DATOLITE

Datolite – CaB(SiO4)(OH) – is a rarer nesosilicate, of no economic importance other than a collector mineral, where it reaches high prices.

It is classified in the Gadolinite Supergroup (Y2Fe2+Be2Si2O10). May contain Mn, Mg, Fe and Al contents. As varieties, “bakerite” (rich in B) and “botryolite” (with botryoidal habit) occur.

Macroscopically, datolite is very similar to several other minerals that occur in the same paragenesis. If the datolite has a massive or coarse granular habit and occurs interspersed with quartz, calcite, prehnite, pectolite and apophyllite, it is practically impossible to recognize it. All these minerals are transparent, have a vitreous luster and generally have light colors with various possible shades.

Datolite very commonly shows a greenish color, which apophyllite and prehnite can also show. The literature reports that datolite fluoresces blue under shortwave ultraviolet waves, but this fluorescence is only occasional and cannot be used as a common diagnostic feature.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Usually with a greenish tinge. White, grayish, pale green, red, yellow, pink, etc.     

Habit: Prismatic short, tabular, botryoidal, compact, granular, lenticular, cryptocrystalline.

Cleavage: No.      

Tenacity:  Brittle.       

Twinning: No.      

Fracture: Irregular, conchoidal.       

Mohs Hardness: 5 – 5.5

Parting: No.          

Streak: White.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.         

Density (g/cm³): 2.96 – 3

 

2. Geology and Deposits

Datolite occurs as a secondary mineral in vesicles (geodes) of mafic igneous rocks.

It can also occur in skarns formed from limestone rocks.

In metamorphic rocks, occurs in serpentinites and in “hornblende” (green amphiboles) schists.

Datolite can associate with ore minerals in metalliferous mineral veins.

 

3. Mineral Associations

Datolite occurs associated with the typical assembly of secondary minerals in vesicles (geodes) of mafic volcanic rocks: quartz, calcite, prenhite, apophyllite, pectolite and zeolites (natrolite, heulandite, thomsonite, laumontite, stilbite and analcime).

It also occurs with sulfides (pyrite, chalcopyrite), hausmannite, native copper, danburite, garnets, tourmaline-Mg (dravite) and axinite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.626   nβ: 1.653 – 1.654    nγ: 1.670

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, without pleochroism.      

Relief: Moderate.          

Cleavage:  No.          

Habits: Short prismatic, massive, granular.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Maximum birefringence of 0.044: colors up to half of the 3rd order, strong colors, colorful, intense. 

Extinction: The generally granular habit does not allow to classify the extinction, which must be oblique, as it crystallizes in the Monoclinic System. 

Elongation sign: Does not apply.            

Twins: No.        

Zoning: No.            

CONVERGENT LIGHT

Character:  B(-)         

2V angle: 74º           

Alterations: it is a product of alteration.          

May be confused with: in the paragenesis in which it occurs, it is the only common mineral with a granular habit and strong interference colors.

Prehnite, depending on the habit, can be similar, but prehnite usually has a fan habit and some anomalous interference colors, in addition to a lower relief.         

Datolite 01: In the image, datolite and calcite in PPL. Both are colorless and of moderate relief, but calcite show cleavage, twins and very different interference colors.
Datolite 01: The same section as of the previous image, now in XPL. Calcite (top right) shows its typical brown to cream interference color while datolite grains show strong and varied colors. The habit of datolites is typically granular. The black grain in CPL (below) is in an extinction position, but also shows strong colors, if rotated.
In XPL, aggregate of datolite grains with some calcite grains. In the paragenesis in which datolite occurs, it is easy to identify, as the characteristics of the other minerals are very different.
In XPL, in the upper part of the image, fans of prehnite and, in the lower part, datolite grains. It is a very typical and easy to recognize association.
In XPL, granular datolite and radial aggregates of prehnite. In PPL, both minerals are colorless and the prehnite has a lower relief and its typical habit, forming beams, fans and aggregates that can be radial, of strong colors and usually with some anomalous interference colors.
In XPL, granular datolite and radial aggregates of prehnite. In PPL, both minerals are colorless and the prehnite has a lower relief and its typical habit, forming beams, fans and aggregates that can be radial, of strong colors and usually with some anomalous interference colors.

5. Reflected Light Microscopy

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

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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