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Fig.1: Agate and quartz geodes (top row); fuchsite, rhodonite and pyrite (bottom row).
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Here is the second post in my new mineralogy series, talking about my mineral collection ✨💎. I
have been a great fan of collecting minerals and gemstones since I was little, and
through the years I've collected quite a few, both from various shops and from
field trips. What really gave me the final nudge to rediscover mineralogy as an active hobby was finding out that the local kiosk was selling a mineral collection, the National Geographic RBA minerals collection (in Spanish), and it included a lot of minerals and gems I didn't have. So in April 2022 I started collecting most of the weekly numbers, and here I am, full on back to mineralogy as a hobby and expanding my existing collection 😃.
In Post 1 of this series I talked about various types of quartz (rose quartz, Tiger's Eye, amethyst and blue agate), as well as gold, fluorite, and celestine. Also check out this same first post for a lengthy rant about what I think about the pseudoscientific branches having to do with rocks, minerals and gems, such as 'crystal healing' (spoiler alert, I'm not a fan).
In this second post we're gonna keep following the order of the RBA collection, and talk about fuchsite, rhodonite, quartz geodes, pyrite and galena. As explained in the first post, in this series I will show the specimens from the RBA collection alongside the ones in my existing collection before collecting the kiosc numbers, and also any pieces from new hauls.
8) Fuchsite:
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Fig. 2 - The fuchsite specimen from the RBA minerals collection. The flash showcases its pearly luster.
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Fig. 3 - The fuchsite specimen from the RBA minerals collection. In this picture we can clearly see the ochre patina formed by alteration minerals.
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The first mineral I got from this collection, this fuchsite specimen originates from Brazil, and I think that it's quite beautiful, with a pale apple green hue (it looks almost sea-green in some lightings), the characteristic pearly sheen, and an ochre patina that is formed by alteration minerals (as well as some silvery muscovite mica). With its low hardness and easy exfoliation, this fuchsite is among the more delicate specimens in this collection.
💎A bit about fuchsite: Source 1, Source 2, Source 3, Source 4, Source 5
Fuchsite, also known as chrome mica*, is a variety of muscovite mica rich in chromium (Cr). In comparison to other muscovites, a variable amount of Cr substitutes for aluminium (Al) in the mineral, and this gives fuchsite its characteristic green hue, ranging from pale green to apple green and a rich emerald green, depending on the amount of Cr substitution.
Fuchsites are most often found as small micaceous aggregates in metamorphic rocks, although rocks composed near entirely of fuchsite can also be found occasionally. These fuchsite-rich rocks are called 'verdite', in relation to their green colour.
*Micas (of which muscovite is the most common member) are a group of silicate minerals common in igneous and metamorphic rocks. Their main characteristic is their perfect cleavage, with individual mica crystals being very easily split into incredibly thin plates along their structural planes. The wide use of micas spreads from numerous uses in electronics to paints, and their common inclusion in cosmetics to create shimmer (the luster and sparkle of shimmery eyeshadows is mainly achieved with micas, for example).
✨Some interesting STEM trivia about fuchsite:
- Under long-wave UV light, fuchsite fluoresces lime green.
Uses: Due to its high fusion point, melting at temperatures above 1300 °C, as well as the dielectric nature of the minerals in the mica group, fuchsite is utilized in the industry area for thermal and electrical insulation purposes. Mica powder - often including fuchsite and other muscovite micas - is often used to make heat-resistant glass (isinglass) for stoves and ovens. Fuchsite is an excellent dielectric material, maintaining a low conductivity in temperatures above 600 °C, and is thus also used in a great variety of electrical appliances. Although it is not often used as a chromium ore due to its relatively low quantity, fuchsite varieties which are especially rich in Cr can also play a part in reinforcing steel.
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Fig. 5 - Fuchsite infographic from the RBA collection.
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9) Rhodonite:
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| Fig. 6.1 - The tumbled rhodonite specimen from the RBA collection (left), alongside a rough rhodonite (right). |
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Fig. 6.2 - Tumbled (left) and rough (right) rhodonites. The tumbled one shows black and brown patches as a result of manganese oxides, while the raw specimen has a green patina that may be malachite or a copper-based mineral.
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Fig. 7.1 - The tumbled rhodonite specimen from the RBA collection.
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| Fig. 7.2 - The rhodonite specimen from the RBA collection, with its characteristic pink hue against black and brown patches corresponding to manganese oxides. |
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| Fig. 7.3 - A small rough rhodonite with its characteristic pink colour and a green patina on the top, possibly malachite or a copper-based mineral? |
The specimen from the RBA, originating from
South Africa, was the first rhodonite in my collection. It's a tumbled rhodonite (see
Figs. 6.1-2 and
7.1-2), with its characteristic
vitreous luster, and showcasing its typical
pink hues against the black and brownish tones caused by the
surface oxidation of manganese. In December 2023 I also got a small
rough rhodonite (in
Figs. 6.1-2 and
7.3-6), a piece displaying the characteristic
pink colour that lacks the brown and black patches of the tumbled specimen, and instead includes a curious
green patina at the top, which I tentatively guess as being a copper-based mineral, possibly
malachite (a curious combination that I personally hadn't seen before. It doesn't look like
epidote or
tephroite, the only green minerals I had seen
associated with rhodonite so far).
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| Fig. 9 - Rhodonite infographic from the RBA collection. |
Here's also a couple of short videos featuring both the RBA tumbled rhodonite (alongside the fuchsite, see above), and the rough rhodonite, in direct sunlight ✨:
10) Quartz and agate geodes:
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| Fig. 10.1 - A Saharan quartz geode specimen from the RBA collection. |
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| Fig. 10.2 - Quartz Saharan geode specimen with no agate banding, featuring some well-formed crystals inside. |
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| Fig. 10.3 - Quartz Saharan geode specimen, alongside other milky quartz pieces in my collection. |
All of my geodes are clear quartz (rock crystal), milky quartz and/or agate geodes. Firstly, we have the specimen from the RBA collection, a quartz geode from the Sahara, this one specifically from Morocco (Figs. 10.1-3 and 11, plus a couple of videos below). It's a geode composed mainly of what looks like milky quartz crystals, without the presence of any agate banding around it. The collection actually describes this specimen as being composed of clear quartz crystals, but personally I think this is a milky quartz geode because the crystals are milky-white rather than crystal-clear, and more transluscent than transparent. Agate banding and the presence of agate in geodes is something very typical in many geodes, as we'll be seeing below with the rest of the specimens in my collection (Figs. 12-15, and the two short videos below in this section).
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| Fig. 10.2 - Saharan geode infographic from the RBA collection. |
💎A bit about milky quartz and white quartzite: Source 1, Source 2, Source 3, Source 4, Source 5. I already introduced quartz in the first post in this series, but here I also wanted to share the rest of my milky quartz and quartzite specimens, alongside this new quartz geode (see Figs. 11.1 as well as Figs. 11.2-9 below).
✨Milky quartz (also known as milk quartz) is the most common variety of crystalline quartz, with its characteristic white colour caused by liquid and/or gas minuscule inclusions trapped during the crystal formation process. It has considerably less value than clear quartz (rock crystal) and other quartz varieties when it comes to gemstone quality and optical applications, but I've always found it very beautiful all the same.
✨A couple of these pieces are white quartzites rather than quartz: Quartzite is a metamorphic rock which consists of at least 80% quartz and is formed by recrystallization of sandstones, sedimentary rocks that are in themselves composed of quartz grains, by means of regional or thermal metamorphism (metaquartzites). This process typically occurs within orogenic belts during tectonic compression, with the intense heat and pressure causing the quartz grains within the sandstone to recrystallize, resulting in strong and low-porosity rocks with a dense interlocking mosaic of quartz crystals. As a result of this structure, quartzite are medium-to-fine grained rocks which typically showcase a characteristic 'sugary' granular texture. The term 'quartzite' is also used sometimes to describe unmetamorphosed sandstones composed of quartz grains and cemented with additional quartz (orthoquartzites).
Pure quartzites are white to grey, while the presence of hematite and iron oxides result in quartzites in various shades of red, orange and pink. Trace amounts of other minerals can result in other colours such as green, yellow and blue. Because of its hardness and variety of colours, this rock is often used with decorative purposes in home construction, from kitchen countertops to flooring and roofing tiles. Historically, quartzite was used in Paleolithic times for making stone tools, alongside flint, quartz and other materials.
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Fig. 11.1 - All the milky quartz and quartzite specimens in my collection so far, featuring the Saharan geode from the RBA collection and some massive pieces of differing sizes, hues and shapes, also including some pieces with orange-yellow and rust tones caused by iron oxide coatings.
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I found all of these milky quartz and quartzite specimens in local parks and mountains regions over the years, and they come in various forms, shapes and sizes, all of them with a massive habit, from very small naturally tumbled stones to larger rough pieces, from more matte specimens to more translucent ones, and featuring some really glittery ones, like the white quartzite in Fig. 11.2, which I found quite recently (Autumn 2023) during a walk near home, and features tiny crystals that really sparkle when moving it from side to side (also see the videos below). In addition, some of these pieces feature yellow and rusty tones that are a result of iron oxide coatings (like hematite and limonite) (see esp. Figs. 11.8-10). I quite like these ones! Ferruginous quartz due to inclusions (this would be iron-stained milky quartz rather than actual ferruginous/hematoid quartz, though) also features in the rarer 'ametrine' specimens, a combo not of amethyst and citrine, but of amethyst and ferruginous quartz (as discussed in this post).
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Fig. 11.3 - Milky quartz pieces in my collection.
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✨Infographic video: On TikTok here (and a slideshow post here).
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Fig. 12 - The rest of geodes in my collection, spanning several sizes from small geodes with clear quartz and agate banding (geodines), to large agate geodes which have been sawn and polished.
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Geodes are independent spherical-to-subspherical hollow rock structures, ranging in size from under 1 cm to several meters in length, with an internal cavity lined with mineral matter, which commonly include crystals. Geodes are created as secondary geological formations within volcanic (igneous) and sedimentary rocks. The crystals fill the cavity mainly through concentric inner growth, a growth that is caused either by the filling of gas bubbles in igneous rocks (such as vesicles in basaltic lava) by minerals which have been deposited from hydrothermal fluids; or in in sedimentary formations, created by the filling and slow growth of minerals precipitated from groundwater or hydrothermal solutions. The durable outer wall of geodes makes them more resistant to weathering than the surrounding bedrock, and allows for geodes to survive intact and be ultimately dug from the soil, or collected in stream beds or from the land surface.
The most common geode type consists of a druse of small crystals lining and filling the cavity, often also including multiple concentric bands of chalcedony (agate or jasper) in different tones (typically brown, tan, grey, white or black). Quartz crystals (especially clear and milky quartz, and amethyst) are dominant in geodes, and white calcite or dolomite are also quite common, while other minerals such as blue silica, pink rhodochrosite, pyrite and colourful opal can be found in rarer specimens. Sometimes the geodes aren't filled with geometric crystals, as is the case with chalcedony-lined geodes (chalcedony being a microcrystalline variety of quartz).
The banding and coloration in geodes is the result of variable impurities. For example, agate banding in its natural form tends to create white and grey tones (as we can see in the geodines of Fig. 13), while brownish and rusty hues are caused by the presence of iron oxides (very present in the large agate geodes in Figs. 14 and 15).
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| Fig. 13 - Two small geodes with clear quartz and agate banding. These small quartz+agate geodines hailing from Brazil are commonly called occo/ocho geodes, displaying sparkly drusy quartz linings on tan, grey, brown or black agate shells. |
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Fig. 14 - A large agate geode, sawn and polished, lined with several layers of colourful agate banding, showing a crystal-filled cavity in the middle, and a central part featuring more agate layers of different hues. Each coloured agate band represents an episode of agate formation.
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| Fig. 15 - Another large agate geode, sawn and polished, bigger than the former one but also thinner. It's also comprised of numerous layers of colourful agate, and we can also see some zones with crystals |
✨Some interesting STEM trivia about geodes:
- The etymology of the name 'geode' comes from the Ancient Greek γεώδης (geṓdēs), "earthlike".
- Geodes are given a variety of names. The word "geode" may be preceded by the name of the mineral filling it (such as "amethyst geode" or "agate geode"), or by a geographic name referring to its place of origin (such as "Sahara geode", in the case of the RBA collection).
- Geodes are massively used as collectable mineral specimens, and for
various ornamentation purposes, from pendants to bookends, paperweights,
and various items of home and office decor. Seeing as agate in its natural form often has white and grey tones (or red and brown if affected by iron oxides), and many people find these hues less appealing for decorative purposes, geodes and geode slices are sometimes dyed with artificial colours, creating vivid and unusual hues of blue, green, purple, red and other bright shades that aren't generally to be found in their natural form (see for example a bright blue agate slice in Post 1 of this series). Personally, I prefer the more natural shades of agate xD.
Finally, here are a couple of short videos showcasing the sparkle and shape of the quartz crystals in one of my geodines (occo/ocho geode), and the textures of one of the large agate geodes:
11) Pyrite:
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Fig. 16.1 - All my pyrite specimens, in various forms: Several cubic pyrite crystals in various sizes, a specimen with pyritohedron-shaped crystals (right, in box), and a disk-shaped pyrite sun.
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| Fig. 16.2 - A cubic
pyrite crystal featuring some intergrowth with another smaller cubic crystal on one of its faces (above, left), and two specimen with pyritohedron-shaped
crystals - the larger one in the right is the specimen from the RBA collection. |
All of the pyrites in Fig. 16.1 are from the rest of my former existing collection, while Fig. 16.2 includes the new specimen from the RBA collection. Most of my pyrite specimens are cuboid crystals (the main crystal habit of pyrite), three small ones, and a very large one. At least two of them originate from La Rioja, one of the main mining sources of pyrite in Spain (mineralogists actually consider La Rioja and Riotinto - in Huelva, Spain - among the sites which provide the best pyrite specimens worldwide). There's also a couple of speciments of the pyritohedron-shaped crystal variety (see below), and a large (and pretty stunning) pyrite sun, a disk-shaped concretion of pyrite arranged in an ortorhombic crystal configuration (a rectangular/stretched cube shape). In pyrite suns, crystals grow radially (laterally) instead of vertically as a result of the weight of overlying sediments, causing flat-shaped pyrites.
💎A bit about pyrite: Source 1, Source 2, Source 3, Source 4, Source 5, Source 6
Pyrite, also known as iron pyrite and fool's gold, is the most abundant sulfide mineral, an iron sulfide variety (chemical formula FeS2). It is a very common mineral that is found ubiquitously around the world, associated with other sulfides or oxides in igneous, sedimentary and metamorphic rocks, as well as in quartz veins, coal beds, and in fossils as a replacement mineral.
Pyrite has a
simple cubic structure, usually forming cuboid crystals. Under certain circumstances, pyrite can also be found in a
pyritohedron shape, forming
irregular dodecahedrons known as
pyritohedra, and also as
octahedrons. Pyrite often occurs
intergrown, and can also be found in
massive, granular, globular, radiated and stalactitic habits.
The name 'fool's gold' originates from the fact that the brass-yellow hue and metallic luster of this mineral give pyrite a superficial resemblance to gold. In spite of this, both minerals can be differentiated relatively easily: Gold is
soft, malleable and dense, while pyrite is hard, brittle and considerably less dense,
and its characteristic cubic crystal structure is also a solid factor
that can distinguish it from the precious metal. Pyrite can however be used as a gold ore, as the two minerals often occur together in the same rocks and form under the same conditions. Even though gold often comprises a very small fraction of the ore, pyrite is still considered as a mining target for gold due to its high value.
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Fig. 17.2 - Pyrite infographic from the RBA collection (in Spanish)
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✨Some interesting STEM and history trivia about pyrite:
- The etymology of the name 'pyrite' is derived from the Ancient Greek πυρίτης λίθος (pyritēs lithos), "stone or mineral which strikes fire", which is in turn derived from πῦρ (pyr), "fire". Ancient Romans used this name to refer to several stones (one of them the mineral pyrite as we know it) which could be used to create sparks when struck against steel or another hard material. Pyrite has indeed been used with flintstone and tinder as a traditional method of starting fires in various parts of the world, and it was also used in the 16th and 17th centuries as an early source of ignition for firearms (sigh).
Pyrite has a wide variety of
uses, among which we can cite the following:
- Ores: As well as an ore of gold, its main use as an ore, pyrite has historically been used as an important ore to produce sulfur. Nowadays, however, pyrite is only a minor ore for sulfur, although it remains in commercial use to produce sulfur dioxide, and sulfuric acid is also produced as a by-product of gold production. Despite containing a sizeable amount of iron in its composition, pyrite is also only considered as a minor ore of iron.
- Electronics and technology: In early 20th century, pyrite was used as a crystal detector in radio receivers.
Crystal detectors consisted of a piece of crystalline material with the
role of rectifying the alternating current radio signal. Galena (see next mineral below!) was the main mineral used in these detectors. Pyrite is a semiconductor material, and one of its newer uses is as the cathode material in non-rechargeable lithium batteries. Pyrite being abundant, non-toxic and inexpensive, this mineral has also been proposed as a fitting material to manufacture photovoltaic solar panels with iron sulfide as a photovoltaic material, as well as thin-film solar cells constructed entirely out of pyrite.
- Ornamentation: Pyrite has been used around the world for centuries with both ornamental and ceremonial purposes: From the Ancient Romans to several Native American peoples, it has been polished as mirrors, and worn in the form of amulets, pins, earrings, lockets and rings. Today it is still used widely as an ornamental stone, as well as a very popular stone among mineralogist collectors. Although it tarnishes easily and is quite brittle, pyrite can be used as a low-cost gemstone, faceted and polished to make jewelry. Pyrite jewelry was especially popular in the Victorian era.
Hazards: In spite of all its uses, pyrite also presents some environmental and construction hazards. The occurrence of sulfur from pyrite in coal beds, for one, can result in sulfur dioxide gas when the coal is burned, contributing to air pollution and acid rain unless removed (coal itself and all fossil fuels are also pretty hazardous to the environment in and for themselves, as we well know). Crushed stone used to make concrete and asphalt paving materials for roads and buildings must also be free of pyrite, as it oxidizes when exposed to air and moisture, resulting in potential maintenance problems as a result of the damage and weakening of these construction materials.
12) Galena:
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Fig. 18 - The galena specimen from the RBA collection, with its characteristic silver colour and metallic luster. It's noticeably quite heavy, due to its high lead content.
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This is my first galena in my collection. The specimen originates from Spain, where galena has been mined extensively since Roman times in Jaén (Andalucía) and Tarragona (Cataluña). The perfect cleavage of galena makes it easy to identify, exhibiting clear planes of exfoliation across which pieces of the mineral can break. Another distinct factor is its weight - This small specimen is easily one of the heaviest specimens in my collection, as a direct result of its high lead content.
A short video with this specimen in direct sunlight:
💎A bit about galena: Source 1, Source 2, Source 3, Source 4, Source 5
Galena, also known as lead glance, is the mineral form of lead sulfide (with a chemical composition of PbS), one of the most abundant and widespread sulfide minerals. This mineral can be found in igneous and metamorphic rocks in hydrothermal veins (in association with several other minerals such as quartz, calcite, dolomite, chalcopyrite and fluorite), and disseminated in sedimentary rocks in the form of veins and isolated grains, and as a replacement of limestone and dolomite.
With its characteristic silver colour, bright metallic luster and perfect cleavage in three directions intersecting at 90 degrees, galena is an easy mineral to identify. It also has a distinct high specific gravity due to its high lead content, and this is immediately noticeable even with small specimens. This mineral crystallizes in the cubic crystal system, often showing cubic and octahedral forms. It can also appear as blocky and tabular crystals.
Galena is the primary ore of lead since Ancient times, where lead was already being smelted from galena in ordinary wood fires. Lead was probably the first metal to be processed from an ore, with manufactured lead artifacts having been found as far in the past as 6500 BC in Turkey. Some galena deposits also contain a small amount of silver, either as a substitution of lead for silver in the atomic structure, or occurring in small grains as a by-product, and this "argentiferous galena" has also been an important ore of silver since Ancient times - Ancient Greeks and Romans already knew how to separate silver from lead 2000 years ago.
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| Fig. 20 - Galena infographic from the RBA collection |
✨Some interesting STEM and history trivia about galena:
- The etymology of the name 'galena' comes from the Latin galēna, “dross from smelting lead”. This name was already being used as far back as 77 AD by the Roman naturalist Plinius the Elder.
- 🌟🪐Astrophysics fact! The planet Venus has a highly inhospitable environment with volcanoes venting superheated gases - featuring sulfur and lead - into the atmosphere. High enough in the atmosphere, these gases condense, generating the so-called "heavy metal snow". This 'snow' seems to be a mineral deposition combining galena (lead sulfide) and bismuth sulfide, precipitated from the atmosphere on the higher elevations of the planet, at altitudes above 2600 m.
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| Maat Mons, on Venus, showcasing this 'metallic snow'. Image: NASA |
As well as an important ore of both lead and silver, these are some other uses of galena:- Cosmetics: As appalling as it sounds, galena with its high content of (toxic 😬) lead has been in widespread use as part of several types of cosmetics since Ancient times, and, as we know today, prolongued exposure to lead can lead to various health problems. In Ancient Egypt, galena was used to make the eye cosmetic kohl (which was applied around the eyes to reduce the glare of the harsh desert Sun, as well as repel flies and ward off eye infections). In pre-Columbian North America, it was also used by First Nations peoples in cosmetics and decorative paints. Lead in make-up has been extensively used in face-whitening 'foundations' in Western Europe since at least Roman times, with a special emphasis during Elizabethan 16th century England.
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Fig. 21- A galena crystal radio. Source.
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Electronics: Galena is a semiconductor material, and as such, it was widely used during the late 19th century and early 20th century in early wireless communication systems. Galena was the main crystal being used as a crystal detector in radio receivers (cf. the similar use of pyrite, above). In these detectors, the alternating current flowed into a sharp wire, called a "cat's whisker", that was in contact with the galena crystal. This crystal served as a point-contact diode that could limit the flow of electricity
to one direction, thus converting the alternating current into a pulsing
direct current to detect radio signals.
- Hazards: Lead, the primary constituent of galena, is a toxic element, and as such, nowadays many lead compounds have been significantly reduced in many areas due to health concerns, from cosmetics to pesticides, plumbing or motor vehicle fuels (among others). Lead is however still being routinely used in paints, electrical acumulators and cables, among some others.
-Finally, here are some infographics from the collection (in Spanish) about fuchsite, rhodonite, geodes, pyrite and galena (click on the pics or open in new tab for larger pics!):
That's it for today! On the next minerals post: Red jasper, Iceland spar, obsidian, azurite, and desert rose 😃💎
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