Calcite with serpentine reportedly from China
- Details
- Created: Tuesday, 25 March 2014 19:15
This cab of unusual physical appearance, a brown glassy material with included aggregates of yellow nodular material was received for determination.
This is the first time such material was presented to us and focused our attention.
Without a raman microscope or an IR microscope, nor with XRD equipment, determination is going to be a challenge due to the possible presence of several materials.
Photo 1: 13.58 ct brown with yellow 'spots' cab| Shape | flat oval cab |
| Size | 28 x 16.5 x 4 mm |
| Color | brown with pale yellow spots |
| Diaphaneity | translucent to opaque |
| Lustre | grasy to glassy |
| Weight | 13.58 ct |
| SG | 2.41 |
| RI | > 1.50 |
| DR | - |
| Pleochroism | - |
| Polariscope / Conoscope | - |
| SWUV | inert |
| LWUV | inert |
| Magnetic susceptibility | weak (~7%) |
Photo 2: 10x details of yellow spots.Aggregates of yellow globular material with cavities melded in a brown glassy material.
Some fractures are also observed.
Infrared reflectance spectroscopy:
Several IR specular reflectance spectra were collected from the top of the cab. Two of them were retained for showing the most significant features and for being distinct from each other.
Figure 1: two spectra (IRS1 & IRS2) obtained from two different area of the top of the cab.
In both spectra, the material belongs to carbonates group with the CO3 radical (880 cm-1 and 1300-1650 cm-1). The reflectance band at 711 cm-1 indicates this carbonate is a calcite. A weaker band at 727 cm-1 indicates the presence of dolomite.
Figure 2: comparison of IRS1 spectrum with one of a lizardite from Vikersund, Norway.
In the spectrum IRS 2, the bands between 520 and 690 cm-1 and 910 and 1120 cm-1, are similar to these of lizardite, antigorite both being serpentine, but they are also similar to these of clinochlore except below 560 cm-1. Preference is given to serpentine (lizardite) even if it is not sufficient to get a final diagnostic.
Another comparison with the lizardite spectrum 'ATR R060006 Lizardite Snarum Norway' from RRUFF gives some analogies, but since it is an Attenuated Total Reflectance spectrum, the bands are slightly shifted to lower frequencies compared to reflectance ones.
UV-VIS-NIR spectroscopy:
Figure 3: uv-vis spectrum of a translucent area of the cab.
The absorption continuum towards higher energies (600-> 380nm) is caused by O2- -> Fe2+ Charge Transfer [1].
The weak 414, 430, 443, 501 nm bands are possibly assigned to Fe2+ Forbidden Transitions [1].
The 600 - 800 nm region does not match exactly the Fe or Ni lizardite spectra as described in [1].
The weak doublet at 944 & 956 nm does exist in serpentines like the bowenite and lizardite. Could this doublet caused by Cr3+ or Fe2+ ?
The UV-VIS absorption spectrum of a Lizardite from Vikersund, Norway is provided in figure 4 for comparison, it was normalized to get similar level of absorption. Even if the match is not perfect, both spectra are very similar.
Figure 4: uv-vis spectrum of a translucent area of the cab compared to that of of a Lizardite from Vikersund, Norway.
Photoluminescence spectroscopy:
No photoluminescence observed with 405nm laser, nor UVL and UVC lamps.
Discussion:
Magnesite is a member of the calcite-type carbonates having the formula MgCO3. It forms dolomite CaMg(CO3)2 with calcite CaCO3 in the system CaCO3-MgCO3. Pure magnesite is not common in nature because there exists a complete series of solid solutions between MgCO3 and FeCO3, which is constantly present in magnesite in its natural occurrence.
Magnesite is usually white, but it may be light to dark brown if iron-bearing. The hardness of magnesite is 3 to 4 on the Mohs scale, and the specific gravity is 3.00.
Magnesite deposits are of two general types: massive and crystalline. Massive magnesite is an alteration product of serpentine which has been subjected to the action of carbonate waters. Crystalline magnesite is usually found in association with dolomite. It is generally thought to be a secondary replacement of magnesite in preexisting dolomite by magnesium-rich fluids.
Because of its magnetic susceptibility, this material contains Fe and possibly Ni, thus this is compatible with lizardite chemistry. However, the fact we here have calcite material (as indicated by IRS spectra) instead of magnesite (which is not detected) is not explained at the moment. We have a very weak reflection band for dolomite which is also a Mg carbonate CaMg(CO3)2 and which often contains Fe that gives it its yellow to brown color. Lizardite and calcite are often found together in veins in kimberlites.
The specific gravity is rather low 2.41 compared to calcites and serpentines, but it might be explained by the small cavities in the yellow material:
| Specific gravity (average) | |
| this sample | 2.41 |
| lizardite | 2.57 |
| calcite | 2.71 |
| magnesite | 3.00 |
Conclusion:
This cab is a mixture of calcite and serpentine possibly lizardite.