Naica
Naica, Chihuahua: a silver-lead-zinc mine where Peñoles miners found the Cave of Crystals in 2000, its gypsum beams 11 to 12 m long, grown at 54 to 58 °C.
Naica, a silver-lead-zinc mine 112 km southeast of Chihuahua City, Mexico, holds the Cave of Crystals, found by Peñoles miners in 2000 about 290 m underground. Its gypsum beams reach nearly 12 m. At Naica they grew at about 54 to 58 °C, and Naica gypsum is soft: hardness 1.5 to 2 on the Handbook sheet.
In short
- Naica is a mine first. Ragon et al. (Frontiers in Microbiology, 2013) describe it as one of the largest chimney-manto (skarn) silver-lead-zinc deposits of Mexico, with an early ore suite of pyrite, galena, sphalerite, chalcopyrite, pyrrhotite, magnetite, rutile and fluorite, dated 30.2 to 26.0 Ma. Workings reach about 890 m below the mine entrance.
- The Cave of Crystals: 2000, 290 m. ACS Central Science (2018) says miners searching for new ore found it in 2000, 290 m underground, in a mine that Industrias Peñoles kept dry by pumping. National Geographic (2007) says two miners cutting a new tunnel, about 300 m down. We print both depths.
- Two lengths, both printed. ACS gives the largest beams as nearly 12 m long and 1 m wide; National Geographic and Ragon et al. give up to 11 m. The Handbook of Mineralogy gypsum sheet gives crystals to 17 m but does not say where, so that figure is not a Naica measurement.
- The 58 °C line. Anhydrite is stable above about 58 °C and gypsum below it. As Naica’s hot water cooled through that line, anhydrite dissolved and fed slow gypsum growth at about 54 to 58 °C. ACS reports a lab-based estimate of nearly a million years for a 1 m thick beam: about 1 micrometre a year, worked.
- The weight figures do not fit the headline size. At the gypsum sheet’s density of 2.317, a 12 m × 1 m × 1 m prism weighs 27.8 t. The published estimates of 40 to 50 t (ACS) and 55 tons (National Geographic) need 17.3 to 23.7 m³ of gypsum. They are estimates of the biggest beams, not of a 1 m square prism.
| Species | Formula (IMA) | Hardness | Density | 20.00 g piece in water | Handbook wording for Naica | IMA status, year, country |
|---|---|---|---|---|---|---|
| Gypsum | Ca(SO4)(H2O)2 | 1.5–2 | 2.317 | 11.37 g | “huge crystals in a cave complex in the Naica Pb–Ag mine” | G, year and country not given |
| Anhydrite | Ca(SO4) | 3–3.5 | 2.98 | 13.29 g | “From Naica, Chihuahua, Mexico” | G, 1804, Austria |
| Galena | PbS | 2.5–2.75 | 7.58 | 17.36 g | “At Naica and Santa Eulalia, Chihuahua” | G, year and country not given |
| Sphalerite | ZnS | 3.5–4 | 3.9–4.1 | 14.87–15.12 g | “from Santa Eulalia and Naica, Chihuahua” | A, 1980 s.p. |
| Fluorite | CaF2 | 4 | 3.175–3.184 | 13.70–13.72 g | “In Mexico, from Naica, Chihuahua” | G, year and country not given |
| Dolomite | CaMg(CO3)2 | 3.5–4 | 2.86 | 13.01 g | “At Naica, Chihuahua, Mexico” | G, 1792, Italy |
Where Naica is and what kind of mine it is
Position. Ragon et al. (Frontiers in Microbiology 4:37, 2013) put the Naica mine 112 km southeast of Chihuahua City in northern Mexico; the geological appendix of the same paper says 100 km. The district sits on the northwest flank of the Sierra de Naica, in a semi-desert basin of about 1,300 km². The host rocks are Albian–Cenomanian limestones and marls in a gentle antiform, cut by quartz–feldspar dikes.
The orebody. The paper calls Naica one of the largest chimney-manto (skarn) silver-lead-zinc deposits of Mexico, formed by hydrothermal flows tied to Tertiary dykes. The main ore-forming process is dated 30.2 to 26.0 Ma, at an estimated 400 to 500 °C. The early ore minerals it lists are pyrite, galena, sphalerite, chalcopyrite, pyrrhotite, magnetite, rutile and fluorite. Quartz, anhydrite and calcite came later, as veins, when the fluids had cooled.
How deep. The same appendix says the workings reach about 890 m below the mine entrance, which is at 1,385 m above sea level, and some 760 m below the original water table. Water in the galleries runs at 50 to 60 °C. Other Mexican localities, Ojuela and Milpillas among them, are on the localities index.
When was the Cave of Crystals found, and how big are the crystals?
Found in 2000. ACS Central Science (Hiolski, 2018) says miners searching for fresh ore found the Cave of Crystals in 2000, a horseshoe-shaped chamber 290 m underground, and credits the discovery to Industrias Peñoles. National Geographic’s 2007 report says two miners cutting a new tunnel for the company found it about 300 m (a thousand feet) down. The two depths are 10 m apart; we print both.
Two lengths, and a third that is not Naica’s. ACS gives the biggest beams as nearly 12 m long and 1 m wide. National Geographic gives 11 m (36 feet), and Ragon et al. say the crystals may reach up to 11 m. The Handbook gypsum sheet gives gypsum crystals to 17 m in its crystal data and describes Naica’s only as huge crystals in a cave complex; it does not tie the 17 m to Naica, so neither do we.
The older cave. The Cave of Swords was found in 1910, at about 120 m, so the Cave of Crystals lies about 170 m deeper on the ACS figures. ACS says its crystals reach 2 m and that collectors took many of the largest; National Geographic calls them typically about a metre long. The selenite variety of gypsum is the one ACS names; see gypsum.
Why are the Naica crystals so big? The 58 °C line
Two calcium sulphates, one temperature. Both fetched articles give the same threshold: anhydrite, CaSO4, is the stable phase above about 58 °C and gypsum, CaSO4·2H2O, below it. Volcanic activity about 26 million years ago filled the mountain with hot water and anhydrite. As the water cooled through 58 °C, the anhydrite began to dissolve and released just enough calcium and sulphate to keep the water barely supersaturated in gypsum. Few crystals nucleated, so the few that did grew very large.
Worked growth rate. ACS reports that Van Driessche, García-Ruiz and colleagues measured the growth of Naica gypsum in Naica mine water in the laboratory, and estimated rates for the 54 to 58 °C range in which the crystals formed. At 55 °C a 1 m thick beam would take nearly a million years. That is an average of 1 micrometre of thickness a year, or about 2.7 nanometres a day. ACS quotes it as the thickness of a sheet of paper every 200 years; 200 years at 1 micrometre is 0.2 mm, which agrees.
The shallow-cave contrast. ACS says the water in the Cave of Swords cooled faster, so more crystals nucleated and each stayed smaller. The question of whether gypsum is a mineral at all is answered on is gypsum a mineral; its anhydrite-to-gypsum volume arithmetic is on the gypsum page and is not repeated here.
How heavy are the Naica crystals? What the weight estimates imply
The published figures. ACS Central Science says the largest beams weigh an estimated 40 to 50 metric tons. National Geographic’s 2007 report says up to 55 tons. Neither source says how the weight was estimated.
The check, worked. The Handbook gypsum sheet gives a measured density of 2.317, so 1 m³ of gypsum weighs 2.317 t. A prism 12 m long and 1 m by 1 m in section would weigh 27.8 t; at 11 m, 25.5 t. To weigh 40 to 55 t a beam needs 17.3 to 23.7 m³ of gypsum: over 11 to 12 m of length, an average section of about 1.4 to 2.2 m².
Documented mismatch, not resolved. The tonnages and the 12 m by 1 m headline do not describe the same prism. Either the heaviest beams are thicker than 1 m along much of their length, or the estimates are high; the sources do not say which, and this page does not guess. The arithmetic uses only the sheet’s density, so anyone can rerun it.
Which minerals the Handbook places at Naica
Six sheets. Method: a script reads the Distribution field of each cached Handbook of Mineralogy sheet (pdftotext -layout, line breaks joined, case-insensitive) for the word Naica. Six match: gypsum, anhydrite, galena, sphalerite, fluorite and dolomite, as tabled above. That is the sheets read for this page, not a full species list for the district.
Documented negatives. Ragon et al. list pyrite, chalcopyrite, pyrrhotite, magnetite and rutile among the early ore minerals, and calcite among the later vein minerals, but the cached Handbook sheets for those six do not name Naica. The hemimorphite sheet names Santa Eulalia, Chihuahua, not Naica. A label that says only “Chihuahua” does not mean Naica: the galena, sphalerite and gypsum sheets also name Santa Eulalia.
The heaviest and the lightest. In water a 20.00 g piece reads 17.36 g as galena and 11.37 g as gypsum. Fluorite sits at 13.70 to 13.72 g and anhydrite at 13.29 g. The other species pages, including sphalerite and dolomite, are on the species index.
Pumping, flooding and what this page does not say
A dry cave is a pumped cave. Ragon et al. say keeping the galleries free of water takes dewatering of about 1 m³ a second. ACS says Industrias Peñoles pumped at a rate that would fill an Olympic pool every 40 minutes. National Geographic wrote in 2007 that the only reason people could enter the caves was the pumping.
Conditions inside. ACS gives the cave about 50 °C and over 90% relative humidity, with visits of 10 to 15 minutes. It reports bassanite, a dehydrated calcium sulphate, on the surfaces of crystals kept in air in a year-long laboratory test, and says foot traffic has worn a blackened path into the floor crystals.
Dated, and limited. ACS reported in 2018 that the company closed the route to the cave in about 2015, after a leak let water in faster than the pumps could remove it. That is the latest statement we fetched. This page says nothing about the mine’s or the cave’s present state, nor about access or visits. On labels, Naica, Chihuahua, Mexico is what the sheets support; see how to label and catalogue a mineral collection. Nothing here is offered for sale; the wanted list is the only commercial route.