CLINOZOISITE

Clinozoisite – Ca2Al3O(SiO4)(Si2O7)(OH) – is a sorosilicate from the Epidote Group. It is a somewhat less common mineral, characteristic of some types of metamorphic rocks. It has no economic importance.

Clinozoisite, in the Epidote Group with epidote and piemontite, is dimorphic with zoisite, forms a series with epidote, and may contain Mn, Mg, Ti, and Fe. It has a variety with Cr and another with Mn.

1. Characteristics

Crystal system:  Monoclinic prismatic. 

Color: Colorless, it can be green, greenish, gray, yellowish-gray, or pink.

Habit: Short prismatic, granular, rarely fibrous. 

Cleavage: {001} perfect. Striations // {010}.

Tenacity: Brittle.        

Twinning: Lamellar in {100}, rare. 

Fracture: Irregular.       

Mohs Hardness: 7

Parting: No.         

Streak: Grayish white.   

Lustre: Vitreous.         

Diaphaneity: Transparent.           

Density (g/cm³):  3.3 – 3.4 

 

2. Geology and Deposits

Clinozoisite is a typical mineral of igneous and sedimentary rocks that have undergone low- to medium-grade regional metamorphism: quartzites, slates, phyllites, chlorite schists, mica schists, and others.

It can also occur in gneisses, amphibolites, and, mainly, in contact and regional metamorphic rocks of calcic composition or associated with calcic rocks: hornfels, skarns, marbles, and calc-silicate rocks.

Clinozoisite is present in subduction zones and in contacts between Ca-rich and Al-rich rocks. If Fe is present, epidote is formed.

Clinozoisite can form as an alteration product of plagioclase, in the alteration known as saussuritization.

 

3. Mineral Associations

It occurs with quartz, anorthite, orthoclase, amphiboles (green hornblende and brown hornblende, actinolite), garnets (grossular), clinopyroxenes (diopside), epidote, prehnite, calcite, chrysocolla, and hessonite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.706 – 1.724    nβ: 1.708 – 1.729     nγ: 1.712 – 1.735

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.   

Relief:  Medium to high positive.          

Cleavage: {001} perfect, perpendicular to the elongation of the crystals, but may be difficult to observe due to the small size of the grains.    

Habits: Prismatic along the b-axis. Anhedral granular with pseudohexagonal basal sections and blade-shaped longitudinal sections. May be acicular, forming radial aggregates.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.004 to 0.011: paler colors between gray, white, and yellow; may exhibit anomalous interference colors of blue (“Prussian blue” or “Berlin blue”) or yellow-green. 

Extinction: always parallel in longitudinal sections parallel to axis b (also parallel and perpendicular to the cleavage), in other sections oblique extinction of approximately 30º.           

Elongation sign: SE(+) or SE(-), it is not diagnostic.            

Twins: rare, tend to be lamellar according to (100).         

Zoning: Frequently zoned. Zoned crystals with an epidote center and clinozoisite borders may occur.   

CONVERGENT LIGHT

Character: B(+) (important, but often difficult to obtain because the crystals are very small.          

2V angle: 40 – 90º           

Alterations: No.          

May be confused with: several other minerals.

Zoisite is similar.

Epidote is very similar, but it is B(-), its birefringence is higher and it may exhibit distinct pleochroism in green colors, which may tend towards citrus green.

Vesuvianite is U(-).

Kyanite shows normal interference colors and is B(-).

Lawsonite and pumpellyite have higher interference colors. When pumpellyite shows no bluish-green color in PPL, it cannot be distinguished from clinozoisite under a petrographic microscope.   

Clinozoisite 01: In PPL, high-relief colorless granular clinozoisite grains.
Clinozoisite 01: The same section as of the previous image, now in XPL. Typical features include granular habit, high relief, evidence of cleavage, interference colors much lower than those of epidote (often anomalous in blue), and a B(+) interference figure.
Clinozoisite 02: In the center of the image, a longitudinal section of clinozoisite in amphibolite, with quartz and brown hornblende (greenish, zoned, etc.).
Clinozoisite 02: The same section as of the previous image, now in XPL. Clinozoisite shows an anomalous blue interference color, reminiscent of the color of jeans, but different from the anomalous blue color of chlorite, "Berlin blue". It has parallel extinction and is B(+).
Clinozoisite 03: In PPL, clinozoisite and epidote are similar, but their interference colors are different, as can be seen in the next image.
Clinozoisite 03: The same section as of the previous image, now in XPL. Clinozoisite displays anomalous blue and white colors, while epidote displays strong colors.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying clinozoisite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with clinozoisite.

Sample preparation: Polishing clinozoisite is simple, easier than polishing associated quartz and feldspars. The quality of the polish is even slightly better than that of feldspars.       

PLANE POLARIZED LIGHT – PPL

Reflection color:  Very light gray, much lighter than quartz and feldspars, it gives the impression of white. Similar to the reflective color of titanite.

Pleochroism: No.

Reflectivity: Slightly higher than feldspars, (~10%?)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed.        

Internal reflections: Widespread, copying the colors of the mineral in hand specimen, usually a dark yellow, with a subtle greenish tint. It is not the green color of epidote, but a burnt yellow in various shades, both lighter and darker.  

May be confused with:  Other transparent minerals with a prismatic habit and similar colors. Clinozoisite’s good polish and almost white reflective color are quite diagnostic.

Epidote polishes much worse, and the internal reflections are greenish in color.

Titanite has a similar reflective color, but the internal reflections are different, and it is anisotropic.

Sphalerite has similar internal reflections, but the reflective color is much darker.

Clinozoisite 01: In the center of the images, clinozoisite crystal and, around it, plagioclase (albite). Clinozoisite shows better polish and a whiter color.
Clinozoisite 01: The same section as of the previous image, now in XPL. The colors of the internal reflections are a dark, caramel yellow, with a subtle touch of green, quite different from the internal reflections of epidote, for example.