Vivianite from the Siglo Veinte Mine is a classic Bolivian phosphate with a very different personality from the sleeker, modern-looking blades of Huanuni or Tomokoni. The best Siglo Veinte pieces have a rugged old-mine character: deep teal to emerald-green prisms and blades, commonly stout rather than needle-thin, perched on iron-stained matrix and sometimes set off by pale childrenite or paravauxite. The color can be glassy and alive in transmitted light, yet many specimens show the subdued satin or matte surface that one expects from vivianite that has spent decades in collections.
Photo: Wikimedia Commons
The mine’s appeal is broader than vivianite alone. Llallagua is one of the great names in Andean mineral collecting, a world-class tin deposit whose specimen suite includes cassiterite, wavellite, fluorapatite, stannite, wurtzite, Japan-law twinned quartz, and the celebrated “vauxite family” phosphates. The phosphate assemblage is especially important: vauxite, paravauxite, metavauxite, ferrivauxite, and sigloite are all tied historically to this locality, and Siglo Veinte is the type locality for several of them. Vivianite belongs in that same late-stage phosphate story, forming in an environment where iron, phosphate-bearing fluids, altered wall rock, and open fractures created some of the most distinctive small-cabinet and miniature phosphate specimens Bolivia has produced.
For collectors, Siglo Veinte vivianite is a locality piece first and a show mineral second. The most desirable specimens combine saturated blue-green to emerald color, recognizable prismatic or bladed habit, intact terminations, and an association that says “Llallagua” at a glance: childrenite, paravauxite, vauxite-family phosphates, quartz, siderite, iron oxides, or old Contacto vein provenance. Single crystals can be elegant, but the strongest examples have context—vivianite rising out of a phosphate-rich matrix rather than floating as an anonymous green blade.
Search for specimens: View all vivianite specimens from Siglo Veinte Mine, Bolivia
Siglo Veinte Mine—also written Siglo XX, and historically associated with the Llallagua and Catavi names—is at Llallagua, Rafael Bustillo Province, Potosí Department, Bolivia. Mindat places the mine at 18° 25' 22" S, 66° 35' 32" W, only a short distance southwest of the Llallagua town center. Its principal commodities are tin, tungsten, and bismuth, but to mineral collectors its fame rests equally on the extraordinary late phosphate and sulfosalt assemblages recovered from a tin system of unusual richness.
The deposit is part of the Bolivian Tin Belt and is centered on the Salvadora stock, a shallow, strongly hydrothermally altered porphyry system intruded into Silurian–Devonian sedimentary and metasedimentary rocks. Published geologic work describes the system as a tin porphyry and hydrothermal vein deposit, with mineralization concentrated in and around a roughly 1.7 by 1.0 km stock. The altered intrusive rocks include rhyodacitic to dacitic porphyry, igneous breccias, and crosscutting dikes; alteration ranges from deep tourmalinization through quartz-sericite assemblages to chloritic and argillic zones outward. Cassiterite mineralization occurs both disseminated and in veins and breccia fillings, while the hydrous iron-aluminum phosphates, including the vauxite family and vivianite, belong to later, lower-temperature stages in open fractures and vein environments.
The mine’s name means “Twentieth Century,” a reference to the beginning of serious tin mining around 1901. It was later acquired by Simón Iturri Patiño, the Bolivian tin magnate often called the “King of Tin.” After the 1952 Bolivian National Revolution, Siglo Veinte and related mines were nationalized under COMIBOL, and the Catavi–Siglo XX complex became one of the central pillars of the state mining system. Large-scale state production ended in 1987 during Bolivia’s economic restructuring; subsequent work has been carried out by cooperative miners working independently or in small groups.
For mineral collecting, that history matters. Many of the finest vivianites and associated phosphates are old-time specimens from the high-production decades of the mine, and pieces with reliable older labels command special respect. The Contacto vein is particularly important in the collecting literature for phosphate specimens, especially paravauxite and sigloite, and dealer records also place some fine vivianite specimens specifically from that vein. Collecting access today is not comparable to a classic open collecting locality: this is an active and historically hazardous mining area, and any underground or dump work requires local authorization and practical mine-safety knowledge.
The mine’s notable finds are not limited to cabinet minerals. Llallagua has produced important scientific material for phosphate mineralogy and geochronology, including studied vivianite, hydrothermal monazite, fluorapatite, xenotime, vauxite-group minerals, and cassiterite-bearing vein assemblages. For the collector, this makes a Siglo Veinte vivianite more than a colorful iron phosphate—it is a hand specimen from one of the benchmark tin-phosphate systems in the Central Andes.
Siglo Veinte vivianite is best known as prismatic to bladed monoclinic crystals, commonly in small sprays, parallel groups, or isolated sword-like crystals on matrix. Published locality data record crystals to 10 cm, but most collectible examples encountered on the market are much smaller: thumbnails and miniatures with crystals from about 1 to 5 cm are more typical, while small-cabinet specimens with several crystals or one larger blade are scarcer. Dealer descriptions of known specimens include 3.3 cm toenails, 4.7 cm single “sword” crystals, 6.8 cm matrix pieces, and small-cabinet association specimens around 7 cm across.
Color is one of the locality’s signatures. Good crystals range from teal-blue and blue-green to deep emerald green. Some pieces are gemmy in the interior but less lustrous at the surface, giving a dark windowed appearance when backlit. Others are semi-lustrous or matte, especially older specimens that have undergone some natural or collection-related surface change. Because vivianite darkens with oxidation and light exposure, the richest color is not always the brightest display color: very dark crystals may once have been paler and more transparent.
The most characteristic associations are childrenite, paravauxite, vauxite, metavauxite, sigloite, wavellite or fluorwavellite, quartz, siderite, pyrite, and iron oxides. Childrenite and paravauxite associations are especially prized because they place the specimen firmly in the Llallagua phosphate suite. A lustrous teal vivianite blade on a sparkling pale childrenite crust is one of the most diagnostic and attractive Siglo Veinte combinations. Vivianite with light green paravauxite is rarer and more desirable, particularly where old labels or Contacto vein provenance can be established.
The quality hierarchy is fairly clear. At the top are sharp, complete, strongly colored crystals with intact tips, visible translucency, and a meaningful phosphate association. A single sharp emerald-green “sword” can be excellent if it is undamaged and well documented. Matrix specimens are generally preferred over loose crystals when the matrix includes childrenite, paravauxite, or other recognizable Siglo Veinte phosphates. Lesser pieces tend to be dark, opaque, bruised, cleaved, or visually confused against brown limonitic matrix.
The locality’s material is not usually valued for mirror-bright luster in the way a fine pyrite or cassiterite is. It is valued for crystal form, saturated blue-green color, rarity of association, provenance, and the unmistakable presence of the Llallagua phosphate environment.
Siglo Veinte vivianite is a classic but not a common market mineral. Old specimens appear periodically through specialist dealers, auctions, and dispersed collections, often described as old-time material from the 1940s through 1970s or as pieces from older Bolivian collections. Recent online dealer records show numerous examples sold rather than sitting in stock, including single crystals, matrix miniatures, and vivianite with childrenite or paravauxite. When available, prices vary strongly with size, sharpness, association, and provenance; fine miniatures and small-cabinet examples can move quickly because serious phosphate collectors, Bolivian-suite collectors, and vivianite specialists all compete for the same limited pool.
Condition is the central issue. Vivianite has perfect cleavage, low hardness, and a strong tendency to darken when exposed to light. Old Siglo Veinte crystals may show edge bruising, repaired tips, cleavage cracks, or dulled faces. Inspect terminations carefully, especially on sword-like crystals where the tip is visually important. Look also for old glue along the base of matrix-mounted crystals, because vivianite blades can detach cleanly along cleavage or from friable limonitic matrix.
Light management is essential. Display under low-intensity, short-duration lighting only, and avoid direct sun or prolonged case lighting. The safest long-term storage is in darkness, with occasional viewing rather than permanent illumination. A specimen that is already dark and opaque may still be collectible, but transparency and internal green color are part of the value, so unnecessary light exposure is a real loss.
No widely cited, locality-specific treatment scandal is associated with Siglo Veinte vivianite in the standard mineralogical sources. The more realistic authenticity concerns are labeling and attribution. Bolivian vivianite also comes from Huanuni, Tomokoni, Morococala, and other districts, and loose green vivianite blades can be difficult to place without matrix or provenance. Siglo Veinte pieces with childrenite, paravauxite, vauxite-family minerals, old Llallagua labels, or a traceable dealer or collection history are stronger purchases than isolated unlabeled crystals.
Be wary of overly broad labels such as “Bolivia” upgraded to “Siglo XX” without evidence. A credible label should include Siglo XX or Siglo Veinte, Llallagua, Potosí, and ideally the older Catavi/Llallagua wording if it came from an old collection. Contacto vein attribution is valuable, but it should be treated as a provenance claim, not assumed from appearance alone.
The mineral story of Siglo Veinte begins with tin, but its collecting legend turns on the strange late phosphates that appeared in the fractures of a giant ore system. Samuel G. Gordon described vauxite and paravauxite from Llallagua in the early 1920s, anchoring the locality in the scientific literature just as the mine was becoming one of the world’s emblematic tin producers. Later, Mark Chance Bandy worked in Bolivia for the Patiño interests and served at Llallagua in senior technical roles, including chief geologist, chief engineer, and general manager of the Siglo XX mine. His collecting and documentation helped give Llallagua specimens their enduring scientific identity. In later museum work, the Bandy Collection was noted for its strong representation of Llallagua phosphates: childrenite, crandallite, fluorapatite, metavauxite, monazite, paravauxite, sigloite, vauxite, vivianite, and wavellite.
One of the most evocative names for collectors is the Contacto vein. Dealer and collection records repeatedly tie the vein to important phosphate specimens, especially paravauxite and sigloite, and a fine vivianite single crystal has also been recorded from Contacto vein material. The best descriptions read like a miniature map of the phosphate stage: deep emerald vivianite, pale green paravauxite, tan sigloite, childrenite crusts, and iron-stained wavellite needles sharing the same broken vein surfaces. These are not merely pretty associations; they are the visual expression of a late-stage chemical environment in which closely related hydrous phosphates crystallized, altered, and overgrew one another in the same open spaces.
The human history is darker. In the early hours of June 24, 1967, during the traditional Night of San Juan, troops moved on the Siglo XX–Catavi mining camps. Víctor Montoya’s account describes families celebrating with bonfires, firecrackers, food, drink, and dynamite as part of the festival, unaware that troops from the Ranger and Camacho regiments had surrounded the camps under cover of night. He gives the toll as about twenty killed and seventy wounded. The event became known as the San Juan Massacre and later formed the basis for Jorge Sanjinés’s 1971 film El coraje del pueblo—The Courage of the People—a reconstruction made with participation from survivors.
That history gives Siglo Veinte specimens an emotional weight absent from many classic localities. A vivianite from Llallagua is a mineral specimen, but it is also a relic of a mining town whose name sits at the intersection of science, labor history, nationalization, collapse of state tin mining, and cooperative survival. The best old labels are small documents of that world.
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