Talk:Calcite: Difference between revisions

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Authors:  [[user:Hschwarz|Hans-Jürgen Schwarz ]], Nils Mainusch
<br>back to [[Carbonate|Carbonate]]
{{Infobox_Salt
|Footnote            =<ref>http://webmineral.com/data/Calcite.shtml viewed on 09/06/2011</ref><ref>http://www.mindat.org/min-859.html viewed on 09/06/2011</ref>
|photo                =[[Image:Calcit Kristalle Musterplatte.jpg|300px]]
|mineralogical_Name  = Calcite
|chemical_Name        =Calcium carbonate
|Trivial_Name        =Calc spar
|chemical_Formula      =CaCO<sub>3</sub>
|Hydratforms          =CaCO<sub>3</sub>•H<sub>2</sub>O<br>CaCO<sub>3</sub>•6H<sub>2</sub>O
|Crystal_System      =trigonal
|Crystal_Structure    =
|Deliqueszenzhumidity =
|Solubility          = 0.014 g/l (25°C)
|Density              =2.72 g/cm<sup>3</sup>
|MolVolume            =
|Molweight            =100.09 g/mol
|Transparency        =transparent to opaque
|Cleavage            =perfect
|Crystal_Habit        =
|Twinning            =
|Refractive_Indices  =n<sub>o</sub> = 1.658<br>n<sub>e</sub> = 1.486
|Birefringence        =Δ = 0.172
|optical_Orientation  =negative
|Pleochroism          =
|Dispersion          =
|Phase_Transition    =
|chemBehavior        =
|Comments            =well soluble in 2M HCl
}}
<!--
== Abstract  ==
== Introduction  ==
<br>
== Preface  ==
-->
== Occurrence of calcite<br>  ==
Calcium carbonate as a natural mineral occurs in the form of calcite and aragonite. Apart from these two modifications, the unstable modification vaterite can be detected in bone, but transforms into calcite under normal conditions.
Calc-spar is a very pure deposit of calcium carbonate. Calcium carbonate as a binder for mortars or paints can be of organic and inorganic origin. A special characteristic of calc-spar, the strong birefringence  was discovered and scientifically explained by C. Huygens in the 17th century.<br><br>
== Information concerning the damage potential and weathering activity of calcite  ==
=== Solubility  ===
Calcite is hardly soluble in water. The influence of temperature on solubility is low. However, if water contains CO<sub>2</sub>, the solubility increases considerably.
{|border="2" cellspacing="0" cellpadding="4" width="30%" align="center" class="wikitable"
|+''Table 1: Calcite: Solubility in dependance of temperature [Data from H. J. Schwarz 1996 and Stark/Stürmer 1993] <bib id="Stark.etal:1996" />''                   
|-
|bgcolor = "#F0F0F0" align="center"| '''18°C'''
|bgcolor = "#F0F0F0" align="center"| '''25°C'''
|-
|bgcolor = "#FFFFEO" align="center"| 0,015g/l
|bgcolor = "#FFFFEO" align="center"| 0,014 g/l
|}
<br>
The system CaCO<sub>3</sub>– H<sub>2</sub>O:<br>Both known hydrate stages of calcium carbonate only exist under special circumstances and dehydrate under normal ambient air-conditions.<br>
<!-- === Crystallization pressure  ===
=== Conversion reaction  ===
== Analytical identification  == -->
== Microscopy<br>  ==
'''Laboratory examination:'''<br> Calcite is only slightly soluble in water, therefore recrystallization cannot be initiated.
<br> '''Refractive indices:'''&nbsp; n<sub>0</sub> = 1.658,&nbsp; n<sub>E</sub> = 1.486 <br>'''Birefringence''':&nbsp;&nbsp;&nbsp; Δ&nbsp; = max. 0,172<br>'''Crystal class''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; trigonal<br>
<br> '''Polarized light microscopic examination:'''<br>
Calcite crystals originating from lime mortar, plaster or paints are usually very fragmented, and morphological characteristics are rarely clearly identifiable with light microscopy. In samples calcite rather appears as a kind of matrix of fine, rounded particles, in which developed tabular rhombohedra, a characteristic for calcite, rarely appear.
If calcite sample materials stem from another source than calc-spar, e.g. chalk or muscles, then significantly larger equidimensional particles can appear. Strongly elongated particles, laths, needles or crystal aggregates do not appear.
The very high birefringence leads to interference phenomena, even with very small particles. In addition the strong relief changes are very noticeable, especially when using an immersion medium with a refractive index near n<sub>O</sub> – value of 1,658. Since calcite is optically uni-axial, independent of the orientation and the viewing direction, in one of the two normal positions it always displays a fixed refractive index value, being effective at n<sub>D</sub> = 1,658.
<br>'''Possible mistakes:'''<br>
Calcite is clearly identified, if the following characteristics are recorded:<br>
*calcite is slightly soluble in water
*acid sensitive
*very strong birefringence
*high relief in most immersion media<br>
*when using an embedding medium with a refractive index of 1,662 it reveals a strong relief change, wherein a crystal nearly disappears in the normal position, when n<sub>o</sub>-refractive index is effective <br>
*only parallel and symmetrical extinction is observable
<br clear="all">
{|border="2" cellspacing="0" cellpadding="4" width="80%" align="left" class="wikitable"
|+''Table 2: Salt phases, with chemical and optical properties similar to calcite''                   
|-
|bgcolor = "#F0F0F0"| '''Salt phase'''
|bgcolor = "#F0F0F0"| '''Distinguishing features to calcite'''
|-
|bgcolor = "#F7F7F7"| '''Dolomite''' MgCa(CO<sub>3</sub>)2&nbsp;
|bgcolor = "#FFFFEO"| Both indices above 1.5
|-
|bgcolor = "#F7F7F7"| '''Magnesite''' MgCO<sub>3</sub>
|bgcolor = "#FFFFEO"| Both indices above 1.5, n<sub>o</sub>- Index with 1.7-1.719 significantly higher than calcite.
|}
<br clear="all">
<!--
== X-ray diffraction  ==
== Raman-Stectroscopy  ==
== DTA / TG  ==
== IR-Spectroscopy  ==
= How to deal with calcite damage  =
-->
== Calcite in Pictures  ==
<!-- === On the object  === 
=== Under the polarizing microscope  === -->
=== Under the scanning electron microscope (SEM)  ===
<gallery caption=" " widths="200px" heights="150px" perrow="3">
Image:Calcit Kristalle Musterplatte.jpg| Calcite crystals on a sample plate
Image: | unter gekreuzten Polarisatoren
</gallery>
== Weblinks<br>  ==
<references/><br>
== Literatur  ==
<biblist />
[[Category:Calcite]] [[Category:Schwarz,Hans-Jürgen]] [[Category:R-MSteiger]] [[Category:inProgress]] [[Category:Carbonate]] [[Category:Salt ]]
[[User:SLeithaeuser|SLeithaeuser]] 09:41, 24 February 2012 (CET)
[[User:SLeithaeuser|SLeithaeuser]] 09:41, 24 February 2012 (CET)

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SLeithaeuser 09:41, 24 February 2012 (CET)

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