SCAPOLITE

Scapolite – (Na,Ca,K)4Al3-6Si9-6O24(Cl,CO3,SO4) – is a rarer tectosilicate, characteristic of some specific geological situations. It is unimportant as an ore, but some varieties are used as gemstones.

“Scapolite” is not a mineral, but a generic term referring to the members that make up the series between marialite (of Na) and meionite (of Ca), classified in the Scapolite Group. The compositional extremes of this series do not occur in nature. “Wernerite” is an obsolete term for an intermediate composition, from Me:Ma 3:1 to 1:2. There are three varieties of “scapolite”.

1. Characteristics

Crystal system: Tetragonal bipiramidal.          

Color: Colorless, white, greenish, less commonly bluish, brownish, violet or yellowish.     

Habit: Prismatic, acicular, granular, massive, poikilitic.      

Cleavage: {100} distinct, {110} distinct.      

Tenacity: Very brittle.        

Twinning: No.       

Fracture: Irregular, conchoidal.       

Mohs Hardness: 5.5 – 6

Parting: On {001}         

Streak:  White.        

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.         

Density (g/cm³):  2.5 – 2.62 

 

2. Geology and Deposits

1) Very often scapolite occurs in limestone and contact metamorphic rocks (skarns), formed by the interaction between magmas and impure limestone rocks.

2) Also in mafic igneous rocks (gabbros, diabases, diorites) that have undergone metasomatism or secondary processes, forming scapolite from plagioclases (labradorite to anorthite).

3) It can occur in rocks of the scapolite-hornblende type formed from gabbros that have undergone pneumatolitic processes.

4) It occurs in metamorphic rocks such as gneisses, granulites, amphibolites and schists, which can be derived from igneous rocks or sedimentary rocks.

5) Occasionally occurs in pyroclastic rocks (tephra), in blocks ejected by volcanoes, as a contact pneumatolytic mineral.

6) It rarely occurs in granites, pegmatites and nepheline-syenites.

 

3. Mineral Associations

1) In skarns and related contact metamorphism rocks (rodingites) it is associated with calcite, diopside, epidote, vesuvianite, phlogopite, garnet, tremolite, wollastonite, graphite, scheelite and pyrite.

2) In mafic igneous rocks it is associated with plagioclases, clinopyroxenes, magnetite and ilmenite.

3) In rocks of the scapolite-hornblende type, it is associated with green and brown (or zoned) hornblendes and fluorapatites, leaving no relics of the original plagioclases and clinopyroxenes.

4) In metamorphic rocks derived from igneous rocks, it is associated with pyroxenes, feldspars, titanite, Fe oxides, quartz, rutile, green hornblende, biotite and sometimes garnet. In rocks derived from sedimentary rocks, it is associated with calcite, wollastonite, diopside, biotite, quartz and alkali feldspar.

5) In pyroclastic rocks occurs with anorthite, nepheline, garnet (melanite) and wollastonite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω:  1,539 – 1,550    nε: 1,532 – 1,541

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  Colorless, very rarely pale yellow or violet, with weak pleochroism. Inclusions often turn the crystal cloudy.   

Relief:  Low to moderate.          

Cleavage: {100} good and {110} distinct. In basal sections the cleavages intersect at 90º. Columnar crystals are partitioned at {001}.  

Habits: Prismatic according to z, more rarely acicular, columnar, may form granular aggregates, may be poikilitic.

Typically contains poikiloblastic inclusions such as quartz, calcite and feldspar; fluid inclusions may occur.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Marialite 0.004 – 0.005; meionite 0.034 – 0.038: 1st and 2nd order interference colors up to light violet at most.

The sodic terms (marialite) have colors between dark gray and white (first order), while the calcic terms (meionite) have stronger colors, up to the 2nd order.           

Extinction: Parallel, in granular aggregates it is not possible to determine.          

Elongation sign: ES(-).            

Twins:  No.        

Zoning: May be zoned.             

CONVERGENT LIGHT

Character:  U(-)         

2V angle: May have anomalous 2V angle from 0 to 10º.         

Alterations: scapolite can undergo several types of alteration.

1) Hydrothermal alteration generates different zeolites (chabazite, stilbite, analcime, etc).

2) Can also change to calcite, prehnite and epidote.

3) Another possible change is to pinnite, a mixture of micas (usually muscovite) and clays.

4) Weathering leads to alteration to kaolinite.          

May be confused with: several other minerals with low relief and low interference colors.

Feldspars have twins, oblique extinction and are biaxial.

Apatite can be very similar, but show lower interference colors.

Quartz has no cleavage, does not change, has lower birefringence, and is U(+).

Feldspathoids have lower relief and other habits.

Nepheline and cancrinite occur in other paragenesis and have different habits.

Cordierite, andalusite and wollastonite are biaxial.        

 

Scapolite 01: In PPL, colorless scapolite crystals. These basal sections show the two cleavage directions at 90° to each other (one cleavage is better than the other cleavage).
Scapolite 01: The same section as of the previous image, now in XPL. These basal sections present very dark gray colors, providing perfect uniaxial negative interference figures. The colored mineral is tremolite.
Scapolite 02: Scapolite, calcite and tremolite in PPL. All are colorless (but tremolite has higher relief). The 90º cleavages are diagnostic for scapolite.
Scapolite 02: The same section as of the previous image, now in XPL. Basal sections of scapolite are gray, calcite is cream and tremolite is colorful.
In XPL, in skarn, scapolite (gray), and tremolite (coloured), a very typical association.
In XPL, in skarn, scapolite (gray), tremolite (colored), and calcite (cream)..
In XPL, scapolite (gray), with idiomorphic tendencies, tending to form squares.
The two cleavages of the scapolite, which intersect at 90º, are generally of only passable quality.
In XPL, scapolite (gray), tending to form square basal sections. Around you, tremolite (colorful).
In XPL, scapolite (gray), tending to form square basal sections. Around you, tremolite (colorful).
A single crystal of scapolite, surrounded by calcite. Stronger interference colors distinguishes it from apatite. XPL.

5. Reflected Light Microscopy

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

Sample preparation: due to the cleavage of the scapolite, its polishing is not good and presents many imperfections (holes). Even with a lot of effort, the polishing will always be of low quality, even more if there are amphiboles (tremolite) and calcite associated, both also with well-developed cleavages.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Dark gray, slightly darker than quartz and feldspar.

Pleochroism: No.      

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections:  Generalized colorless, clear and white, intense.     

May be confused with: many other transparent and light colored minerals. It is not possible to identify scapolite under Reflected Light.       

Scapolite 01: In the center of the image, in PPL, scapolite grain with an approximately square shape and well-developed cleavage. Around it, other scapolites, tremolite, calcite and holes (black).
Scapolite 01: The same image as of the previous image, now in XPL. It is not possible to identify the scapolite because the internal reflections of the other minerals that make up the paragenesis are of the same appearance.