Sanidine

Sanidine is a feldspar mineral made of potassium, aluminium, silicon and oxygen (KAlSi₃O₈). It differs from orthoclase and microcline, which have the same composition, in the arrangement of its atoms and the temperature at which it forms: sanidine forms at high temperature and keeps this arrangement because lava or ash cools quickly. That is why it appears in volcanic rocks such as rhyolite and trachyte as white, tablet-shaped crystals (phenocrysts). Being rich in potassium, it is also used to date volcanic eruptions.

Dating: Because it is rich in potassium, and radioactive potassium-40 decays to argon-40, sanidine lets scientists measure the ages of volcanic eruptions very precisely. According to the USGS, sanidine is abundant in rhyolite ash and lava flows at volcanoes such as Yellowstone and Valles Caldera and is ideal for argon dating. A feldspar source: Trachytes are an alternative source of potassium feldspar for the ceramic and glass industries. Sanidine is put to the same uses as other potassium feldspars such as orthoclase and microcline.

It has a hardness of 6 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.52–2.62 g/cm³.

Its best-known localities: Drachenfels, Siebengebirge, North Rhine-Westphalia, Germany; Laacher See area (Mendig, Mayen), Eifel, Germany; Puy de Sancy, Monts Dore, Auvergne, France; Vesuvius and Monte Somma, Campania, Italy; Monte Cimino, Lazio, Italy.

Known localities in Türkiye: Kınık, Dinar, Afyonkarahisar, Türkiye.

In volcanic rocks (trachyte, rhyolite) it shows up as colourless or white, glassy, tablet-shaped crystals that stand out clearly from the fine-grained groundmass. It scratches glass and a steel knife hardly scratches it (hardness 6); it splits along flat surfaces in two nearly perpendicular directions, and these surfaces flash like mirrors in the light. Its crystals often form Carlsbad twins: two crystals fused together as if one were turned half a revolution relative to the other. Sanidine is confused with orthoclase, microcline and plagioclase. Only a laboratory can say for certain which feldspar it is.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Sanidine is mainly a rock-forming mineral rather than an ornamental stone, and has no price of its own. Collectors look for large, clear, undamaged crystals and well-developed twins such as Carlsbad twins; cut stones are very rare. In industry, its use as a potassium feldspar depends on its K₂O and Na₂O content, the absence of colour-spoiling impurities such as iron, its grain size and the cost of separating it from the rock.

Crystal-therapy websites claim that sanidine calms anxiety, sadness and anger and releases emotional blocks. The same sites say it helps with menstrual pain, circulation, the nervous system and skin irritation. No study supports these claims. Sanidine is an ordinary potassium feldspar, very common in volcanic rocks. If there is a feeling of relief, it comes from spending a quiet moment looking at and touching the stone. Someone with severe period pain, circulation problems or skin trouble should see a doctor. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.