CYLINDRITE
Cylindrite – Pb3Sn4FeSb2S14 – is an extremely rare sulfosalt found in hydrothermal Sn-Ag veins. The type locality for cylindrite are the Santa Cruz and Trinacria mines (Poopó, Oruro, Bolivia).
It is classified within the Cylindrite Group. Its composition is variable; silver (Ag) is a common impurity.
1. Characteristics
Crystal system: Triclinic pinacoidal.
Color: Blackish-gray, grayish-white.
Habit: Cylindrite forms triclinic pinacoidal crystals that appear as thin plates. These thin plates curl up to form cylinders. Under pressure, the cylinders can be unrolled. The cylinders are rarely well-terminated. The largest cylinders found measured 5 cm in thickness and 2–3 cm in length. The arrangement of the cylinders is usually completely random, though they may sometimes be sub-parallel or radial. The spaces between the cylinders are filled with cassiterite, sphalerite, tennantite-tetrahedrite, and other minerals.
Cleavage: {100} excellent.
Tenacity: Something malleable.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 2.5
Parting: No.
Streak: Black.
Lustre: Intense metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.4
2. Geology and Deposits
Cylindrite occurs as a late-stage formation in hydrothermal Sn-Ag veins.
3. Mineral Associations
It occurs associated with quartz and some common sulfides such as pyrite, galena, and sphalerite.
Cylindrite also occurs with rarer sulfides such as stannite, wurtzite, and teallite.
In its specific paragenetic assemblage, it occurs with other Pb-Sn-Sb-Ag sulfosalts such as boulangerite, franckeite, incaite, jamesonite, and potosiite.
It also occurs with oxides such as cassiterite.
4. Transmitted Light Microscopy
Not applicable, as cylindrite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Polishing cylindrite is very difficult due to its low hardness and the tendency of the plates to flake off when the section is cut tangentially to the cylinders. The polishing hardness of cylindrite is low—lower than that of stannite, sphalerite, and chalcopyrite, and slightly higher than that of franckeite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White.
Compared to the color of galena, the color of cylindrite is almost the same, though slightly darker.
Compared to the color of stannite, the color of cylindrite is a very light gray.
Compared to the color of sphalerite and wurtzite, the color of cylindrite is yellowish-white.
Compared to the color of pyrrhotite, the color of cylindrite is grayish-white.
Compared to the color of franckeite, the color of cylindrite is more gray.
Pleochroism: Very weak.
Parallel to the axis of the cream-white cylinders.
Perpendicular to the axis of the white-blue-gray cylinders.
Reflectivity: 30.82 – 35.22%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct but not high anisotropy, with colors ranging from brown to blue-gray and yellow-gray.
The parallel plates forming the cylinders – which individually range from 2 to 5 microns in thickness – exhibit oblique extinction, and the mineral never becomes completely dark in the “extinction” position.
If the cylinders are sectioned perpendicular to the axis, a “spherulite cross” is formed.
Internal reflections: No.
May be confused with: No other mineral. Since cylindrite almost always occurs in characteristic cylinders, it is impossible to confuse it with other minerals. Other minerals that form cylinders are chlorite and serpentine, which are completely different.
General Characteristics:
Grain shape: cross-sections show a concentric texture with layers of varying thicknesses. In longitudinal sections, these layers appear as twinning lamellae that converge toward the ends of the crystals. Layers of franckeite (characterized by higher reflectivity and more pronounced anisotropy) – often appearing as circular segments – may be interspersed among the cylindrite layers. The cores of the cylinders typically consist of a very fine-grained mixture of cylindrite, sphalerite, quartz, and occasionally cassiterite.
Radial twinning can be observed within the cylindrite layers, interrupted by franckeite layers.
Incipient alteration primarily produces secondary cassiterite, as well as marcasite and significant amounts of cerussite, galena, and boulangerite. These minerals initially appear at grain boundaries or along the individual rolled lamellae.
Inclusions 1: cylindrite may contain inclusions of stannite, pyrite, cassiterite, and sphalerite.
Inclusions 2: cylindrite can occur as inclusions – in the form of droplets or subhedral grains – within chalcopyrite and in the cores of wurtzite globules.
