Cover Photo: Mineral Collection 2015.
I dedicate this post to my dear friend and colleague Klaus,
who recently won a great battle
and to Luiz Alberto Dias Menezes Filho In Memoriam.
By Maurício Pinheiro
As a child, I would often play in my father’s office, and I was always mesmerized by his impressive collection of minerals, which included a variety of gemstones, natural metals, Brazilian Neolithic tools, and fossils. The prized possessions of the collection were tourmalines, aquamarines, and beryls that hailed from the pegmatite-rich region of North-East Minas Gerais. My grandfather was born in the same region (Itambacuri, MG), known for its abundant tourmaline mines. He was a physician who had a keen interest in tourmalines and started his collection by receiving them as gifts from grateful patients. Later, my father, who went on to become a nuclear engineer, continued to build upon the family legacy by adding more exotic minerals to the collection. While I pursued a Ph.D. in Physics, I returned to my roots in minerals and gemstones by applying various optical techniques to study them, with the help of a fellow professor Klaus, at UFMG. I also continued the family tradition of contributing to the collection by adding cut gemstones.
For those who are more interested in what I do, I asked ChatGPT has to explain…
The colors of gemstones can be caused by a variety of factors, including the presence of certain trace elements, structural defects, and the crystal lattice of the mineral. For example, chromium gives the green color to emeralds, while iron is responsible for the yellow and brown tones in citrine and topaz. Additionally, some gemstones, such as opal, exhibit iridescence due to the presence of microstructures within the mineral. To study the causes of color in gemstones, various physical techniques are employed. One of the most common techniques is UV-Vis spectroscopy, which involves shining a beam of light through a sample and measuring the amount of light absorbed at different wavelengths. This can provide information about the electronic transitions that give rise to a particular color. Another technique electron paramagnetic resonance (EPR) spectroscopy is a powerful technique for studying the effects of impurities and structural defects on the electronic properties of gemstones. By detecting the presence of unpaired electrons in the sample, EPR can provide valuable insights into the color-causing mechanisms in a wide range of gemstones. Overall, the study of gemstone coloration is a complex and fascinating field that requires a multidisciplinary approach, incorporating expertise in materials science, chemistry, physics, and mineralogy. The development of new analytical techniques and the refinement of existing methods are helping to shed light on the mysteries of these beautiful and precious minerals.
I am pleased to report that our efforts have been successful, and after nearly 20 years in this field, Klaus and I have many stories to share. We have also made many friends, primarily geologists, gemologists, and gemstone dealers from all over the world. Through this collaboration, we obtained some rare gems to study, whose name ironically is Brazilianite, from the Conselheiro Pena pegmatite district in northeastern Minas Gerais. We wrote a nice paper on that (see reference at the end).
Then I asked ChatGPT to do create a collection entry for the samples that we studied and here is what it delivered (correctly):
| Gemstone | Number | Cut | treatment | Color | Occurrence |
|---|---|---|---|---|---|
| Brazilianite | 371 | Emerald cut | 200kGy gamma-irradiation | Yellow | Conselheiro Pena pegmatite district, northeastern Minas Gerais |
Tecnical data
| Mineral | Brazilianite |
|---|---|
| Chemical formula | NaAl3(PO4)2(OH)4 |
| Color | Greenish yellow, yellow, brownish yellow |
| Crystal system | Monoclinic |
| Crystal habit | Prismatic crystals with a wedge-shaped termination |
| Twinning | Common on {100} |
| Cleavage | Perfect on {100} and {010} |
| Fracture | Uneven to conchoidal |
| Mohs hardness | 5.5 – 6 |
| Specific gravity | 2.98 – 3.08 |
| Refractive index | 1.598 – 1.616 |
| Birefringence | 0.018 |
| Pleochroism | Strong, yellow-green to yellow-brown |
| Luster | Vitreous |
| Streak | White |
| Density | 2.98 – 3.08 g/cm³ |
| Transparency | Transparent to translucent |
| Occurrences | Brazil, USA, Russia, Canada, Namibia |
| Crystal lattice | |
| Lattice parameters | a = 6.919 Å, b = 11.800 Å, c = 6.631 Å, β = 99.53° |
| Space group | P21/n |
| Unit cell volume | 534.65 ų |
Recently, during a walk in the countryside at our farm with my father, my son, my brother, and his sons, we stumbled upon a plethora of minerals, including quartz, mica, feldspar, and tourmalines, as expected for pegmatites.
As the sun began to set, the country road shone with a magical glow, reflecting off the mica crystals. I’m glad I was born in Minas Gerais!
References:
Pinheiro, M. V. B., Scholz, R., Karfunkel, J., Chaves, M. L. S. C., & Krambrock, K. (2021). On the yellow color of gamma-irradiated brazilianite from Minas Gerais (Brazil). Physics and Chemistry of Minerals, 48(9), 33.
More about Gems and Minerals of Minas Gerais, Brazil:
http://recursomineralmg.codemge.com.br/wp-content/uploads/2018/10/GemasMineraisColecao.pdf
