HOPEITE
Hopeite – Zn3(PO4)2·4H2O – is a rare phosphate that occurs in some very specific geological environments. It is part of Zn ores, but is never the main ore mineral in the deposit.
It is classified in the Hopeite Group and is the phosphate analog of arsenohopeite. It has two varieties, α-hopeite and β-hopeite. It is readily soluble in dilute hydrochloric acid.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: White, yellow, colorless, brown, or gray.
Habit: Tabular, prismatic. Crystals up to 6 mm, individual or in tufts, divergent aggregates and in crusts. Reniform, compact.
Cleavage: {010} perfect,, {100} good, {001} poor.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 3.5
Parting: No.
Streak: White.
Lustre: Vitreous to pearly.
Diaphaneity: Transparent.
Density (g/cm³): 3 – 3.1
2. Geology and Deposits
Hopeite is a mineral that occurs as a late mineral in some hydrothermal Zn deposits based on sphalerite. It is also found in complex phosphatic granitic pegmatites.
It was found forming crusts over a bone breccia in a limestone cave. It can form in oxidation zones of Zn ores.
3. Mineral Associations
In the type locality (Altenberg Mine, Belgium), it occurs associated with hemimorphite, smithsonite, willemite, sulfides (sphalerite, galena, pyrite, marcasite), carbonates (calcite, dolomite, siderite), quartz, gypsum, native sulfur, and various secondary minerals of Zn, Pb, and Fe, totaling 24 different minerals.
It may be associated with sulfides such as sphalerite, galena, and pyrite.
It occurs with primary Zn minerals such as sphalerite and with secondary Zn minerals such as hemimorphite, smithsonite, as well as with various other Zn phosphates such as parahopeite, phosphophyllite, spencerite, tarbuttite, scholzite, and parascholzite.
In granitic pegmatites, it is associated with sphalerite and phosphates such as triphyllite, ferrisicklerite, leucophosphite, laueite, robertsite, and hydroxyl apatite.
It is also associated with native copper and secondary Pb minerals such as vanadinite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.572 – 1.574 nβ: 1.582 – 1.591 nγ: 1.590 – 1.592
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: A perfect cleavage, one good and one poor, can be visible in larger crystals.
Habits: Tabular, prismatic. Crystals up to 6 mm, individual or in tufts, divergent aggregates and in crusts. Reniform, compact.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.018, corresponding to interference colors up to the end of the 1st order, such as gray, white, yellow, orange and red.
Extinction: No information available, propably paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 26 – 82º (calculated)
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected Light microscopy is clearly not the recommended analytical method for identifying hopeite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with hopeite, such as pyrite and galena.
It is not possible to recognize hopeite under Reflected Light, but the technique allows visualization of replacement textures in the ore in relation to the opaque minerals that occur there.