PbAl2Si2O8 crystallizes in the triclinic P-1 space group. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(4), one O(5), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. In the second Al site, Al(2) is bonded to one O(2), one O(4), one O(6), and one O(8) atom to form AlO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. Pb(1) is bonded in a 3-coordinate geometry to one O(3), one O(4), and one O(7) atom. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(3), one O(6), and one O(8) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Al(2)O4 tetrahedra. In the second Si site, Si(2) is bonded to one O(2), one O(3), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Al(1)O4 tetrahedra. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Al(2) and one Si(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Pb(1), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to one Al(1), one Al(2), and one Pb(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Al(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Al(2) and one Si(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 150 degrees geometry to one Al(1), one Pb(1), and one Si(2) atom. In the eighth O site, O(8) is bonded in a bent 150 degrees geometry to one Al(2) and one Si(1) atom.
PbAl2Si2O8 crystallizes in the triclinic P-1 space group. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(4), one O(5), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(2)O4 tetrahedra. The Al(1)-O(1) bond length is 1.74 Å. The Al(1)-O(4) bond length is 1.77 Å. The Al(1)-O(5) bond length is 1.74 Å. The Al(1)-O(7) bond length is 1.78 Å. In the second Al site, Al(2) is bonded to one O(2), one O(4), one O(6), and one O(8) atom to form AlO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Si(1)O4 tetrahedra. The Al(2)-O(2) bond length is 1.76 Å. The Al(2)-O(4) bond length is 1.77 Å. The Al(2)-O(6) bond length is 1.76 Å. The Al(2)-O(8) bond length is 1.76 Å. Pb(1) is bonded in a 3-coordinate geometry to one O(3), one O(4), and one O(7) atom. The Pb(1)-O(3) bond length is 2.68 Å. The Pb(1)-O(4) bond length is 2.44 Å. The Pb(1)-O(7) bond length is 2.64 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(3), one O(6), and one O(8) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Al(2)O4 tetrahedra. The Si(1)-O(1) bond length is 1.62 Å. The Si(1)-O(3) bond length is 1.66 Å. The Si(1)-O(6) bond length is 1.64 Å. The Si(1)-O(8) bond length is 1.63 Å. In the second Si site, Si(2) is bonded to one O(2), one O(3), one O(5), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Al(1)O4 tetrahedra. The Si(2)-O(2) bond length is 1.64 Å. The Si(2)-O(3) bond length is 1.66 Å. The Si(2)-O(5) bond length is 1.62 Å. The Si(2)-O(7) bond length is 1.64 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Al(2) and one Si(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Pb(1), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to one Al(1), one Al(2), and one Pb(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Al(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Al(2) and one Si(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 150 degrees geometry to one Al(1), one Pb(1), and one Si(2) atom. In the eighth O site, O(8) is bonded in a bent 150 degrees geometry to one Al(2) and one Si(1) atom.