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The Mines and Minerals of Lavrion - Allophane

Last Updated: 27th Jan 2020

By Branko Rieck

ALLOPHANE, (Al2O3)(SiO2)1.3-2 · 2.5-3H2O, amorphous



General Appearance
The mineral allophane is easily confused with a number of minerals like chrysocolla, gibbsite, nordstrandite, and glaucocerinite. All confirmed specimen of allophane that I am aware of are glassy, transparent and show a splintery fracture if broken. Most minerals with which allophane is often confused, exhibit a spherical, botryoidal structure while allophane usually forms homogenous masses that often fill entire cavities. In open spaces the surface of allophane also appears botryoidal, but its structure is never radial. Depending on the amount of copper (and sometimes further) impurities its color ranges from colorless to sky blue.

Paragenesis
Allophane appears in paragenesis with almost all secondary minerals found in Lavrion. It is usually one of the latest – if not the latest – mineral to form in the paragenesis.

Localities
Some of the nicest specimens of allophane come from the 3rd level of the Hilarion area where obviously very late-stage silica-rich solutions impregnated already formed secondary paragenetic sequences and led to the formation of pseudomorphs after minerals like adamite, azurite, malachite, spangolite, and mixite-group minerals. The easily soluble copper-bearing precursor minerals provided enough copper ions to give the allophane a pleasing sky-blue color (see Fig. 1). Obviously, the same history is true for the occurrence of allophane at the Serpieri Nr. 5 Mine. There was however too little copper to noticeably color the allophane. At this locality – also known for the appearance of Al-bearing adamite - zincaluminite has grown as an even later formation on top of allophane.
Allophane appears in literally hundreds of places in the Lavrion Mining District, so new discoveries can be expected any time. One note of caution however: one locality known locally as "allophane-place" has not yielded a single piece of this mineral. All analyzed specimens turned out to be a mixture of gibbsite and nordstrandite.


Fig. 1: This allophane contains traces of copper – hence the sky-blue color. Notice however, as can be seen at the very right edge of the photographed view, that not all the intensity of the color comes from impurities but also from azurite that has grown beneath the allophane and shines through.


Fig. 2: Colorless to slightly blue, glassy allophane with splintery fracture covering the inside of small cavities in gossan matrix from the Serpieri #5 Mine. Late-stage pearly white, hexagonal platelets of zincaluminite are grown on top of the allophane. Also visible are indistinct green adamite grains and powdery white hemimorphite.


Fig. 3: Colorless to slightly blue, glassy allophane with splintery fracture covering the inside of a small cavity in gossan matrix from the Serpieri #5 Mine. At the bottom left, allophane has formed a pseudomorph after an unknown, needle-like precursor mineral. Late-stage pearly white, hexagonal platelets of zincaluminite are grown on top of the allophane. Also visible are indistinct green adamite grains and powdery white hemimorphite.




Acknowledgements
Thanks go to Dr. Uwe Kolitsch for constructive comments and diligent proofreading to improve this article.




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