CLINOHUMITE

Clinohumite – Mg9(SiO4)4(OH,F)2 – is a very rare nesosilicate. It does not constitute ore.

 It is the monoclinic member of the Humite Group and may contain Ca, Mn, Fe, Ti and (OH) as impurities. Clinohumite forms an isomorphic series with hydroxyclinohumite (defined only in 1998, so care is needed with older literature). It is likely that in many localities both clinohumite and hydroxyclinohumite occur. It has a variety rich in Ti, titanoclinohumite, formerly called titanolivine.

In the Humite Group there are four minerals with similar structure, chemical composition, optical properties and paragenesis: norbergite, chondrodite, humite and clinohumite. Structurally, they are similar to olivines, but do not carry Fe and Mg in larger amounts, only Ti.

Macroscopically, clinohumite can be confused with several other minerals of the paragenesis in which it occurs, mainly with other minerals from the Humite Group, which generally occur in the same environment and localities. It is similar to chondrodite, olivine, vesuvianite, garnet and very similar, only more orange and less red, to apatites with alteration to Fe (hydro)oxides.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: White, yellow to brown as the Ti content increases. Orange, pink.

Habit: Rounded crystals with complex shapes, with many faces. It can be massive and tabular.

Cleavage: {100} poor.       

Tenacity: Brittle.        

Twinning: Common on {100}, simple or lamellar.

Fracture: Irregular to subconchoidal.       

Mohs Hardness: 6

Parting: No.         

Streak:  White.        

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.17 – 3.35

 

2. Geology and Deposits

Clinohumite typically occurs in contact metamorphism and metasomatism rocks such as limestones and metasomatic dolomites (skarnites), necessarily magnesian: calcitic marbles and dolomitic marbles, in contact with felsic plutonic rocks (granites).

It also occurs in metamorphosed ultramafic rocks (peridotites, talc-schists, serpentinites) and in basic-ultrabasic complexes.

It can occur in volcanic ejecta, carbonatites and kimberlites.

 

3. Mineral Associations

It occurs with rock-forming minerals such as carbonates (calcite, dolomite) and silicates such as quartz, sanidine, nepheline, biotite, talc, cordierite and clinochlor.

With sulfides like pyrite and to oxides like magnetite and spinel.

Associates with minerals typical of contact metasomatism rocks: vesuvianite, wollastonite, garnets (grossular), olivine (forsterite), tremolite and meionite (a very rare tectosilicate from the Escapolite Group).

It is also associated with rare to extremely rare borates (szaibélyite, ludwigite and fluoborite), monticellite (a rare nesosilicate) and cuspidine (a very rare sorosilicate).

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.628 – 1.632   nβ: 1.641 – 1.654     nγ: 1.662 – 1.674  (Indices vary in the literature)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  Colorless, pale yellow to golden yellow. The color becomes more intense with increasing Fe and/or Ti content.

Weak pleochroism between:

X = golden yellow, brown yellow, deep reddish yellow.

Y = pale yellow, orange yellow, light yellow.

Z = pale yellow, yellow orange, colorless. 

Relief: Moderate to high.           

Cleavage:  {100} poor, usually not visible.          

Habits: Granular, rounded, polygonal shapes possible, but generally tends to rounded with subhedral to anhedral shapes. It can be presented in a tabular form.

Lamellar intergrowths with olivine and monticellite are frequent.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.028, corresponding to colors up to the end of the second order: gray, yellow, red, purple, violet, blue and greenish. They are strong and diverse colors. The stronger the color of the mineral in PPL, the less evident will be the interference colors in CPL, as with titanite, for example.

If the clinohumite has strong own colors, the interference colors are masked and the appearance of the mineral is almost the same in PPL and in XPL           

Extinction: Oblique with angles between 9 and 15º.           

Elongation sign: ES(+) or ES(-), can rarely be determined as a function of the rounded shapes that the grains usually have. When the grains are tabular, determination is possible, but not diagnostic.

Twins: May occur, simple or lamellar.         

Zoning: No.             

CONVERGENT LIGHT

Character:  B(+)         

2V angle: 52-90º or 73-76º; literature with conflicting data.         

Alterations: clinohumite alters to serpentine or chlorite.          

May be confused with: several other minerals, including several from the same paragenesis.

Olivine is very similar, especially similar to colorless clinohumites. But olivine has an angle of 2V close to 90º and is B(-).

Staurolite shows similar pleochroism, but it occurs in another paragenesis (medium grade pelitic metamorphic rocks such as schists) and its interference colors are much lower.

Altered apatites may appear red, but they are well-defined red colors, do not have the yellow and orange tones typical of clinohumite, nor do they show pleochroism.

The other members of the Humite Group – chondrodite, humite and norbergite – are very similar and careful analysis is necessary. Probably it will be necessary to use another analytical technique for accurate identification.         

Clinohumite 01: In PPL, in the center of the image, clinohumite grain showing the first of its pleochroism colors. The next image shows the second color.
Clinohumite 01: In PPL, the same grain as of the previous image, now rotated 90 degrees to show the second color of pleochroism. The colors vary between faint yellow, almost colorless, and very strong yellow.
Clinohumite 01: The same grain as of the two previous images, now in XPL. The interference colors are strong, with a predominantly yellow color.
In XPL, in the center of the image, clinohumite grain with lamellar twins. Around him, olivine.
Clinohumite 02: In PPL, clinohumite grain with its typical yellow color. Above it, olivine, Below, calcite. Opaques are magnetite, often in octahedrons.
Clinohumite 02: The same section as of the previous image, now in XPL. The clinohumite grain shows well developed twins. Lamellar twins appear to be common in clinohumite.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of clinohumite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with clinohumite.

Sample preparation: the polishing of clinohumite does not present any problems and is of good quality, comparable to the polishing of carbonates. There are no polishing scratches, only occasional holes.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, like calcite.       

Pleochroism: No.     

Reflectivity: Low.        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy between yellowish and bluish.  

Internal reflections: Widespread.      

May be confused with: No information available. 

Clinohumite 01: In PPL, olivine (bottom), calcite (top) and clinohumite (middle). There is no diagnostic feature for clinohumite in PPL.
Clinohumite 01: The same section as of the previous image, now in XPL. Clinohumite exhibits generalized internal reflections in yellowish, which is the color of the mineral in hand specimen.
Clinohumite 01: The same section as of the two previous images, in XPL and now rotated 90 degrees to show the second anisotropy color, a light bluish. Olivine with darker reflections and calcite with white reflections.