MONTICELLITE
Monticellite – CaMgSiO4 – is a rare nesosilicate that occurs mainly in magnesian skarns. It has no importance as an ore.
It is classified in the Olivine Group and may contain Al, Fe, Ti, Zn, Mn, and H2O as impurities. Forms a series with kirschsteinite.
1. Characteristics
Crystal system: Orthorhombic pipyramidal.
Color: Colorless, gray, greenish gray or yellowish gray.
Habit: Prismatic, granular, massive. Crystals up to 5 cm.
Cleavage: {010} poor.
Tenacity: Brittle.
Twinning: It can form cyclic twins through {031}, generating twins that resemble, in section, six-pointed stars.
Fracture: Subconchoidal to irregular.
Mohs Hardness: 5.5
Parting: No.
Streak: White.
Lustre: Vitreous, somewhat resinous.
Diaphaneity: Transparent.
Density (g/cm³): 3.03 – 3.27
2. Geology and Deposits
Monticellite occurs mainly in magnesian skarns, when siliceous dolomitic limestones have been subjected to high-grade metamorphism and metasomatism. It was found in the contact zones between olivine-gabbros and limestones and is frequent in the contact zones between granites and dolomitic limestones.
It also occurs in carbonatites and ultrabasic rocks such as kimberlites and alnöites.
In mafic and ultramafic igneous rocks it forms isolated grains or rims around olivine.
3. Mineral Associations
It occurs associated with carbonates (calcite, dolomite, harkerite, malachite), feldspathoids (nepheline, hauyne (in alnoites)), micas (biotite), other nesosilicates (merwinite, garnets, spurrite and minerals of the Humite Group (norbergite, chondrodite, humite and clinohumite)) and sorosilicates (vesuvianite, melilite, cuspidine),
Also with inosilicates (diopside, tremolite, wollastonite, hillebrandite, foshagite), oxides (perovskite, dysanalyte, spinel, ilmenite, magnetite) and apatite.
It never occurs with feldspars.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.638 – 1.663 nβ: 1.645 – 1.674 nγ: 1.653 – 1.680
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate to high.
Cleavage: {010} poor, it is not visible under a microscope.
Habits: Generally granular, anhedral, rounded or irregular. Massive, prismatic. In volcanic rocks it can be idiomorphic, but it is usually hypidiomorphic. In skarns its habit is granular.
It may exhibit intergrowths with minerals of the Humite Group (norbergite, chondrodite, humite and clinohumite).
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.014 to 0.020, showing interference colors up to first-order orange.
Extinction: Usually parallel.
Elongation sign: ES(+) or ES(-), depending on the section orientation, is not diagnostic. ES(+) with regard to cleavage, which is usually not visible.
Twins: Sometimes present, cyclic, by {031}, generating shapes that resemble, in section, six-pointed stars.
Zoning: No information visible.
CONVERGENT LIGHT
Character: B(-)
2V angle: 69–88º, decreases with increasing Fe or Mn.
Alterations: Monticellite readily alters under hydrothermal conditions to serpentine, tremolite, calcium clinopyroxenes, and other minerals.
May be confused with: Some other minerals.
Olivine is very similar, but the birefringence is less pronounced.
Clinopyroxenes exhibit well-developed cleavage.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying monticellite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with monticellite, such as magnetite and ilmenite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: