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Zoisite

Crystal system · Orthorhombic

Zoisite is a silicate mineral prized by collectors for its exceptional color range, with known Chinese sources.

About Zoisiteextended article

Elemental Composition (by mass)
ElementMass %Visual
O Oxygen45.78%
Si Silicon18.54%
Al Aluminum17.82%
Ca Calcium17.64%
H Hydrogen0.22%
Computed from simplified end-member formula. Solid-solution series, water content, and trace substitutions cause real-world variation.
IMA Abbreviation (Whitney-Evans 2010)
Zo
→ Zoisite
Sorosilicate
Standard symbol from American Mineralogist (Whitney & Evans, 2010). Used in thin-section labeling, phase diagrams, and IMA-style species records.
Pronunciation
/ˈzɔɪzaɪt/
↔ ZOY-zite
named for Sigmund von Zois
Diaphaneity (Transparency)
translucent-to-opaque
Massive zoisite (anyolite) opaque; tanzanite variety transparent.
Type Locality
Saualpe, Carinthia — Austria
Described 1805 by Werner (named for S. Zois)
Mohs 6–7
Vickers (~) 820 HV
Knoop (~) 870 HK
Element composition by mass

Formula: Ca₂Al₃(SiO₄)(Si₂O₇)O(OH) · molar mass: 454.35 g/mol

O 45.78%
Si 18.54%
Al 17.82%
Ca 17.64%
H 0.22%

Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.

GroupEpidote Group
Related members: Epidote · Clinozoisite · Tanzanite
Optical Effects
Pleochroic
Mohs Hardness 6–7

Zoisite sits at 6–7 on the Mohs scale — harder than glass; scratches steel.

Colors:
Streak
White
Crystal system
Orthorhombic
SilicatesSilicates (Sorosilicates)
TL;DR · 1 min read
Zoisite (Ca₂Al₃(SiO₄)(Si₂O₇)O(OH)) is the orthorhombic sister to monoclinic epidote and a calcium-aluminum sorosilicate of major collector and gem importance. The species hosts three iconic gem varieties: tanzanite (vivid blue-violet, V/Cr-bearing) from the Merelani Hills in Tanzania, thulite (vivid pink, Mn-bearing) from Norway, and anyolite ("ruby in zoisite," massive green Zoisite enclosing ruby crystals).

Zoisite (Ca₂Al₃(SiO₄)(Si₂O₇)O(OH)) is the orthorhombic sister to monoclinic epidote and a calcium-aluminum sorosilicate of major collector and gem importance. The species hosts three iconic gem varieties: tanzanite (vivid blue-violet, V/Cr-bearing) from the Merelani Hills in Tanzania, thulite (vivid pink, Mn-bearing) from Norway, and anyolite (“ruby in zoisite,” massive green Zoisite enclosing ruby crystals). Tanzanite is the second-most popular blue gem after sapphire and is mined exclusively from a small region of Tanzania.

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About Zoisite

Zoisite is a silicate mineral in the epidote group and has the chemical formula Ca2Al3(Si2O7)(SiO4)O(OH). It crystallizes in the orthorhombic system and is one of the most visually varied minerals in the collector market. Its combination of structural character and global distribution make it a recognized species in both systematic and aesthetic collections.

Identification & care

Specimens usually show prismatic crystals with striated prism faces; also massive, granular. Its color range is broad, including gray, white, greenish-gray, yellowish (non-gem), blue-purple (tanzanite gem variety), pink-red (thulite — now separate), and green (chrome-bearing from tanzania). The luster is vitreous, pearly, the streak is white to colorless, and specimens range from transparent (gem) to translucent/opaque (massive). The cleavage is perfect {010}, imperfect {100}. The fracture is uneven to conchoidal, which aids identification.

Collector context

How it forms

The geological setting for Zoisite is typically medium-grade regional metamorphism of calcium-rich rocks; hydrothermal alteration; calc-silicate rocks (skarns); the gem varieties (tanzanite) form in graphitic gneisses under specific p-t conditions. It is commonly found in association with epidote, clinozoisite, hornblende, corundum (ruby), pyrite, calcite, phlogopite.

Classic Chinese localities

Zoisite is widely represented across Chinese provinces, including Gansu, Anhui, Guangdong, Guangxi.

Why collectors care

Collectors pursue Zoisite for the clarity of its crystal form and, in good material, saturated color that reads instantly across a display case. A well-terminated zoisite on clean matrix photographs well, identifies quickly, and anchors a cabinet piece. Top Chinese specimens over the last two decades have reset the bar for what zoisite looks like at collector grade.

What affects value

Value in Zoisite is assessed, in typical order of weight, against: (1) locality provenance; (2) size relative to the species norm; (3) crystal form and termination sharpness; (4) color saturation and zoning; (5) transparency and internal clarity; (6) matrix quality and aesthetic balance; (7) condition (absence of damage, chips, or repair). Cleaning quality and verified locality documentation act as multipliers across the above.

Naming history

The name Zoisite has a specific etymological and historical context — see Mindat's reference entry for provenance details. We have retained naming data at the record level; published prose is paraphrased from factual fields rather than copied from source.