Pseudomorphs from Peru: pseudomorphs, crystals that have been replaced by another species while retaining their original shape, have been found in remarkable variety in the mines and mineral deposits of Peru. They provide evidence of changing chemical and physical conditions within ore deposits, and also make fascinating specimens.
Pseudomorphs (replaced crystals) and perimorphs (molds of dissolved crystals) are a rare but very interesting part of the mineral kingdom. They are scientifically important, some of them are aesthetic, and they are very popular among mineral collectors. This article summarizes occurrences of pseudomorphs and perimorphs in Peru, much as a previous study by Hyrsl and Petrov (1998) summarized Bolivian pseudomorphs. Most of the cited examples were acquired by the author during regular trips to Peru since 1995, and later analyzed by X-ray diffraction.
The classification of pseudomorphs is very difficult, because in many cases the original mineral cannot be verified. For this reason, they are listed here according to the chemical composition of the replacing mineral. For more detailed description of the localities in Peru see Crowley et al. (1997) and Hyrsl and Rosales (2003, 2005).
Acanthite after Silver
The Uchucchacua mine has produced excellent specimens of silver minerals, especially in 2001. Black acanthite pseudomorphs after thick native silver wires to about 1 cm long rise from calcite druses, together with small crystals of acanthite paramorphous after argentite.
Arsenopyrite after Pyrrhotite
The Huanzala mine has occasionally produced nice pseudomorphs of arsenopyrite after thin, platy pyrrhotite crystals.
Chalcopyrite/Pyrite/Stannite (?) after Chalcopyrite
The Animon mine (also called the Alimon mine, in the Huaron district) has yielded Peru's most beautiful chalcopyrite specimens. In 2001 it produced strange pseudomorphs of a mixture of sulfide species after chalcopyrite. These are yellow rounded crystals to 2 cm with dull luster, on thin quartz needles with black sphalerite. X-ray diffraction analysis indicates that the replacing substance is a mixture of chalcopyrite, pyrite and a mineral close to stannite.
Galena after Apatite
Galena is an unstable mineral in the mines of Quiruvilca; Crowley et al. (1997) mention that pseudomorphs of sphalerite, tetrahedrite and even gratonite after galena are known from the locality. However, in a specimen collected in December 2002, a mixture of galena and calcite forms a pseudomorph after three parallel hexagonal crystals (with prism and pinacoid faces), to 1 + 3 cm, of an unknown mineral on a pink manganocalcite crystal. The shape would suggest that apatite was most probably the original mineral. Galena forms tiny rounded crystals, and calcite forms gray, distorted rhombohedrons of saddle-like shape to about 1 mm. Three bournonite crystals to 4 mm are seen as floaters in the galena-calcite mixture.
Galena after Barite
Pink platy barite crystals have been found quite commonly in the Jesus vein of the Julcani mine since 2004. The crystals reach about 5 cm long and are comonly found covered by younger siderite and galena. Several specimens, almost certainly from the Jesus vein, show original platy barite crystals covered by fine-grained galena and by later-generation beige-colored siderite. The barite was later replaced by galena. The youngest barite generation forms small, colorless, transparent crystals on the surface of the pseudomorph.
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At least two interesting specimens of galena perimorphs after barite crystals were found at Quiruvilca in 2002. These are hollow, rectangular galena casts. The larger specimen, now in the collection of Rock Currier, measures 3 X 8 X 8 cm.
Galena after Stibnite
A very aesthetic pseudomorph which the author purchased in 2000 comes reportedly from the Julcani district, but Crowley et al. (1997) mention the same type from the Cuadalosa mine in the Castrovirreyna district. These two mining districts lie very close to each other, and it may be that the pseudomorphs in question come from both places. The studied specimen is a druse of fine-grained gray galena pseudomorphs after pointed stibnite crystals up to about 2 cm in length. Some of the pseudomorphs are hollow.
Pararealgar after Realgar
In 2000 the Palomo mine in Huancavelica Department produced nice single crystals of transparent red realgar up to about 2 cm. Most of the crystals have two symmetrical superficial zones of alteration to orange pararealgar (see the accompanying article in this issue).
Pyrite after Arsenopyrite
One of the pyrite after pyrrhotite pseudomorphs from Huanzala contains also an unusual pseudomorph of pyrite after elongated 1.5-cm arsenopyrite crystals.
Pyrite after Barite
Barite is common in the Julcani district, and commonly is replaced by galena and sphalerite. One specimen collected in 2005 shows platy pyrite pseudomorphs up to 1 cm after (most probably) barite.
Quartz perimorphs after barite have been noted commonly in the Pachapaqui mine since 2004. One specimen found in December 2005 has one side of the perimorph entirely composed of pyrite crystals (combining the cube and pyritohedron forms); the remaining sides of the perimorph are formed by a thin quartz layer.
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Pyrite after Chalcopyrite
One of the most interesting Peruvian pseudomorphs came in 2001 from the Pachapaqui mine, a locality noted especially for producing many habits of calcite. Sharp pseudomorphs of fine-grained to coarse-grained pyrite after chalcopyrite are covered by brightly lustrous tetrahedrite crystals to 1 cm. The original chalcopyrite crystals were up to 5 cm, and in some specimens chalcopyrite is still present as grains in the centers of the pseudomorphs. Tetrahedrite has grown epitactically on the pseudomorph's surfaces as parallel crystals. The tetrahedrite covered the chalcopyrite first, and then chalcopyrite was replaced by pyrite.
Even more aesthetic, sharp pseudomorphs of pyrite after chalcopyrite to 4.5 cm came at the end of 2005 from the Casapalca district. The pseudomorphs are intergrown with quartz prisms, tennantite crystals to 1 cm, and black sphalerite crystals to 3 cm covered by tiny tennantite tetrahedrons. Rarely, pseudomorphs of tennantite after enargite rest on the tops of the pseudomorphs.
Pyrite after Enargite
Between 1984 and 1987, hollow cast pseudomorphs of pyrite after enargite up to 7 cm long were found at Quiruvilca (Crowley et al., 1997).
Pyrite after Pyrrhotite
The Huanzala mine is probably the world's biggest producer of pyrite crystals. It has also yielded many interesting pseudomorphs of pyrite after platy hexagonal pyrrhotite crystals to more than 5 cm. Some of the pseudomorphs are hollow, and some are covered by small galena crystals.
Many specimens whose locality is given as the Huanzala mine may well be from the small Pucarrajo mine, which has a similar assemblage and is situated near the pass between the Huanzala and Pachapaqui mines. On studied specimens of pyrite pseudomorphs after pyrrhotite from the area, those from Pucarrajo are more columnar than the platy ones from Huanzala.
A specimen with unusually elongated fine-grained pyrite pseudomorphs after pyrrhotite comes reportedly from the Colquijirca mine near Cerro de Pasco. Of the several pseudocrystals, the largest is 4 cm long but only 1 cm in diameter.
I visited the San Cristobal mine in January 2006, and inside the mine I saw a wall 3 meters high and partially covered by galena crystals to 2 cm; a 1 square meter area of the wall was completely covered by pyrite pseudomorphs after pyrrhotite up to 1 cm, a few of these pseudocrystals reaching 2 cm across. Unfortunately, the rock was too hard to allow extraction of a good specimen.
Pyrite pseudomorphs after flat pyrrhotite crystals to 5 mm associated with small pyrite crystals were found at Uchucchacua in 2002.
Pyrite after Siderite
Casapalca has produced specimens showing pyrite cast pseudomorphs after a carbonate, most probably siderite. These are rhombohedrons to 2 cm, formed by 1.5-mm crusts of fine-grained pyrite overgrown by small quartz crystals; inside the casts are abundant small pyrite cubes.
Pyrite after Tetrahedrite
A few strange specimens were found in the Casapalca district in September 2001: rounded crystals covered by a thin, lustrous crust, analyzed as a tetrahedrite-tennantite mixture with a ratio close to 1:1. The underlying pseudocrystals consist of fine-grained pyrite and, to judge from their shape, they are probably pseudomorphs after tetrahedrite-tennantite or after chalcopyrite. The pseudomorphs, reaching about 1.5 cm in diameter, rest on druses of short-prismatic quartz crystals.
Sphalerite after Barite
Goodell (1974) shows a picture of a colloidal epimorph of sphalerite after flat barite crystals from the Julcani district; the specimen is 15 cm across. Another specimen, collected in the Jesus vein in Julcani in 2005, shows plates of fine-grained sphalerite covered by younger, pale brown siderite. A sphere of colloidal pyrite, typical for Julcani, is sitting on the top of the specimen.
Sphalerite and Arsenopyrite after Calcite
A few unusual pseudomorphs were found in the Carmen mine in the Casapalca district in the summer of 2005. They show hollow, pointed triangular shapes which very probably are molds after calcite scalenohedrons. The molds are composed of tiny black sphalerite crystals mixed with larger galena crystals up to 5 mm, coated by tin-white arsenopyrite crystals on their inner surfaces. The pseudomorphs reach 3 cm long and almost 2 cm wide, and are partially covered by younger, elongated, white calcite crystals. Probably the molds grew over the original calcite crystals from only one direction, because all are open on the same side.
Sphalerite after Galena
On a specimen probably found in the Julcani district, a black cube, 1.9 cm on edge, occurs with platy barite crystals and a younger calcite crust. The cube is composed of coarse-grained sphalerite and surely is a pseudomorph after galena.
Another specimen, collected in 2005 in the Animon mine, has several cubic pseudocrystals of brown sphalerite up to 1 cm on edge. One is broken and is seen to consist entirely of sphalerite, proving that the replacement is complete. The original mineral was almost surely galena, as fresh cubic crystals of pyrite are also present.
Sphalerite after Wurtzite (?)
A nice specimen which came from Huanzala in 2004 shows two types of pseudomorphs. In one of them, thick hexagonal-tabular crystals of sphalerite to 1 X 3 cm are covered by small galena crystals. In the other, pyrite appears as hexagonal-tabular pseudo-crystals to 2 cm in diameter but only 3-5 mm thick; some of the pseudocrystals are slightly twisted and several are hollow. It seems clear that in the latter specimens pyrite has replaced pyrrhotite, but in the former specimens sphalerite must have replaced a different hexagonal or trigonal mineral. Wurtzite is a possibility, as Huanzala is the only Peruvian locality known for wurtzite crystals. In this case the original crystal would have been one of the world's finest wurtzites, had it survived unaltered.
Tetrahedrite after Bournonite
Pseudomorphs of tetrahedrite after columnar, striated bournonite crystals from the Julcani district are also interesting; they were collected around 1996. The pseudocrystals are up to about 1 cm long, and grow on white barite together with acicular crystals of bismuthinite.
Tennantite after Enargite
One of the most beautiful Peruvian pseudomorphs was found in the Julcani district in 1999, and specimens were quite abundant for a short time. They show tennantite replacing columnar enargite crystals, accompanied by white barite plates. The size of the pseudocrystals reaches at least 5 cm, but most of them are severely damaged. Many of the pseudocrystals are hollow, with enargite remaining in the centers of some of them as black cleavable grains. A few of the pseudocrystals, to about 1 cm long, are covered by tiny pyrite crystals. X-ray diffraction studies show that the replacing material has a tetrahedrite : tennantite ratio close to 1:1.
Pseudomorphs of tennantite after enargite were found also in the Casapalca district in November 2005. They are columnar and hollow, and reach 2 cm long. Some of the pseudocrystals rest on nice sharp pyrite pseudomorphs after chalcopyrite, intergrown with black sphalerite crystals.
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Crowley et al. (1997) mention many pseudomorphs from Quiruvilca. The most abundant of these are probably partial to complete pseudomorphs of tetrahedrite after enargite, very similar to the more recent specimens from Julcani. According to Rock Currier, they were not analyzed, but it is reasonable to suppose that, like the Julcani specimens, they are tennantite and not tetrahedrite; moreover, enargite and tennantite contain the same elements.
The extremely important Cerro de Pasco silver deposit is still exploited by a huge open pit, but in recent years almost no mineral specimens from this locality have appeared on the market. Ward (1961) describes many types of pseudomorphic replacements from Cerro de Pasco, including tennantite, chalcopyrite, chalcocite and pyrite after enargite, as well as enargite and chalcocite after pyrite. Unfortunately, the author has seen no examples of any of these recently.
Paratacamite after Atacamite
The small Lily copper prospect, about 50 km east of Pisco, yielded beautiful specimens of atacamite, malachite, chrysocolla and gypsum around 1998-2000, as well as the world's best paratacamite in dark green cuboidal crystals to 1.5 cm. The best known Lily prospect specimen of atacamite included in gypsum appears on the cover of the Mineralogical Record, vol. 31, no. 3. A few pale green columnar aggregates to about 1 cm long were found to be paratacamite; these must be pseudomorphs after atacamite.
Fluorite after Calcite
Several attractive specimens found in the Huanzala mine in 2005 show thin purple fluorite crusts on calcite crystals. Most of the calcite has dissolved away, but in some cases the perimorphs are filled with recrystallized calcite crystals, proving that the perimorphs are natural, not products of artificial leaching.
Calcite after Galena
One specimen found at the Huanzala mine shows a hollow calcite perimorph about 1.5 cm across with patches of corroded galena on its inner surface. Most probably, therefore, the original mineral was galena.
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Dolomite after Calcite
A specimen showing dolomite perimorphs after calcite rhombohedrons to 3 cm is most probably from the Huanzala mine. The dolomite crust is 2 mm thick and covered by colorless fluorite cubes measuring 2 to 10 mm.
Rhodochrosite after Barite
In recent years, until 2002, Huaron has produced only a very few interesting specimens. Surprisingly, in October 2002 a cavity was discovered with pink rhodochrosite forming perimorphs after dissolved barite. In most cases the flat perimorphs form the bottoms of specimens, but some complete rhodochrosite pseudomorphs after barite to about 6 cm were also found. In some cases the rhodochrosite is covered by hemispherical aggregates of black sphalerite to 3 cm in diameter, accompanied by small pyrite and rarely even tiny pyrargyrite crystals.
Rhodochrosite after Galena
One very interesting specimen found in the Morococha district in January 2004 is a matrix entirely covered by a white crust of tiny quartz crystals, a perimorph after a crust of cuboctahedral crystals to about 1 cm. To judge from their shape, the original crystals were galena, but now all galena has leached away, leaving cavities which are filled completely by massive rhodochrosite or partially by pink rhombohedral rhodochrosite crystals to 3 mm.
Siderite after Galena
Casapalca is a big mine close to Lima which has produced some pseudomorphs. For instance, siderite pseudomorphous after galena occurs as hollow brown molds of cuboctahedral form to 2.5 cm. The molds rest on tetrahedrite crystals and small, corroded galena remnants.
Smithsonite after Calcite
A discovery at Casapalca in 2001 produced a few specimens with druses of small, sharp, gray scalenohedrons to about 3 mm, with a few gypsum crystals to 5 cm resting on the druses. The scalenohedrons are hollow and X-ray diffraction analysis has identified them as smithsonite. This is surprising, because secondary minerals are almost unknown in Peruvian deposits situated at very high altitudes. The specimens probably represent smithsonite perimorphs after calcite.
Hubnerite after Scheelite (?)
The mines of the Pasto Bueno district have produced the world's best hubnerite crystals, and they have also yielded a few nice specimens of hollow hubnerite molds after bipyramidal or octahedral crystals resting on a mixture of quartz with greenish muscovite. The pseudocrystals, reaching about 2 cm, are composed of small dark red hubnerite crystals. There are two possibilities for the identity of the original dissolved crystals: scheelite and fluorite, both common at Pasto Bueno. Scheelite is more probable because its chemistry is closer to that of hubnerite.
Quartz after Barite
A very unusual and aesthetic specimen from the Pachapaqui district was acquired in Lima in December 2004. It is a quartz perimorph after platy rectangular crystals, probably of barite. The specimen measures 18 X 24 cm and the original barite plates were up to 11 cm long and 2 cm thick. On the front side were originally two layers of parallel barite crystals at an angle of 150 degrees. Barite was covered by quartz crystals to about 1.5 cm and by pyrite cubes to 5 mm, then dissolved away, forming a quartz perimorph. Similar but much inferior specimens from the Pachapaqui district were quite abundant for some time; some of these show younger pink manganese-rich calcite crystallized in cavities. Specimens found at the end of 2005 have abundant pyrite crystals on their surfaces.
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Huaron produced good pseudomoprhs of rhodochrosite after barite in 2002 (see earlier). A few specimens from the same vein show platy quartz perimorphs after barite to 7 cm long, covered by barrel-shaped quartz crystals and druses of small rhodochrosite crystals.
Two specimens showing quartz pseudomorphs after barite were collected a long time ago in the Julcani district. Both consist entirely of quartz, which replaced original barite crystals up to 1 X 4 X 8 cm.
The small Palomo mine between Julcani and San Genaro has been producing good stibnite and gypsum specimens since the end of 2003. Specimens found there in December 2005 show well-formed quartz pseudomorphs after barite to 2 cm, some of them with greenish yellow orpiment or hemispheres of stibnite needles resting on them. One pseudomorph is broken and can be seen to be hollow.
Quartz after Calcite
The Milpo and Atacocha mines are both situated near Cerro de Pasco, and available lots of specimens from them are usually mixed together. One of the mines produced a quartz perimorph specimen showing a very thin quartz crust over a vanished 1.5-cm calcite rhombohedron. The perimorph is intergrown with barrel-shaped quartz crystals of the kind that are typical for both mines.
One specimen from the Casapalca district in the Rock Currier collection has two rhombohedral pseudocrystals to 5 cm, each composed of small, thin quartz crystals to about 1 cm long; it is surely a pseudomorph after a carbonate, most probably calcite. A similar specimen found in 2002 has a perimorph of small quartz crystals after two calcite rhombohedrons, and measures about 4 X 6 X 7 cm. Small acicular arsenopyrite crystals occur on the inner surface of the perimorph, and yellow calcite scalenohedrons occur on its outer surface.
Quartz pseudomorphs after very thin, platy calcite crystals are present on a few specimens of pyrite after chalcopyrite from the Pachapaqui district. The largest of these measures about 3 X 5 cm.
Quartz after Fluorite
A few interesting quartz after fluorite pseudomorphs were found a few years ago in the Animon mine. They are hollow molds of small, transparent quartz crystals on black sphalerite which originally grew on octahedral fluorite crystals to 4 cm, after which the fluorite was dissolved completely.
Quartz after Pyrrhotite
One quartz pseudomorph after a hexagonal-tabular pyrrhotite crystal 7 mm wide and 2 mm thick was found among pyrite pseudomorphs after pyrrhotite from the San Cristobal mine in January 2006.
K-feldspar after Fluorite
One of the strangest of the new Peruvian pseudomorphs was found at Pucarrajo in 1999 and again in 2000. The pseudocrystals are snow-white octahedrons to 8 mm, composed, according to X-ray diffraction analysis, of fine-grained microcline or orthoclase. They rest on quartz and are covered by tiny, younger pyrite cubes. To judge by their shape, they are pseudomorphs after fluorite.
Muscovite after Epidote
Pampa Blanca, a famous locality about 80 km east of Pisco, produced a huge number of fan-shaped aggregates of epidote a few years ago, as well as excellent Japan-law twins of quartz to 50 cm. From a border of the deposit came a few specimens showing fan-shaped muscovite pseudomorphs to about 12 cm with quartz crystals. To judge by their shapes, the muscovite must have replaced epidote.
Neotocite and Quartz after Unknown
Some specimens from Uchucchacua collected in December 2004 show unusual pseudomorphs associated with red scalenohedral crystals of rhodochrosite. The pseudocrystals are sharp, simple tetrahedrons up to 1 cm, formed by a thin quartz crust at the surface and filled with amorphous black neotocite inside. The identity of the original mineral remains a mystery.
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CROWLEY, J. A., CURRIER, R. H., and SZENICS, T. (1997) Mines and Minerals of Peru. Mineralogical Record, 28, 1-98.
GOODELL, P. (1974) A typical sulfosalt environment: the mineralogy of the Julcani District, Peru. Mineralogical Record, 5, 215-221.
HYRSL J., and PETROV A. (1998) Pseudomorphs from Bolivia, a review. Rocks and Minerals, 73, (6), 410-414.
HYRSL, J., and ROSALES, Z. (2003) Peruvian minerals: an update. Mineralogical Record, 34, 241-254.
HYRSL, J., and ROSALES, Z. (2005) Neue Mineralienfunde in Peru 2000-2005. Mineralien-Welt, 16, (6), 48-57.
WARD, H. J. (1961) The pyrite body and copper orebodies at the Cerro de Pasco mine, central Peru. Economic Geology, 56, 402-422.
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|Publication:||The Mineralogical Record|
|Date:||Mar 1, 2008|
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