ZOISITE

Zoisite – (Ca2)(Al3)(Si2O7)(SiO4)O(OH) – is a rarer sorosilicate from the Epidote Supergroup, characteristic of some types of metamorphic rocks. It has no importance as an ore.

Zoisite is the orthorhombic polymorph of clinozoisite. As impurities it may contain Ca, Na, Fe, Mn, Mg, Cr, Ti, V., Sr and H2O.

There are four varieties: with Cr, with Mn (pink – “Thulite”), blue-violet and with anomalous optical properties. The famous gemstone “Tanzanite” is a zoisite.

1. Characteristics

Crystal system:  Orthorhombic bipiramidal.         

Color: Colorless, grey, grey-white, green, yellow. It can be yellow brown, purple or pink.     

Habit:  Prismatic, columnar, can be compact to massive.      

Cleavage: {010} perfect, {100} imperfect. Striations // to {010}       

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular, conchoidal.       

Mohs Hardness: 6 – 7

Parting: No.         

Streak: White.         

Lustre:  Vitreous, pearly on cleavage.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.15 – 3.36

 

2. Geology and Deposits

Zoisite occurs mainly in medium-grade metamorphic rocks (epidote–amphibolite), of regional metamorphism, formed from Ca-rich and Mg-poor sediments: schists, gneisses, eclogites and marbles. It is also found in blueschists, but is not as common. It can form a high-grade metamorphic rock called zoisitite. In the type locality, it occurs in eclogite pegmatite.

In igneous rocks, it occurs as an accessory mineral in mafic and ultramafic rocks.

Thulite occurs only in pegmatites and hydrothermal veins.

Plagioclases can change to saussurite, a very fine-grained hydrothermal alteration product composed of albite, zoisite or clinozoisite, sericite (muscovite) and other silicates, in addition to calcite.

 

3. Mineral Associations

Associates with:

– quartz and feldspars (albite, adularia),

– pyroxenes (diopside),

– amphiboles (“hornblende”, actinolite, pargasite, micas (biotite, muscovite, chlorite),

– other members of the Epidote Supergroup (epidote, clinozoisite) and

– garnet, prehnite, calcite and corundum (including the ruby variety).

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.696 – 1.700    nβ: 1.696 – 1.702    nγ: 1.702 – 1.718

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, does not show pleochroism.

Thulite (variety with Mn) has a pink color and moderate pleochroism between colorless, pink, violet, deep blue, red-violet to yellow and greenish-yellow.

Relief: High.           

Cleavage: {010} perfect, {010} imperfect.           

Habits: Columnar, fibrous, acicular, granular, prismatic crystals, massive. Basal sections are rhombic or hexagonal. May form oriented intergrowths with epidote or clinozoisite.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence 0.005 – 0.018: 1st order interference colors between grey, white and straw yellow, or anomalous deep blue-gray or brown (leather brown) colors.

Birefringence increases with increasing Fe content.

Zoysite-β= normal birefringence.           

Extinction: Parallel (is the only orthorhombic member of the Epidote Supergroup). Basal sections show symmetrical extinction. 

Elongation sign: ES(-).            

Twins: No.         

Zoning: Often zoned. Thulite may have a darker pink core and a lighter pink border as a function of varying Mn content.             

CONVERGENT LIGHT

Character:  B(+).         

2V angle: 0 – 69º, it can vary in the same grain due to variations in the chemical composition.         

Alterations: is generally not altered.          

May be confused with: its habit is very similar to that of epidote and clinozoisite, but these are monoclinic and have oblique extinction. Characteristic for zoisite are the high relief, the basal cleavage parallel to the elongation of euhedral crystals and the fact that it is colorless in PPL.

Clinozoisite, in addition to oblique extinction, shows anomalous interference colors in very intense and luminous blue (“Prussian blue”), a color that is more intense and stronger than the anomalous interference colors of zoisite, which tend more towards gray-blue. little luminous.

Vesuvianite and apatite are uniaxial. Vesuvianite also has two good cleavages and not just one.

Melilite may be similar, but has much lower relief and is U(-).

Pink andalusite is quite similar, but is B(-) and has lower relief.

Sillimanite has more intense interference colors and other habits.     

 

Zoisite 01: In the center of the images, diagonally, approximately rhombic basal section of zoisite. It is colorless, shows high relief and a sharp cleavage. PPL.
Zoisite 01: The same section as of the previous image, now in XPL. Zoisite shows low (gray) or anomalous interference colors. Around it, a green hornblende and plagioclases.
Zoisite 02: In PPL, rhombic basal sections of zoisite with green hornblendes and plagioclases. The rhombic section on top will show symmetrical extinction in the next image.
Zoisite 02: The same section as of the previous image, now in XPL. Low interference colors are typical for zoisite.
Zoned zoisite in XPL. The zonation pattern changes from grain to grain, but generally the grains show regions with anomalous interference colors of different colors or the same color but different shades.
In XPL, zoned longitudinal section of zoisite.

5. Reflected Light Microscopy

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

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: 

Caption
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