevi. Um solo, e sua fertilidade química, não se produz apenas a partir de intemperismo físico de rochas. A trituração em partículas de tamanho muito pequeno é uma imitação apenas do intemperismo físico. As argilas do solo, no entanto, originam-se do intemperismo químico de minerais primários, ou seja, de sua alteração química e mineralógica, e isso não é facilmente imitável. A recuperação de áreas degradadas é um processo muito mais complexo. Dependendo do clima e do tipo de rocha que você usasse, possivelmente o efeito desta “fertilização” que você propõe só seria sentida depois de dezenas ou talvez centenas de anos. Um solo, como já escrevi em outros lugares, é resultado de uma série de fatores interagindo ao longo do tempo, e a rocha de origem é apenas um deles. Não existem fórmulas fáceis e milagrosas para recuperar um solo, tenha sido ele degradado por agricultura intensiva ou outro uso qualquer. Não duvido que haja por aí charlatões que vendam este tipo de solução, mas não é nosso caso. Prestamos consultoria em recuperação de áreas degradadas, se desejar entrar em contato conosco com algum caso concreto, sinta-se à vontade.

  19. Jose Salles disse:

    Muito boa a reportagem, com conteúdo, elucidativa e muito informativa. Sendo assim eu gostaria de deixar algumas questoes….Quer dizer que se eu triturar algumas rochas silicatadas a tamanhos ínfimos pode ser possível sua aplicação na recuperação de solos de lavouras intensivas? Basicamente só me faltaria uma fonte de materia orgânica para reconstituir todas as propriedades do solo de maneira ecologica e sustentável? Saudações.

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