Horse Hill locality, Norway, Oxford Co., Maine, USA

(Na,Ca)[Al(Si,Al)Si₂O₈]

Crystal System: triclinic 
Colour: colourless, white, very pale yellow, blue with inclusions
Lustre: vitreous, sub-vitreous
Habitus: usually as cleavable, granular or massive aggregates formed by tabular crystals. Twinning is common
Hardness: 6 - 6½
Fracture: irregular/uneven, sub-conchoidal
Cleavage: perfect {001} perfect, {010} good, {110} poor
Density: 2.63-2.66 g/cm³
Origin and geological occurrence: “oligoclase” is a member of the plagioclase series, with composition ranging between An₁₀ to An₃₀.It occurs in felsic igneous rocks. It is rarer in metamorphic rocks. It may be a detrital mineral in sedimentary rocks.

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Samples:

9F.6 Oligoclase White tabular twinned crystals. Redondo Mt., Leira, Portugal #tectosilicates