Caysichite-(Y)

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Formula: (Ca,Yb,Er)₄Y₄(Si₈O₂₀)(CO₃)₆(OH).7H₂O
Inosilicate (chain silicate), yttrium-, ytterbium- and erbium- bearing mineral
Crystal System: Orthorhombic
Specific gravity: 3.03 measured, 3.029 calculated
Hardness: 4½
Streak: White
Colour: Colourless, white, pale yellow, rarely greenish
Luminescence: Not fluorescent in long or short wave UV, but has a faint green cathodoluminescence under the electron beam
Solubility: Decomposed by cold hydrochloric acid with evolution of carbon dioxide and formation of silica residue
Mildly RADIOACTIVE
Environments

Pegmatites

Localities

At the type locality, the Evans-Lou mine, Lac Saint-Pierre, Val-des-Monts, Les Collines-de-l'Outaouais RCM, Outaouais, Quebec, Canada, caysichite occurs in a granite pegmatite. It is especially common in quartz-rich areas of the giant perthite zone which surrounds the quartz core, but has not been found in the core itself. Caysichite coats fractures and encrusts or completely fills cavities in quartz, perthite and hellandite, and is apparently of supergene origin. It is most closely associated with hellandite but is also found with the other rare-earth bearing minerals fergusonite, kainosite-(Y), tengerite-(Y), lokkaite-(Y) and yttrian thorogummite; less commonly it is associated with yttrian spessartine, xenotime, wakefieldite and synchysite-(Y). Most commonly, caysichite-(Y) lines cavities as a dull white pulverulent coating or as a cream stain. More rarely it is found as thin, brittle encrustations. Additionally, the mineral occurs as radiating groups and terminated crystals, and, at one locality, as stalactites up to one cm in length.
The close spatial relationship of caysichite and hellandite suggests that caysichite is secondary after hellandite. It is suggested that caysichite was formed at low or moderate temperatures. Solution and transportation may have taken place in mildly acidic carbonated groundwaters, with precipitation occurring when the pH was increased to a value approaching neutrality (CM 12.293-298)

Caysichite-(Y) from the Evans-Lou Mine

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