Bones are primarily made of calcium and phosphorus, combined into a mineral crystal called hydroxyapatite. This single compound accounts for about 60% of bone’s total weight. The remaining 30% is organic material (mostly collagen protein), and the final 10% is water. Beyond calcium and phosphorus, bones also store meaningful amounts of magnesium, sodium, and several trace minerals that play supporting roles in keeping the skeleton strong.
Hydroxyapatite: The Main Mineral
The mineral that gives bone its hardness and rigidity is hydroxyapatite, a crystalline compound with the chemical formula Ca₅(PO₄)₃(OH). It’s essentially calcium bonded to phosphate groups and a hydroxide ion, packed into tiny crystals that sit within and between fibers of collagen protein. Think of collagen as the flexible scaffolding and hydroxyapatite as the cement that makes that scaffolding rigid. Without the mineral, bone would bend like cartilage. Without the collagen, it would shatter like chalk.
By volume, the split looks a bit different than by weight: roughly 40% mineral, 35% organic material, and 25% water. That water content matters because it contributes to bone’s slight flexibility and helps transport ions in and out of the crystal structure.
Calcium and Phosphorus
Calcium and phosphorus are the two dominant minerals in bone, and their balance matters. The ideal dietary ratio is about 1.3 parts calcium to 1 part phosphorus by weight. Your skeleton holds roughly 99% of your body’s total calcium and about 85% of its phosphorus, making bone the largest mineral reservoir in the body. When blood calcium drops, your body pulls it from bone to keep nerves and muscles functioning. Chronic withdrawals without adequate replacement are what eventually thin bones.
Phosphorus does more than just partner with calcium in hydroxyapatite. Specialized enzymes on the surface of bone-building cells break down a compound called pyrophosphate, which normally blocks mineralization. This releases free phosphate ions that combine with calcium and trigger crystal formation. The ratio of phosphate to pyrophosphate in the local environment around bone cells is one of the key switches that determines whether new mineral gets deposited or not.
How Minerals Get Deposited Into Bone
Collagen fibers in bone aren’t packed perfectly tight. Their hierarchical structure leaves nanoscale gaps both inside and between fibrils. These gaps are small enough that large proteins that would inhibit mineralization can’t fit inside, but calcium and phosphate ions can. Once those ions reach sufficient concentrations in and around the collagen scaffold, hydroxyapatite crystals begin to nucleate and grow.
Bone-building cells also release tiny packages called matrix vesicles into the surrounding tissue. Inside these vesicles, enzymes break down membrane components to liberate phosphate. As phosphate accumulates inside the vesicle, it combines with calcium to form the first seed crystals of hydroxyapatite. These seed crystals then serve as starting points for further mineral growth once the vesicle breaks open into the collagen matrix.
Magnesium’s Role in Crystal Quality
About one-third of skeletal magnesium sits on the surface of hydroxyapatite crystals or in the thin water layer surrounding them. The rest is built directly into the crystal lattice. Magnesium influences something subtle but important: the size and organization of those crystals.
When magnesium levels are adequate, it competes with calcium for binding sites and keeps hydroxyapatite crystals relatively small. Smaller crystals actually produce better bone because they distribute stress more evenly and give bone a degree of flexibility. In magnesium-deficient animals and in osteoporotic women with low magnesium, the crystals grow larger and more perfectly structured, which sounds like a good thing but actually makes bone stiffer and more brittle. High magnesium also inhibits excessive crystal formation by binding to pyrophosphate and forming a compound that resists enzymatic breakdown, providing a natural brake on runaway mineralization.
Sodium, Potassium, and Other Stored Minerals
Bone isn’t just a calcium and phosphorus bank. Your skeleton stores roughly 35% to 40% of your body’s total sodium, about 1,400 milliequivalents tucked into the mineral matrix. Potassium is stored in smaller amounts. These electrolytes can be mobilized when the body needs them, which is one reason bone is sometimes described as a mineral homeostasis organ rather than just a structural one.
Several trace minerals also accumulate in bone tissue and influence its metabolism. Zinc, copper, boron, iron, and selenium all play roles in how bone cells function, how collagen cross-links form, and how efficiently the mineralization process runs. None of these are present in large quantities, but deficiencies in any of them can impair bone turnover and quality.
Unwanted Minerals That Displace the Good Ones
Because hydroxyapatite readily accepts ions that are similar in size and charge to calcium, toxic metals can substitute into the crystal structure. Lead is the most well-studied example. It mimics calcium so effectively that the bone matrix becomes its primary storage site in the body. Lead can also displace magnesium and iron from bone. Aluminum and iron in their oxidized forms can similarly swap into spots normally occupied by calcium in the crystal lattice. Once embedded, these metals can linger for years or decades, slowly releasing back into the bloodstream during normal bone remodeling.
How Mineral Content Changes With Age
Bone mineral content increases steadily throughout childhood and adolescence as the skeleton grows in size. During the second and third decades of life, mineral accumulation continues even after you stop getting taller, eventually reaching what’s called peak bone mass. This plateau, typically in the mid-to-late twenties, represents your lifetime maximum of mineral density. It’s one of the strongest predictors of whether you’ll develop osteoporosis later.
After peak bone mass, mineral content holds relatively steady for a while, then begins a gradual decline. Women experience a sharper drop in the first few years after menopause due to falling estrogen levels, which normally help suppress excessive bone breakdown. Both men and women then continue to lose mineral more slowly with advancing age.
Bone mineral density tests measure this progression using a T-score, which compares your density to that of a healthy young adult. A T-score of negative 1 or higher is considered healthy. Between negative 1 and negative 2.5 indicates osteopenia, a milder form of bone loss. At negative 2.5 or below, the diagnosis shifts to osteoporosis. Each single-point drop in T-score increases fracture risk by 1.5 to 2 times, which is why even modest mineral losses carry real consequences.