ANDORITE
Andorite – PbAgSb3S6 – is a rather rare sulfosalt characteristic of hydrothermal veins. It is generally a secondary ore but can occur in economically significant volumes, such as in certain deposits in the Andes.
Andorite crystals are actually polycrystals composed of “andorite-IV” and “andorite-VI.”
Andorite may contain Cu, Fe, and Zn.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Dark gray; it can tarnish to yellow or become iridescent.
Habit: Short, tabular prismatic crystals; may be massive.
Cleavage: No. Striations // to [001].
Tenacity: Very brittle.
Twinning: On {110}.
Fracture: Conchoidal.
Mohs Hardness: 3.5
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.33 – 5.37
2. Geology and Deposits
Andorite occurs in low-temperature polymetallic hydrothermal veins containing Pb, Sn, Sb, and Ag, always very close to the surface.
3. Mineral Associations
It occurs with gangue minerals typical of hydrothermal veins, such as quartz, barite, fluorite, and carbonates (siderite, calcite).
It is also associated with:
– common sulfides such as pyrite, sphalerite, arsenopyrite, and galena,
– native elements such as Sb, Pb, and Ag,
– Sn minerals such as cassiterite, stannite, ferrokësterite, and rhodostannite,
– Sb sulfides and oxides such as stibnite, berthierite, cervantite, and pyrargyrite,
– Cu sulfides such as enargite,
– polymetallic sulfides such as franckeite, minerals of the Freibergite Group, bournonite, and various Pb-Sb sulfides, with or without Ag (e.g., zinkenite, jamesonite, plagionite, diaphorite),
– as well as alunite and rhodochrosite.
4. Transmitted Light Microscopy
Not applicable, as andorite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Andorite readily takes an excellent polish. Its polishing hardness is low, but:
– greater than that of pyrargyrite and stibnite,
– approximately the same as that of jamesonite,
– lower than the hardness of semseyite and bournonite, and
– significantly lower than the hardness of sphalerite and stannite.
It is quite brittle, which makes preparing a polished section difficult. If the mineral is fractured, it is hard to produce a polished section free of polishing scratches.
PLANE POLARIZED LIGHT – PPL
Reflection color: Very light gray, quite white, almost like galena.
Compared to the color of galena, the color of andorite is darker and has a greenish-blue tint.
Compared to the color of bismuthinite, the color of andorite is darker and more bluish.
Compared to the color of jamesonite in its lightest orientation, the color of andorite is much darker.
Pleochroism: Very weak, difficult to see, and perceptible only at intergranular contacts and in twin lamellae.
Reflectivity: 35.37 – 39.33
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct to moderate anisotropy in various shades of gray: greenish-gray to bluish-gray, violet-gray, or pinkish-gray.
Internal reflections: It rarely exhibits sparse internal red reflections along cleavage planes and fractures, visible only under intense lighting.
May be confused with: Various other minerals in the paragenesis, the identification of andorite is quite complicated. Abundant fractures are, to a certain extent, diagnostic.
Stibnite exhibits much stronger pleochroism and stronger anisotropy.
General Characteristics:
Grain shape: Andorite typically forms idiomorphic tabular and prismatic crystals (similar to stibnite!). May occur in rounded grain aggregates.
Cleavage can rarely be visualized.
Fractures are quite common.
Lamellar twins are very common, often in several different directions, which can end up forming a texture that resembles checkerboard (parquet).
Substitutions 1: Andorite replaces stibnite, plagionite and zinkenite and remains (relicts) of these minerals may occur in andorite grains. In these cases, a native antimony crown may occur between the andorite and the inclusions of these minerals.
Substitutions 2: Andorite replaces freieslebenite, freibergite, diaphorite and stephanite.
Substitutions 3: Andorite is replaced by pyrargyrite, zinkenite and other Pb-Ag minerals.
