Hydroxylherderite, Ca2Be2P2O8(OH)2, is among the most common beryllophosphates in nature and could play a substantial role in Be geochemical cycle. Hydroxylherderite P–T stability and crystal structure behavior were studied under extreme conditions (up to 750°C/100 GPa) using in situ single-crystal and powder X-ray diffraction and Raman spectroscopy. The mineral demonstrated high stability under high-pressure conditions (up to ∼100 GPa) without any phase transitions. Under high-temperature conditions, it was stable up to about 700°C, when it decomposed with the formation of fluorapatite Ca5(PO4)3F and hurlbutite CaBe2P2O8. The beryllophosphate member of the gadolinite supergroup is the most stable mineral (material) under high-pressure conditions, compared to aluminum-, boro- and beryllosilicates.